UNIVERSITY OF CALIFORNIA 
 
 COLLEGE OF AGRICULTURE 
 
 AGRICULTURAL EXPERIMENT STATION 
 
 BERKELEY, CALIFORNIA 
 
 BREEDING 
 FOR EGG PRODUCTION 
 
 LEWIS W. TAYLOR and I. MICHAEL LERNER 
 
 BULLETIN 626 
 
 October, 1938 
 
 UNIVERSITY OF CALIFORNIA 
 BERKELEY, CALIFORNIA 
 
CONTENTS 
 
 PAGE 
 
 Explanation of terms used in breeding 3 
 
 Historical development of methods of poultry breeding 5 
 
 Characters governing the annual egg record 8 
 
 Sexual maturity 10 
 
 Pauses 13 
 
 Persistency 16 
 
 Rate of production 17 
 
 General considerations of production characters 20 
 
 Characters affecting viability 21 
 
 Longevity 22 
 
 Disease resistance 22 
 
 Methods of measuring flock production 23 
 
 Characters governing the quality of eggs 24 
 
 Egg size 24 
 
 Egg shape 27 
 
 Egg color 27 
 
 Shell quality 28 
 
 Albumen quality 29 
 
 Hatchability 30 
 
 Breeding systems 30 
 
 Close inbreeding and line breeding 31 
 
 Outbreeding and crossbreeding 32 
 
 Grading 33 
 
 Value of breeding systems 33 
 
 Relation of genetics to poultry-breeding methods 33 
 
 Pedigree breeding 33 
 
 Pedigree records 34 
 
 Sib and progeny testing 35 
 
 Multiplication of superior stock 37 
 
 A breeding program for multipliers 39 
 
 Age of breeding stock 40 
 
 What breeding can accomplish 41 
 
 Egg production 41 
 
 Viability 43 
 
 Importance of other conditions 44 
 
 Literature cited 45 
 
BREEDING FOR EGG PRODUCTION 
 
 LEWIS W. TAYLOR 2 and I. MICHAEL LEKNER 3 
 
 The purpose of this bulletin is twofold : (1) to review the present knowl- 
 edge of the genetics of egg-production and egg-quality characters and 
 (2) to demonstrate what may be accomplished by the application of the 
 principles and methods discussed in the text in bringing about rapid im- 
 provement in egg production. For this purpose, data from the California 
 Agricultural Experiment Station flock, as well as from literature on 
 poultry breeding, are used. 
 
 In the limited space of a bulletin, only a small part of the published 
 work on the subject can be reviewed. Only those papers which are di- 
 rectly drawn upon are cited. Similarly, the analysis of the Experiment 
 Station data has necessarily been confined to the more important general 
 considerations, with many details omitted. These data confirm in a strik- 
 ing manner the adequacy of the progeny-test method of breeding poultry, 
 when this method is applied to the individual characters affecting pro- 
 duction and viability. We believe that the application of similar princi- 
 ples should enable a poultry breeder to accomplish substantially the same 
 type of flock improvement. The presentation of dogmatic methods of pro- 
 cedure for pedigree breeders has purposely been avoided because con- 
 ditions vary widely from flock to flock. Emphasis is thus laid on broad 
 principles which may be adapted to specific circumstances found in dif- 
 ferent flocks. However, a more definite breeding program for multipliers' 
 flocks is advanced. 
 
 EXPLANATION OF TERMS USED IN BREEDING 
 
 Inheritance of characters is based on the transmission of genes from 
 parent to offspring. Genes are defined as the determiners of hereditary 
 characters. They occur in pairs (with a few exceptions), one member of 
 each pair being derived from the sire, and its mate from the dam. 
 
 One member of the pair may be different from its mate, as in the case 
 of a cross between two breeds. For example : A bird resulting from a cross 
 between a purebred rose-combed and a single-combed bird will receive a 
 gene for rose comb from one parent and for single comb from the other. 
 The crossbred bird will exhibit a rose comb, since the rose-comb gene is 
 
 1 Eeceived for publication April 19, 1938. 
 
 2 Associate Professor of Poultry Husbandry and Associate Poultry Husbandman 
 in the Experiment Station. 
 
 3 Instructor in Poultry Husbandry and Junior Poultry Husbandman in the Experi- 
 ment Station. 
 
 [3] 
 
4 University of California — Experiment Station 
 
 dominant and prevents the expression of the single-comb gene, which is 
 known as a recessive. If two such crossbred birds are mated together, in 
 the next generation both rose- and single-combed individuals will appear 
 in the ratio of three rose-combed birds to one single-combed bird. Of the 
 rose-combed birds, one-third will be pure-breeding, and two-thirds will 
 not breed true (possessing like their parents, one rose-comb gene and one 
 single-comb gene) . The birds which possess two like genes and breed true 
 are homozygous for this gene. In the example given, the pure-breed- 
 ing rose-combed and the single-combed birds are homozygous for comb- 
 shape genes. Those rose-combed birds which possess two unlike genes will 
 not breed true and are heterozygous. 
 
 Various genes are usually designated by arbitrary letters. Capital let- 
 ters indicate the dominant condition, and small letters the recessive : the 
 gene for rose comb, for example, is represented as B and that for single 
 comb as r. The actual constitution of an individual with respect to these 
 genes is known as the genotype, which is designated by the genes of which 
 it is composed : the genotype of the single-combed bird, for example, is 
 represented by rr, that of the pure-breeding rose-combed bird by BB, and 
 that of the heterozygous rose-combed bird by Br. 
 
 The appearance of the bird with respect to these characters is called 
 the phenotype. In the example given, there are only two phenotypes — 
 single-comb, to which the bird with the rr genotype belongs, and the rose- 
 comb, to which the birds with both the BB and Br genotypes belong. The 
 two genotypes that show the rose-comb phenotype can be distinguished 
 only by breeding tests. If a rose-combed bird, bred to a single-combed 
 bird gives only rose-combed offspring, then it must belong to the BB 
 genotype, but if it gives some single-combed offspring, then it must be- 
 long to the Br genotype. 
 
 At times, when two unlike genes are present in the pair governing the 
 expression of a character, neither gene completely suppresses the action 
 of its mate. The case is then one of incomplete dominance. When this 
 occurs, the mating of two such heterozygous individuals produces three 
 phenotypes in the offspring, which correspond to the three genotypes 
 present. 
 
 In cases where characters are produced by the action of a large num- 
 ber of genes, such are known as multiple genes. 
 
 Sex-linked inheritance can probably be best explained by an example. 
 A simple sex-linked character in poultry is the barred plumage of the 
 Barred Plymouth Rock. When a male of this variety is crossed with a 
 black female, all the progeny is found to be barred. In a reciprocal cross, 
 however, when a black male is mated to a barred female, in the first gen- 
 
Bul. 626] Breeding for Egg Production 5 
 
 eration, the females are found to be black, while the males are barred. 
 Sex-linked crosses for early sex identification are based on this principle. 
 The reason for the behavior of the sex-linked genes lies in the fact that 
 the male possesses a paired complement, as in the case of all other genes, 
 while the female has only one member of a sex-linked gene pair. Thus the 
 male genotype in the case of the pure Barred Plymouth Rock is BB, 
 while the female genotype is B-. In the case of the black birds, the male's 
 constitution is bb and the female's b-. In the second cross as described 
 above, bb x B- (black male x barred female), the male progeny will be 
 heterozygous barred (Bb) and the females will be black (5-) . 
 
 HISTORICAL DEVELOPMENT OF METHODS OF 
 POULTRY BREEDING 
 
 The history of the development of egg-production characters in strains 
 of chickens goes back centuries before there was any indication of a com- 
 mercial poultry industry. The types of fowl developed in the countries 
 bordering on the Mediterranean Sea were early recognized to have a 
 greater inherent capacity for producing eggs than birds from the Orient. 
 From these higher-producing fowl, the breeds of the Mediterranean class 
 of chickens, which includes Leghorns, Minorcas, and Anconas, were de- 
 veloped. During an intensive period of breed creation in the nineteenth 
 century, fowls of the Mediterranean class were extensively used to 
 cross with Asiatic breeds or with mongrel stocks to produce new breeds 
 and varieties, which are found in the American and English classes of 
 chickens. 
 
 With the development of improved systems of transportation and 
 marketing, accompanied by an increased demand for eggs for human 
 consumption at seasons other than those of normal high egg production, 
 an increasing importance was attached to the inherent ability of birds to 
 lay. Under some conditions of poultry keeping, the value of a breed no 
 longer rested entirely on its size and quality of meat, on its plumage 
 color or pattern, or on its peculiar appearance as evidenced by structural 
 differences in comb, feathers, or number of toes. Thus, by the middle of 
 the nineteenth century, writers on poultry subjects began to emphasize 
 the ability of certain breeds to lay a relatively large number of eggs. 
 Before the end of that century, the idea of trapnesting birds in order to 
 obtain exact records had been conceived and suitable devices to accom- 
 plish this work had been invented. 
 
 At the close of the nineteenth century, an experimental approach to 
 the solution of problems involved in improving egg production was made 
 by Gowell at the Maine Agricultural Experiment Station on a flock of 
 
6 University of California — Experiment Station 
 
 Barred Plymouth Rocks (Pearl and Surface, 1909)/ His method of 
 breeding involved the selection and mating of his better-producing fe- 
 males in one line and of the poorer in another. No means of evaluating 
 the contributions of the breeding males was employed. After a period of 
 years, the egg production in the two lines was approximately the same. 
 
 Pearl succeeded Go well and was able to improve the production of the 
 same flock by the use of different methods of selection. He noticed that 
 there was a close correlation between the winter and the annual produc- 
 tion of any bird. The more eggs produced in winter, the higher was the 
 annual record. Using winter production as a criterion of selection for 
 mating, and analyzing his results genetically, Pearl (1912) developed his 
 theory of the inheritance of egg production, which proposed that two 
 pairs of genes were primarily responsible for high egg production. 
 
 A few years after Pearl's work was published, Goodale (1918) intro- 
 duced a new concept regarding the mode of inheritance of egg production 
 obtained from studies on the Rhode Island Red flock of the Massachusetts 
 Agricultural Experiment Station. Egg production was considered by 
 him to be the final expression of the action and interaction of a number 
 of diverse production characters. Of some ten characters first listed by 
 Goodale, five are important in contributing to an individual's annual 
 record. A sixth has been suggested recently. These characters may be 
 divided into two general groups : those affecting the length of time the 
 bird lays eggs, and rate or intensity of production when the bird is lay- 
 ing. The first group of characters include : the time when a pullet begins 
 to lay eggs, known as sexual maturity; the time when a hen ceases to lay 
 at the end of the pullet year of production, known as persistency ; and the 
 intervening nonlaying periods within the time limits of sexual maturity 
 and persistency produced by broodiness and winter pause. Spring or sum- 
 mer pause may be considered as another character (Lerner and Taylor, 
 1936) . These characters governing the length of time in production, com- 
 bined with rate of production, determine the annual record of the hen. 
 
 After the development of the idea of breaking egg production down 
 into a number of component parts, Goodale and later Hays carried on 
 studies to determine the method of inheritance of these characters. As a 
 result of these studies, Hays has advanced a series of hypotheses regard- 
 ing the number and nature of the genes that control the characters deter- 
 mining egg production (see Jull, 1932, for a bibliography of papers on 
 this subject) . Hays and Sanborn (1934) also have published data on the 
 improvement in the flock egg production and the various production 
 
 4 See "Literature Cited" at the end of this paper for complete data on citations, 
 which are referred to in the text by author and date of publication. 
 
Bui,. 626] BREEDING FOR EGG PRODUCTION 7 
 
 characters in the Massachusetts Station flock over a period of twenty 
 years of breeding conducted by Goodale and Hays. 
 
 Several other investigators have published data on improvement in 
 egg production. Dryden (1921), by breeding from hens with high egg 
 records, was able to increase greatly the average production in the Barred 
 Plymouth Rock, White Leghorn, and crossbred flocks at the Oregon Agri- 
 cultural Experiment Station. He also commented that the use of the 
 progeny test in selection might be expected to hasten the progress to 
 higher production averages. Asmundson (1927a, 19275, 1928) has re- 
 ported results from six years of breeding Rhode Island Reds, Barred 
 Plymouth Rocks, and White Leghorns for improved egg production at 
 the University of British Columbia. The results of twenty years of mass 
 selection of high- and low-egg-production lines at the Cornell Agricul- 
 tural Experiment Station have been reported by Hall (1935) . Hall's con- 
 clusion that egg production has been improved by this method of selection 
 has been challenged by Petrov (1935) who has pointed out that, with the 
 exception of the first four years of selection, the increase in egg produc- 
 tion seemed to be due to improved environmental conditions, since both 
 high and low lines improved in average egg production. 
 
 During the early part of the present century, practical poultry breed- 
 ers selected birds for mating largely on the bases of physical appearance 
 and breed characters stressed by the Standard of Perfection. When trap- 
 nesting and pedigree systems came into common use, strong reliance was 
 placed on the ancestry of the individuals mated. This was soon followed 
 by a combined consideration of the individual's performance along with 
 its ancestry. During this period, the poultry industry began to strive for 
 high individual records. The production of 300-egg hens became the goal 
 of breeders. 
 
 Recently, there has been a tendency to add progeny testing to the bases 
 for selection of birds for breeding. This last step, introduced to the field 
 of poultry breeding by Pearl, combines all of the information available 
 from ancestry and individual records of performance with an actual test 
 of the bird's breeding ability. This type of selection has also tended to 
 change the emphasis from an individual basis of selection to a family 
 basis. Not only good individuals, but good individuals from good fam- 
 ilies, are wanted. 
 
 Prom a genetic standpoint, the progeny test is based on sound princi- 
 ples. Many good individuals with similar phenotypes are known to be 
 genotypically dissimilar, some being homozygous and some heterozygous 
 for different important genes controlling production characters. A good 
 bird from a uniformly good family is likely to be homozygous for and 
 
8 University of California — Experiment Station 
 
 breed true for a greater number of desirable genes than a good individual 
 from a poor family. The final proof of the value of a breeding bird is, 
 then, found in the offspring, which show the various combinations of 
 hereditary factors that the parent can actually transmit to the next gen- 
 eration. This is especially so, since the actual number of genes involved 
 in the inheritance of egg production is not known. Estimates vary from 
 8 pairs postulated by Hays and Sanborn (1934) to 200-300 suggested by 
 Munro (1937) . Whatever the exact number may be, the phenotypes dis- 
 played by the progeny or by the sisters of the individuals considered give 
 a means of estimating the relative values of their genotypes. 
 
 CHARACTERS GOVERNING THE ANNUAL EGG RECORD 
 
 The annual record is a measure of the productivity of the individual and 
 reflects the contributions of all egg-production characters. The ideal con- 
 dition to strive for is the best combination of all of the desirable char- 
 acters involved. The number and importance of the independent factors 
 entering into the annual record can be adequately illustrated by actual 
 trapnest records of some individuals. A number of birds may have the 
 same annual record, but it may be conditioned by entirely different char- 
 acters in each case. Five representative individual trapnest sheets se- 
 lected to illustrate this point appear in figure 1. The five birds laid very 
 nearly the same number of eggs in the thirteen calendar months begin- 
 ning with September 1. Yet an examination of the course of egg produc- 
 tion in these birds will show five different reasons why the figure of 
 approximately 200 eggs in each case was not exceeded. 
 
 The bird whose record is shown in figure 1, A, was a late-maturing in- 
 dividual, not starting production until she was 305 days old. Although 
 she laid at a high rate, the late sexual maturity limited her record for 
 the year. 
 
 Similarly, the length of time in production of the bird in figure 1, B, 
 was limited by an early molt. She evidently was a nonpersistent bird and, 
 though fairly early-maturing and laying with reasonable intensity, did 
 not exceed the 200-egg mark because of early cessation of production. 
 
 Figure 1, C, on the other hand, illustrates the loss of time in production 
 during the laying year, rather than at its beginning or end as in the 
 previously mentioned cases. This bird went into a pullet molt or, in other 
 words, exhibited winter pause, during the months of December and Jan- 
 uary, which limited her possibilities of attaining a high record. 
 
 The case of the bird in figure 1, D, which became broody on four occa- 
 sions during the year, also shows a similar loss of time in production. 
 
 Finally, figure 1, E, shows the record of a bird that possesses most of 
 
Bul. 626] 
 
 Breeding for Egg Production 
 
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 E, Leg band No. J 10; hatching date Mar.20,1934; age at first egg, 176 days 
 
 Fig. 1. — Conditions lowering the annual record: 
 A, late maturity; B, poor persistency; C, winter 
 pause; D, broodiness; E, low rate. 
 
10 University of California — Experiment Station 
 
 the desirable characters governing time in production — her maturity 
 and persistency are satisfactory, she was nonbroody and lost very little 
 time by winter pauses. And yet her record was as low as those of the other 
 four. The explanation lies in her inability to lay at a high rate. While 
 the bird in figure 1, A exhibited monthly production figures of 24, 26, 
 28, 27, 27, and so on, this bird laid only 19, 24, 15, 18, and 12 eggs respec- 
 tively. Although the factors which need improvement in these five birds 
 are different, no indication that this is the case could be obtained from 
 the total annual egg records alone. 
 
 However, the conditions existing in reality are even more complicated. 
 The birds in figure 1 each exhibited one principal defect. Most birds 
 show various combinations of these defects, often two and sometimes 
 three or more at a time. Yet it is possible both for the trapnesting breeder 
 and the nontrapping multiplier to detect and estimate the severity of 
 these defects in the flock. 
 
 Methods of detecting and measuring each of these defects with and 
 without trapnesting are discussed in the following sections. 
 
 Sexual Maturity. — Sexual maturity is the character which expresses 
 the rate of sexual development of the bird and the onset of egg produc- 
 tion. This factor is the one that Pearl principally studied when he investi- 
 gated winter production. In his stock it was the early-maturing birds 
 which laid the most eggs before March 1 of their first laying year. When 
 maturity is considered on a time scale, a less arbitrary criterion of this 
 character is available. If all of the birds considered are hatched on the 
 same date, the date of the first egg laid provides such a measure. If, how- 
 ever, the date of hatch is variable, maturity may more logically be meas- 
 ured by the age at first egg. 
 
 There is, however, a complication involved here too. Climatic condi- 
 tions are known to affect the rate of growth of individuals ; early-hatched 
 chicks, as a rule, grow and develop faster than do late-hatched ones 
 (Kempster and Parker, 1936). The age at first egg is affected by a sea- 
 sonal retardation of growth, associated with high temperature. The Cali- 
 fornia Station flock is usually hatched in March and April, but even in 
 this short season, differences develop : each week's delay in hatching de- 
 lays the date of first egg by 10 to 12 days, or, in other words, the average 
 age at first egg is increased by 3 to 5 days. In a similar manner, nutrition 
 affects the age at first egg : for example, the addition of 15 per cent wheat 
 bran to a ration containing no bran used at the California Station low- 
 ered the average age at sexual maturity by approximately 15 days. 
 
 Artificial light is another environmental factor which may decrease 
 both the age and the date of sexual maturity. Thus Scott and Payne 
 
Bul. 626] Breeding for Egg Production 11 
 
 (1937), by subjecting turkeys to artificial light, accelerated the onset of 
 production by nearly two months. Similarly, Bissonnette and Csech 
 (1937) and also Clark, Leonard, and Bump (1937) obtained earlier eggs 
 from pheasants by providing artificial light. 
 
 Age at first egg, however, seems to be a satisfactory criterion of sexual 
 maturity in chickens, and with the recognition of and provision for the 
 influence of hatching date, it can be safely used, when other management 
 factors are uniform. In turkeys, Asmundson (1938) has found date of 
 first egg to be superior to age at first egg as a measure of maturity. 
 
 That sexual maturity affects annual production is clearly seen from 
 figure 2, which illustrates the effect of the date of first egg on the length 
 
 AUG SEP OCT NOV DEC JAN FEB MAR APR MAY JUN JUL AUG SEP OCT 
 
 414- DAYS 
 
 374 DAYS 
 
 336 DAYS 
 
 269 DAYS 
 
 2 24 DAYS 
 
 DATE OF FIRST EGG BEGINNING OF FALL MOLT 
 
 Fig. 2. — Length of laying year in relation to date of 
 first egg. Birds are grouped according to date of first egg, 
 in 50-day periods. 
 
 of laying year of a flock of pullets. The actual production is not shown 
 here largely because there are inherent differences in intensity of laying 
 between the individuals whose records were used for this chart. If equal 
 rates of production were found in all groups, the length of time in pro- 
 duction would be directly related to the annual record. Figure 2 indicates 
 that birds maturing in February have only slightly more than 50 per 
 cent as much time in production previous to the annual fall molt as birds 
 maturing in August. The fall molt occurs at about the same date regard- 
 less of date of maturity. In commercial practice, the factor of early ma- 
 turity is of added importance, for most of the birds are hatched in the 
 spring months, and hence those that are early-maturing usually lay more 
 eggs at the time of high egg prices. 
 
 That sexual maturity is inherited has been demonstrated definitely by 
 Pearl (1912), by Hays (1924), and by "Warren (1934). In determining 
 this character both sex-linked and non-sex-linked genes are involved. 
 Sex-linked inheritance in chickens operates in such a manner that the 
 sire exerts more influence than the dams on the character observed in the 
 daughters (see p. 4 in section, "Explanation of Terms Used in Breed- 
 
12 University of California — Experiment Station 
 
 ing"). Consequently it is of paramount importance to use sires from 
 early-maturing families if one desires to improve his flock for this char- 
 acter. 
 
 Hays (1924) has suggested that two pairs of genes are involved in the 
 inheritance of early maturity : EE, which is sex-linked (E- in females) 
 and E'E', which is not. According to Hays, either gene in the dominant 
 state will produce early maturity, while late maturity will ensue when 
 both are recessive. Warren (1934) has produced evidence that both E 
 and E' are necessary for a satisfactory early maturity. The situation may 
 be even somewhat more complicated, but in any case, at least two pairs 
 of genes are involved, one of which is sex-linked. 
 
 There is a possibility of having birds which mature too early. Preco- 
 cious maturity is associated with small body size, which is one of the fac- 
 tors contributing to small egg size (see section entitled "Egg Size," p. 24) . 
 Precaution must therefore be taken to guard against developing too 
 early maturity as well as against too late maturity. 
 
 Hays (1936a) has produced evidence that the earliest phenotypic 
 group of Rhode Island Reds mature at less than 180 days of age. These 
 birds were assumed to carry genes E and E', since they produced off- 
 spring maturing on the average earlier than the offspring of dams ma- 
 turing at 180 days or more. The border line between early maturity and 
 precocity has not yet been determined. However, general breeding ex- 
 perience would indicate that birds maturing at less than 150 days of age 
 are apt to be small and to produce small eggs. Furthermore, at least in 
 one case on record (Knox, 1930), such birds did not produce as many 
 eggs as the birds maturing at between 160 and 210 days of age. 
 
 Identification of both early- and late-maturing birds is accomplished 
 very easily. The trapnest record, of course, is the best guide, but physical 
 selection of early-maturing birds is relatively simple. Any culling guide 
 gives directions for selecting early-maturing birds. 5 The late-maturing 
 birds should be marked in order to eliminate them from any future use 
 as breeders. They may, however, still prove to be fairly profitable egg 
 producers, if they are healthy and of average size. 
 
 5 Most general texts on poultry contain instructions for culling, for example : 
 Jull, M. A. Poultry husbandry. 2d ed. viii + 548 p. Illus. McGraw-Hill Book Com- 
 pany, Inc., New York, N. Y. 1938. 
 
 Lippincott, W. A., and L. E. Card. Poultry production. 723 p. Illus. Lea and Febi- 
 ger, Philadelphia. 1934. 
 
 There are also books dealing specifically with the subject of culling methods: 
 Eice, J. E., G. O. Hall, and D. E. Marble. Judging poultry for production, rru + 425 p. 
 Illus. John Wiley and Sons, New York, N. Y. 1930. 
 
 Payne, L. F., and H. M. Scott. International poultry guide for flock selection. 
 142 p. Illus. International Baby Chick Association, Kansas City, Mo. 1934. 
 
Bul. 626] Breeding for Egg Production 13 
 
 Pauses. — Pauses occurring in the production record between the time 
 of the first egg laid and the normal fall molt may be broadly classified 
 into three groups : winter pause, pause due to broodiness, and spring and 
 summer pause. 
 
 Winter Pause : One very important problem in connection with pauses 
 in the winter as well as those in the spring and summer is the debatable 
 question as to what constitutes a pause. At various times, different work- 
 ers on the subject have used some given number of successive days of 
 nonlaying for a definition of pause, the number of such consecutive days 
 varying from 4 to 15. As yet no clear-cut definition, which would not be 
 arbitrary, has been suggested, the major difficulty being the fact that the 
 underlying reasons for such pauses are not well understood. In many 
 cases, pauses can be recognized in the individuals of a flock by head, neck, 
 or body molt. As has long been recognized, birds going out of production 
 usually start molting. Often the severity of the molt — which is judged by 
 knowledge of the order of the normal molt (head-neck; breast-abdomen- 
 back ; wings ; tail) — serves to indicate the length of the pause. The win- 
 ter-pause molt is usually confined to the head and neck. Some individ- 
 uals, however, lay through a molt, and others pause for a short period of 
 time without molting. In a trap-nested flock, all types of pausing birds 
 can be accurately recognized, but in a flock which is not being trapped, 
 some are improperly classified even by those well trained in the use of 
 known physical methods of culling. 
 
 Little is known about the factors involved in winter pausing. That 
 pauses are brought on by environmental factors in many cases is well 
 recognized, for sudden changes in management, extremely rough han- 
 dling, and sudden climatic changes often result in pause. Since earlier- 
 hatched groups of pullets of the same strain regularly show a greater 
 amount of pause than later-hatched lots, there is probably a direct rela- 
 tion between age of the bird and the internal conditions which cause her 
 to cease egg production. Yet at the same time that some birds in any 
 flock of a given age show a pause, others may go through such disturb- 
 ances and continue to lay. This gives evidence that there are differences 
 between birds in their thresholds of response to conditions producing a 
 pause. Evidence from the California Station flock indicates that such 
 differences are, at least in part, hereditary. If such is the case, the possi- 
 bility of selection and breeding for absence of pause immediately pre- 
 sents itself. 
 
 That such procedure is wise can be illustrated by the losses a poultry- 
 man sustains by having many pausing birds in his flock. Thus, some years 
 ago in the California Station flock, birds showing a pause of 7 or more 
 
14 University of California — Experiment Station 
 
 days were found to lay on the average about 56 eggs during the four 
 winter months (November through February), while the nonpausing 
 birds during the same time laid on the average 77 eggs ; the winter pro- 
 duction of the nonpausing birds was over one-third greater. 
 
 Hays (1924) has postulated that the inheritance of winter pause de- 
 pends on a dominant gene M for its expression. Thus, birds of the con- 
 stitution mm would be free from pause. However, the mode of inheri- 
 tance of the factors responsible for the occurrence of winter pauses is 
 not so simple as this, for, if it were, the elimination of pause from a flock 
 would be easily obtained by breeding only from nonpausing {mm) fam- 
 ilies. At present, the best a breeder or multiplier can do is to select for 
 mating those families or individuals which show greatest freedom from 
 pause. For breeding selection against pause, pullets which are from 7 to 
 8 months of age in November will probably give a better differentiation 
 between genetic pausers and nonpausers than either older or younger 
 birds. The trapnest breeder can base his selection on periods of nonpro- 
 duction in the egg records. In the nontrapnested flock, reliance must be 
 placed on the appearance of molt in birds. Although some of the pausing 
 birds may prove profitable egg producers, they should be identified so as 
 to be eliminated from the breeding flock. 
 
 Broodiness : This type of pause is of greater importance in some breeds 
 than in others. The Asiatic breeds are very broody as a rule. The Ameri- 
 can breeds, such as Plymouth Rocks, Rhode Island Reds, and Wyan- 
 dottes, are subject to considerable broodiness, while most of the Mediter- 
 ranean breeds seem to be comparatively free from it ; but no strain of 
 any breed has ever been reported to be completely nonbroody. Selection 
 against broodiness should be very rigorous, since if unchecked it may 
 become an important problem, especially in small breeding flocks. 
 
 Of the different characters involved in egg production, broodiness is 
 one upon which considerable genetic and physiological data have been 
 obtained. Two pairs of dominant complementary genes, A and C, seem 
 to be involved in producing broodiness, according to Goodale, Sanborn, 
 and White (1920). The presence of either A or C alone will produce no 
 effect, but the two brought into combination will result in a broody bird. 
 Thus two nonbroody lines crossed together may produce broody progeny, 
 if one of the lines possesses the factor complementary to the one carried 
 by the other line. For instance, a cross between AAcc and aaCC will re- 
 sult in a broody bird, AaCc, heterozygous for both genes. In the Cornish 
 breed, Roberts and Card (1934) have obtained evidence of an additional 
 sex-linked gene for broodiness. 
 
 A situation involving complementary genes makes it difficult for any 
 
Bul. 626] Breeding for Egg Production 15 
 
 breeder to eliminate broodiness completely from a strain. The incidence 
 of broodiness may be greatly reduced, however, by a method of testing 
 the genotype of breeding birds. This method involves the isolation of two 
 nonbroody strains : one carrying gene A without gene C, the other carry- 
 ing C without A. When crossed, these strains produce broody daughters. 
 Prospective breeding males and females can be mated to first one and 
 then the other of the test strains. If no broody daughters are produced 
 from the mating with either strain, then the prospective breeding bird 
 carries no genes for broodiness and is of the constitution aacc. 
 
 In a flock where such a program is not feasible, elimination of the 
 broody families or, in the case of nontrapnested birds, of broody individ- 
 uals from the breeding flock, should be made. Since broodiness is a very 
 conspicuous character, this plan can easily be carried out. In the simplest 
 form it involves placing a colored legband on a bird that at any time was 
 taken off the nest as being broody. Once identified, such birds can be dis- 
 posed of in any manner desired. 
 
 The importance of broodiness in the annual record of a bird depends 
 both on the number of times during the year that a bird becomes broody 
 and on the length of time before the bird resumes laying. Both of these 
 factors are extremely variable ; the latter is in part dependent on man- 
 agement, since the sooner a broody bird is detected and prevented from 
 sitting, the more quickly she will return to production. 
 
 The physiological basis of broodiness is better understood than that of 
 most of the other characters discussed. Broodiness is produced by pro- 
 lactin, a hormone secreted by the anterior pituitary body. This is the 
 same substance that controls the production of crop milk in pigeons as 
 well as of the milk of nursing mammals (Riddle, Bates and Lahr, 1935) . 
 Byerly and Burrows (1936) have found that broody females and males 
 from broody strains produce more of this hormone than nonbroody birds. 
 
 Spring and Summer Pauses : Besides the pauses occurring in the win- 
 ter months and pauses due to broodiness, there are other periods of in- 
 terruption of egg production, which may appear during the spring or 
 summer. Superficially they seem to be of the same nature as the winter 
 pauses, but actually they have been demonstrated (Lerner and Taylor, 
 1936) to be the result of other causes which have not as yet been deter- 
 mined. Whether hereditary factors are concerned in the production of 
 these pauses is not known at present, consequently neither is it known 
 whether or not selection against them is feasible. Superior birds, how- 
 ever, do not exhibit this condition, so that discriminating against spring 
 and summer pausers in a breeding flock is a safe policy because of the 
 possibility that this defect may be transmitted to their progeny. 
 
16 University of California — Experiment Station 
 
 Persistency. — The character limiting or extending egg production at 
 the end of the laying period is known as "persistency" ; the cessation of 
 production is usually accompanied by the onset of the annual (fall) molt. 
 Its importance, recognized by Rice (1914) before the development of the 
 Goodale-Hays hypothesis, can be readily demonstrated in the same man- 
 ner as that of early maturity. Thus figure 3, based on the same stock as 
 used for figure 2, shows that the early -molting birds have a much shorter 
 period of lay than the late molters. Thus the birds molting in June were 
 in production about 60 per cent as long as those molting in December. 
 Figure 3, like figure 2, indicates that there is no relation between date 
 of first egg and the time of onset of fall molt, 
 
 OCT NOV DEC JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC 
 
 
 248 
 
 DAYS 
 
 1 
 
 
 1 
 
 
 
 1 
 
 I 
 
 302 
 
 DAYS 
 
 
 
 1 
 
 
 
 
 
 
 
 336 
 
 DAYS 
 
 
 
 
 
 
 1 
 
 390 
 
 DAYS 
 
 
 1 
 
 
 
 
 
 
 429 DAYS 
 
 DATE OF FIRST EGG BEGINNING OF FALL MOLT 
 
 Fig. 3. — Length of laying year in relation to beginning 
 of annual molt. Birds are grouped according to date of 
 molt, in 50-day periods. 
 
 A rather important question in connection with persistency is the 
 selection of an adequate method of measurement. There is a large number 
 of these available, the most prominent ones being the length of biological 
 laying year, suggested by Hays and Sanborn (1926) ; production in 
 August and September, used by Knox, Jull, and Quinn (1935) ; and age 
 at last egg or date at last egg, originally mentioned by Goodale (1918) 
 and recently brought back to usage by Lerner and Taylor (1937a). The 
 latter measures are deemed more adequate than that of Hays and San- 
 born on the grounds that length of biological laying year depends on 
 sexual maturity as much as on persistency and hence is not a true meas- 
 ure of the latter (Lerner and Taylor, 1937^) . In view of the above criti- 
 cism of the use of the biological laying year as a measure of persistency, 
 Hays's (1936&) claim that persistent production results from the action 
 of a single dominant gene P should be submitted to further experimental 
 test. The measurement used by Knox, Jull, and Quinn reflects the in- 
 herent rate of production as well as the ability of the bird to lay late in 
 the fall, and thus also fails to give an uncomplicated measurement of per- 
 sistency. 
 
Bul. 626] Breeding for Egg Production 17 
 
 As far as the preference between age at or date of last egg is concerned, 
 there seems to be a greater effect of date of hatch on the age at last egg 
 than on date at last egg. Both can be demonstrated to be inherited to an 
 equal extent, when a population hatched during a limited season is con- 
 sidered. By analogy with sexual maturity, the age measure may be used 
 in preference to the date measure, especially if correction for hatching 
 date is made. Probably breeders hatching stock over a period of several 
 months will find it easier to use the date of last egg as a measure of per- 
 sistency. In turkeys, Asmundson (1938) has used this measure as an 
 adequate criterion of persistency. 
 
 Whichever method is used, selection by the breeder can be readily 
 practiced for this character. Such selection, if properly carried out, 
 should be effective in increasing the length of time the flock is in lay, but 
 caution must be exercised, just as in the case of early maturity, not to 
 overdo it. 
 
 In commercial practice, a flock of yearling hens which will lay through 
 the late fall at the time of the highest egg prices will be highly profitable. 
 A breeding flock, on the other hand, if in lay at a late date, may not return 
 to full production after the molt by the time hatching eggs are desired. 
 A practice of forced molting might be necessary in highly persistent 
 breeding flocks, but it would tend to nullify any attempt to select for 
 extreme persistency. In order to make possible adequate selection of 
 birds for persistency, forced molting probably should not be started 
 before the end of September. 
 
 Rate of Production. — The factor of intensity, or rate of production, is 
 also one for which a number of measurements have been proposed. The 
 most common way of representing rate is by the use of percentage pro- 
 duction. This may be designated as gross rate and has the distinct dis- 
 advantage of permitting confusion of nonpausing birds possessing low 
 intensity with birds which show pause but have a high rate of production 
 when in lay. Since some types of pause are inherited independently from 
 rate, they should be measured separately. The net rate of production 
 accomplishes this by eliminating pausing days from consideration. Thus, 
 instead of dividing the number of eggs laid in the period considered by 
 the total number of days involved, it is divided by the latter number less 
 the days broody and in pause. The difficulty arising here is again the one 
 of deciding just what constitutes a pause. Until more information on the 
 subject is available, only arbitrary standards can be used. In some of the 
 work carried out at this station, where 7 days has been used as a minimum 
 standard for pause, the comparative figures in table 1 were obtained. 
 
 As may be seen, the winter pauses are distinct from rate, since the net 
 
18 
 
 University of California — Experiment Station 
 
 winter rate is almost the same irrespective of whether the birds paused 
 in the winter. The gross rate in the two cases, however, is different, the 
 winter-pausing birds falling about 20 per cent lower than the nonpausing 
 birds. Spring and summer pauses, however, seem to have a somewhat 
 depressing effect on rate, this being one of the points of evidence for a 
 distinction between them and the winter pause. The main point these 
 data illustrate, however, is that the gross rate may be a misleading cri- 
 terion of measurement of the inherent intensity of production. 
 
 Average clutch size has been used by Hays and Sanborn (1932) as a 
 measure of intensity. It is, however, somewhat more laborious to calcu- 
 
 TABLE 1 
 Gross and Net Rates of Production for Pausing and Nonpausing Hens* 
 
 Winter-pausing birds 
 
 Spring- and summer-pausing birds 
 
 Birds pausing both winter and spring or summer. 
 Nonpausing birds 
 
 Winter rate 
 
 percent 
 46 
 63 
 47 
 65 
 
 Net 
 
 percent 
 65 
 64 
 64 
 67 
 
 Spring rate 
 
 Gross 
 
 percent 
 74 
 58 
 56 
 74 
 
 Net 
 
 percent 
 77 
 70 
 71 
 75 
 
 * From Lerner and Taylor (1936). 
 
 late and, furthermore, does not reflect the number of nonlaying days 
 between clutches of eggs, which should enter intrinsically into the cal- 
 culation of rate. A hen laying an egg each third day would have the same 
 average clutch size as a bird laying an egg every other day, but the latter 
 would have the higher net rate. The suggestion of Hays and Sanborn 
 (1934) that two dominant genes I and V determine clutch size does not 
 necessarily solve the problem of inheritance of rate of production, par- 
 ticularly because of the arbitrary assignment of clutch sizes to specific 
 genotypes. 
 
 In using net rate, one should choose a short period, rather than the 
 whole year. This is largely because the average annual rate may also lead 
 one to an erroneous evaluation of the inherent intensity of a bird. Thus, 
 a bird pausing during the winter months will have undue weight laid on 
 her spring production, which is normally higher than the winter produc- 
 tion ; and a nonpausing bird will be penalized by the inclusion of her 
 winter record in the calculation of her rate. Since, however, rates at 
 different periods of the year bear a definite relation to each other, a 
 shorter period of evaluation can be used with a fair degree of precision. 
 In general, the rate during the spring months (March to June) is found 
 to give a higher correlation with the annual egg record than does the 
 
Bul. 626] 
 
 Breeding for Egg Production 
 
 19 
 
 winter or the summer and fall rates ; but the correlation between winter 
 rate and annual production is sufficiently high to make it feasible to rely 
 upon winter rate. This has the additional advantage that, outside of the 
 birds which pause through most of the winter months, an evaluation of 
 the inherent rate can be thus made in pullets before the breeding season. 
 
 as 
 
 
 A, HIGH WINTER RATE 
 
 / \ B, UNKNOWN 
 / /—•—.- \s\ WINTER RATE 
 
 / / -~X/ v J 
 
 / / / K \y 
 / if ^ \ 
 
 so 
 
 
 
 
 7J- 
 
 
 1 \ M 
 
 
 
 70 
 
 
 / / \ A 
 
 ' v \\ 
 
 \ V- 
 
 
 i 
 
 / C, LOW WINTER RATE \ \ 
 
 es 
 
 \ 
 
 W, \\ 
 
 
 \ 
 \ 
 
 V \ 
 
 
 \ 
 
 ' % — . 
 
 
 \ 
 
 / \^ 
 
 
 \ 
 
 ' \ 
 
 
 \ 
 
 » 
 
 60 
 
 V' 
 
 \ 
 
 \ 
 \ 
 \ 
 
 s& 
 
 
 
 JO 
 
 1 
 
 —1 1 1 1 1 1 I I l 
 
 A/QV. A£C ^PIAL /Z& MA*. APR. MAW JUA/£ JVLY AUG. SEPT. OCT. 
 
 Fig. 4. — Relation of fall and winter rate to annual 
 rate: A, birds laying 21 or more eggs in each of any two 
 of the winter months (November, December, January) ; 
 C, birds laying less than 21 eggs in two of these months; 
 B, birds, information on which is lacking for these months. 
 
 Figure 4 illustrates the fair degree of accuracy obtained by such a 
 method of classification with nonpausing birds. An arbitrary standard 
 of 21 eggs a month in any two of the three winter months (November, 
 December, January) is used for high rate. Birds fulfilling such a require- 
 ment exhibit on the average a higher rate of lay throughout the rest of 
 the year than those falling below it. The third curve on the graph repre- 
 sents the production of birds which either paused during the winter 
 months or were extremely late-maturing. Birds with both low and high 
 
20 University of California — Experiment Station 
 
 rates were present in this group, since the average production is found 
 to be intermediate between the other groups. The rate of such birds would 
 have to be judged by their performance in the spring. Since, however, 
 these birds would in all likelihood not be bred from, because they ex- 
 hibited a winter pause or exceedingly late maturity, no disadvantage in 
 delaying the collection of information on the rate of such birds is in- 
 curred. In all groups, the highest rate of production occurred from 
 March to May, and the differences in rate between high- and low-rate 
 groups were least in the spring and summer months. 
 
 The above methods of identifying birds possessing high or low rate are 
 obviously suitable only for trapnested flocks. Unfortunately, there are 
 no precise physical criteria by which rate can be judged, so that indi- 
 vidual selection is impossible in nontrapped flocks. A short trapnesting 
 period will achieve the desired ends; otherwise only pen selection is 
 possible. Where pause is not a significant factor, percentage production 
 of one pen as measured against another may serve a useful purpose. 
 
 General Considerations of Production Characters. — The general dis- 
 cussion of the genetic factors affecting egg production in relation to 
 practices of selection can now be summarized. Some of the characters 
 controlling time in production, such as maturity, persistency, and broodi- 
 ness, are inherited on a basis such as to make selection for the desirable 
 genes possible. The trapnester should have no difficulty in interpreting 
 his records on these characters and in setting up standards of selection. 
 Further, where trapnesting is not practiced, certain physical manifesta- 
 tions of maturity, persistency, and broodiness are easily observed ; and 
 most culling guides provide complete instructions for the recognition 
 both of superior individuals and of those to be eliminated from the breed- 
 ing flock. 
 
 Manifestations of pauses are not so readily interpreted. While neck 
 molt is a fair criterion of the expression of the winter-pause character, 
 there is considerable difficulty in setting up satisfactory and efficient 
 standards of selection in these cases. This applies less to the trapnesting 
 breeder than to the nontrapnesting multiplier. 
 
 The condition of summer pause is still more difficult to interpret, since 
 it shows some association with low rate, and rate is the hardest character 
 to measure unless some trapnesting is practiced. If at least part-time 
 trapnesting is practiced, selection for high rate is feasible. Otherwise, 
 little reliability can be placed on the use of physical standards of selec- 
 tion, and improvement of the inherent intensity of production in a flock 
 is beset with great difficulties. 
 
 The best birds are not the ones which are superior in only one par- 
 
BUD. 626] BREEDING FOR EGG PRODUCTION 21 
 
 ticular character, but those which exhibit the best combination of all of 
 the characters considered. The annual record and the monthly and sea- 
 sonal distribution of egg production throughout the year are in the final 
 analysis the main criteria of the profitableness of a bird's production. 
 Their expression is conditioned by the combination of the characters 
 discussed ; hence, the latter should always be considered, not only singly, 
 but also in relation to each other and to their contribution to the annual 
 record. 
 
 When flock efficiency is considered, however, a whole new series of 
 characters, some of which are undoubtedly inherited, comes into the 
 picture. These are the characters affecting the viability of birds. Flock 
 efficiency will, of course, be lowered if mortality is high. Selection for re- 
 sistance to adverse environmental conditions or for the ability of the 
 bird to live must, therefore, now be considered. 
 
 CHARACTERS AFFECTING VIABILITY 
 
 The influence of the conditions affecting viability on the efficiency of pro- 
 duction of the flock begins with the egg and continues throughout the 
 productive life of the bird. The number of chicks hatched from a given 
 number of eggs, the proportion of these raised to maturity, and finally 
 the mortality of the laying birds, all have an effect on the size of the flock 
 and on its productivity. Just how much of the embryonic and post-em- 
 bryonic mortality is due to inherited factors cannot be estimated accu- 
 rately, but there is good evidence in many cases that at least part of the 
 mortality up to the completion of the first laying year is controlled by 
 genetic factors, and hence the condition is amenable to improvement by 
 proper breeding methods. 
 
 The mortality most closely related to egg production is that occurring 
 during the first laying year. This is evidenced by reports available on 
 commercial flocks where as high as 100 per cent replacement has been 
 necessary during the first laying year. The replacements are not entirely 
 due to mortality, since some of the birds removed, though nonproductive, 
 appear anatomically normal, and others are unprofitable because of non- 
 fatal pathological conditions. However, at least half of the replacement 
 in an average well-cared-for flock is likely to result from actual deaths 
 from nonspecific pathological conditions, that is, conditions not related 
 to definite infectious or dietary factors. This point was brought out by 
 Lubbehusen and Beach (1938) in an analysis of mortality in the Cali- 
 fornia Station flock. 
 
 The presence of birds that are doomed to die during the laying year 
 and also of birds which fail to lay because of specific diseases causes a 
 
22 University of California — Experiment Station 
 
 flock to be less profitable. To illustrate this point, limited evidence on the 
 California Station flock indicates that birds which succumb to paralysis 
 and associated conditions in the spring months show a reduced produc- 
 tion during the previous fall months although they appear normal in 
 every other respect. In a similar manner Asmundson and Biely (1930), 
 among others, have reported that adult carriers of pullorum disease show 
 a reduction in egg production. On the other hand, we have evidence that 
 birds developing disturbances of the reproductive system seem to lay 
 normally up to the time of onset of the outward manifestations of the 
 condition. 
 
 In any case, if any of the various diseases to which poultry is subject 
 have a specific hereditary basis of susceptibility or are conditioned even 
 in part by a general constitutional weakness, breeding methods may be of 
 value in increasing the flock efficiency by the creation of lines more re- 
 sistant toward such conditions. 
 
 Longevity. — Partly on these grounds, Pearl (1923) suggested breed- 
 ing from aged birds only. He believed that the productivity of the breed- 
 ing birds would be improved along with longevity. Greenwood (1932), 
 however, has found that very old birds showed no marked superiority in 
 offspring, while their progeny was limited in numbers by low fertility 
 and hatchability. Whether or not such is the case, greater viability will 
 ensue in a flock as a consequence of selection of longer-lived strains. Dif- 
 ferences in mortality between families in the same flock are known to 
 persist for several generations, as they have in the California Station 
 flock, and hence are probably heritable. Despite the poultryman's inter- 
 est in obtaining long-lived birds, comparatively few reports are available 
 on the results of selection for longevity. This fact may indicate that 
 breeding for characters influencing longevity is a complex process. 
 
 Disease Resistance. — Some experimental evidence of the inheritance 
 of resistance to specific infections in chickens has been obtained. Roberts 
 and Card (1926 and 1936) have demonstrated inherited differences in 
 susceptibility to pullorum disease, and Lambert and Knox (1932) ob- 
 tained similar results with fowl typhoid. They failed, however, to obtain 
 strains in which all the birds were resistant. Furthermore, such diseases 
 can be more efficiently controlled by other means. On the other hand, 
 there is evidence on the hereditary nature of resistance to certain other 
 disease conditions for which methods of prevention and control are not 
 yet known. This applies particularly to neurolymphomatosis (paralysis) 
 and may also be true of other forms of lymphomatosis (big-liver disease 
 or leukemia), leucosis, and certain types of tumors. 
 
 Present conceptions of the nature of such pathological conditions are 
 
Bul. 626] Breeding for Egg Production 23 
 
 conflicting, but there is considerable experimental and statistical data 
 from British Columbia (Asmundson and Biely, 1932), Louisiana (Upp 
 and Tower, 1936) , Idaho (Gildow, Williams, and Lampman, 1936), Iowa 
 (Wilcke, Lee, and Murray, 1938) and from the California Station flock, 
 to indicate the existence of definite differences in degree of occurrence of 
 this group of diseases between different families. These data point to the 
 possibility of partial control by selective breeding. Results obtained with 
 the California Station flock having a bearing on this problem are dis- 
 cussed in a later section (p. 43) . 
 
 A suitable method of selection against these diseases in an affected 
 flock involves breeding from families which are as free as possible from 
 paralysis and other neoplasms, rather than the selection of survivors 
 from susceptible families. Since the genetic nature of disease resistance 
 is usually fairly complicated, the latter method would result in perpetu- 
 ation of factors for susceptibility in the flock. The former method, on the 
 other hand, would tend to eliminate any family of birds which shows an 
 undue concentration of such undesirable factors. 
 
 All available evidence indicates that disease resistance is specific in 
 nature. Creating a strain resistant to one disease does not mean that the 
 strain will be resistant to any other disease. Nor does it mean that the 
 strain will be unusually susceptible to some other disease. Genetically, 
 every disease presents a separate problem of breeding for resistance. 
 Whether or not genetics can be effectively applied to the control of all 
 diseases is questionable. Those for which specific control measures have 
 been developed can undoubtedly be handled more easily by such means, 
 rather than by breeding. Extension Circular 8 a should be consulted for 
 other more direct methods of control of diseases. 
 
 METHODS OF MEASURING FLOCK PRODUCTION 
 
 Several ways of expressing the average egg production of a flock have 
 been commonly used. The least-accurate method is that of the average of 
 the records of surviving birds. A better method involves the computation 
 of hen-day averages. This method is well suited for cost-account work, 
 since the denominator represents the average number of birds in the flock 
 for a given period of time. 
 
 A measure of breeding-flock values known as the production index has 
 been developed at the Kimber Poultry Breeding Farm. This measure is 
 the quotient of the number of eggs laid by the group under consideration 
 (family, flock, etc.) over the original number of pullets in the group. Any 
 
 6 Beach, J. R., and S. B. Freeborn. Diseases and parasites of poultry in California. 
 California Agr. Ext. Cir. 8:1-110. 5th ed. 1936. 
 
24 University of California — Experiment Station 
 
 birds that die at any time during the laying year are thus considered 
 equally with the survivors as members of the flock or family. In that 
 manner, a flock or family exhibiting high mortality will be penalized. 
 Just as the annual egg record reflects all the various characters influ- 
 encing the egg production of an individual bird, the production index 
 expresses them for a group of birds. While neither the productivity nor 
 viability of a family is reflected separately in the production index, this 
 index gives a true picture of the interaction of all of the factors involved. 
 To obtain measurements on the individual characters contributing to 
 production, one has to resort to the separate determination of all of the 
 variables previously discussed. 
 
 The production index is superior to both the average of the survivors 
 and the hen-day average as a measure of the productive efficiency of 
 breeding flocks. The average of the survivors completely ignores the im- 
 portant factor of mortality in the flock; the hen-day average fluctuates 
 greatly with the use of various culling practices, so that a heavily culled 
 flock may show a better average production than an unculled flock of the 
 same inherent productivity. Both mortality and culling decrease the 
 average egg production expressed as a production index, which thereby 
 shows the prevalence of nonproductive or nonviable birds in the flock. 
 
 CHARACTERS GOVERNING THE QUALITY OF EGGS 
 
 All of the discussion to this point has referred to the quantity of produc- 
 tion. Factors affecting quality of production may be discussed next. 
 
 Egg Size. — Probably the most important character concerned in the 
 quality of commercial eggs is that of size. The production of large eggs 
 within a reasonable period of time after sexual maturity is now recog- 
 nized as an essential in a good egg-production strain. Rigid selection in 
 breeding flocks has brought about during the last fifteen years a remark- 
 able improvement in the size of egg produced by many strains. 
 
 That there is a direct correlation between body size and egg size, within 
 a breed or within a strain, has long been known. Very small birds rarely 
 produce large eggs. Table 2 illustrates the relation between body weight 
 and egg weight. The 595 birds involved were weighed in December, and 
 the December and January egg weights were calculated for birds at dif- 
 ferent body weights. Sexually mature pullets which are still growing will 
 improve their egg size as they become larger. 
 
 Seasonal effects expressed through temperature also affect egg size : 
 Lorenz and Almquist (1936) have shown an average decrease of 1 ounce 
 per dozen in mature birds for each 30° F increase in maximum daily tem- 
 perature. 
 
Bul, 626] 
 
 Breeding for Egg Production 
 
 25 
 
 Rate also may be related to egg size, producers of very small eggs often 
 having a phenomenal rate, while producers of exceptionally large eggs 
 seldom have a high rate. This relation is illustrated in table 3. However, 
 this does not mean that all birds laying small eggs have a high rate of pro- 
 duction. Since egg size is the result of the sum of the weights of the 
 various parts of the egg, any circumstance reducing the weight of a com- 
 
 TABLE 2 
 
 Relation between Body Weight and Egg Weight in 595 
 
 White Leghorn Pullets 
 
 Body weight at 8 months of age 
 
 Class 
 average 
 
 Number 
 of birds 
 
 Average 
 egg weight in 
 
 December 
 and January* 
 
 grams 
 1,100-1,399 
 
 pounds 
 2.76 
 3.42 
 4.08 
 4.74 
 5.40 
 
 8 
 
 236 
 
 272 
 
 75 
 
 4 
 
 ounces 
 1.621 
 
 1,400-1,699 
 
 1.781 
 
 1,700-1,999 
 
 1.900 
 
 2,000-2,299 
 
 1.975 
 
 2,300-2,599 
 
 2.000 
 
 
 
 * From 8 to 10 months of age. 
 
 TABLE 3 
 
 Relation between Winter Rate of Production and Egg 
 Weight in 145 White Leghorn Pullets 
 
 Number of birds 
 
 Net 
 winter rate 
 (November- 
 February 
 inclusive) 
 
 Winter 
 egg weight 
 (November- 
 February 
 inclusive) 
 
 14 
 
 per cent 
 56.0-63.9 
 64.0-71.9 
 72.0-79.9 
 80.0-87.9 
 
 ounces 
 1.852 
 
 44 
 
 1.855 
 
 69 
 
 1.841 
 
 18 
 
 1.810 
 
 
 
 ponent part reduces egg size. Thus, Buckner, Martin, and Insko (1930) 
 have shown that a deficiency of calcium in the diet reduces the amount of 
 shell per egg and also results in smaller egg size. 
 
 From a breeding standpoint, the most satisfactory short period in 
 which to determine the egg size of individuals is during the late winter 
 or early spring or, more specifically, in California during February and 
 March. By that time, pullets hatched in the previous breeding season are 
 approximately one year old, have reached adult body size, and produce 
 eggs under a moderate range of temperatures in most areas in California. 
 Egg size during this period is also reasonably close to the average size of 
 egg produced during the pullet year of production, as can be seen from 
 
26 
 
 University of California — Experiment Station 
 
 table 4. Comparative inherent egg size can be determined at any season, 
 but the early spring period is least subject to adverse environmental in- 
 fluences. Where greater accuracy is desired, about 5 eggs a month may 
 be weighed. Weighing every egg scarcely adds sufficient accuracy to 
 warrant the labor involved. 
 
 Hays (1929, 1937) has advanced a hypothesis for the inheritance of 
 egg size involving a dominant gene A producing small eggs and dominant 
 genes B and C for large eggs. When A, B, and C are present, an inter- 
 
 TABLE 4 
 
 Monthly Variation in Egg Weight in a Flock of 116 
 
 White Leghorn Pullets 
 
 Month 
 
 Average 
 egg weight 
 
 Month 
 
 Average 
 egg weight 
 
 
 ounces 
 1.753 
 1.834 
 1.926 
 1.965 
 1.972 
 1.972 
 1.961 
 
 
 ounces 
 1.997 
 
 
 July 
 
 1.997 
 
 
 
 2.021 
 
 
 
 2.067 
 
 
 October 
 
 Annual average 
 
 2.081 
 
 
 
 May 
 
 1.962 
 
 
 
 mediate, but predominantly small size of egg is obtained. According to 
 this hypothesis, the genotypic constitution for largest egg size should be 
 aaBBCC, which would represent a pure-breeding large-egg type. Rigid 
 selection of both male and female breeders for egg size should act to 
 eliminate the undesirable A genes rather rapidly, and cause B and C 
 genes to become homozygous at the same time in an increasingly large 
 portion of the flock. While Hays' data lend considerable support to this 
 hypothesis, it is questionable whether or not the gene combinations sug- 
 gested by him have universal application. One practical difficulty in 
 flock breeding lies in the fact that the contribution of the male for all 
 egg characters can be determined accurately only by progeny tests. Prac- 
 ticing rigid selection of breeding males on the basis of egg size exhibited 
 by their female ancestors or their sisters is, however, fully as important 
 as selecting females with large egg size for mating. 
 
 The discrimination against small eggs on the market is sufficient to 
 encourage the production of large eggs. However, egg size may be se- 
 lected to the point where it becomes undesirably large. Minorca varieties 
 frequently produce eggs averaging 30 or more ounces per dozen. These 
 eggs are too large for the standard types of fillers and cases used for 
 chicken eggs. Occasional premiums for "jumbo" sizes may influence some 
 poultrymen to attempt commercial production of this type of egg. The 
 
Bul, 626] Breeding for Egg Production 27 
 
 low hatchability associated with extremely large eggs (Dunn, 1922), 
 however, should be sufficient argument against breeding for this size. 
 Better hatchability results are usually obtained by selecting for breeders 
 hens with average egg size ranging from 24 to 28 ounces per dozen. 
 
 Egg Shape. — There are three common shapes of eggs : long, ovoid 
 (typically egg-shaped), and round. Long types are not commercially 
 desirable since the fillers of egg cases are not deep enough to take a large 
 long egg. Similarly, large round eggs are too wide for the ordinary filler. 
 From a hatchability standpoint, furthermore, ovoid eggs in common ex- 
 perience give better average results than either very long (Landauer, 
 1937) or round types. 
 
 The inheritance of egg shapes is not clearly understood. Benjamin 
 (1920) states that long-shaped types are dominant over normal types, 
 but Kopec (1924) has presented contrary evidence. Some casual observa- 
 tions made by the senior author would lead us to believe that round may 
 be dominant to ovoid. Axelsson (1936), however, has found no domi- 
 nance of any one shape over another in several breed crosses. Rigid selec- 
 tion for ovoid types tends to bring rapid improvement in the character 
 of the eggs produced by the flock. It is important that the breeding males 
 come from families producing ovoid-shaped eggs. 
 
 Many types of misshapen eggs seem to be produced by temporary or 
 chronic inflammation of the shell gland of the oviduct and are probably 
 not inherited. Unless the breeding hens displaying such conditions have 
 proved to be valuable breeders, such birds should probably be removed 
 from the breeding flock. Misshapen eggs show notoriously poor hatch- 
 ability. 
 
 Egg Color. — In the common breeds of chicken, egg color is usually 
 some shade of either brown or white. One peculiar breed, the Araucana, 
 lays a blue egg. Breeds of the Mediterranean class, such as Leghorns, 
 Minorcas, and Ancona, should lay a pure-white egg. Breeds of the Asiatic 
 and American classes should produce a brown egg. In these breeds a 
 uniform, medium-brown color is desired. Pale-brown or creamy colors 
 should be discriminated against. 
 
 The New York market has long been noted for its premium on pure- 
 white-shelled eggs, while the Boston market has similarly placed a pre- 
 mium on brown-shelled eggs. In recent years these premiums have been 
 lower than previously. However, in choosing a breed or strain for com- 
 mercial egg production, local market preferences for egg color must still 
 be considered. 
 
 Birds producing brown-shelled eggs and also individuals from white- 
 shelled varieties which produce tinted or creamy eggs characteristically 
 
28 University of California — Experiment Station 
 
 show a decreasing intensity of color with advancing production (Benja- 
 min, 1920). Immediately after sexual maturity or after any period of 
 nonproduction, the color of the shell tends to be deeper in shade than 
 after long periods of production. Where tints and creaminess are to be 
 selected against, it is important to take records of color when the bird 
 first begins to lay and after pauses and the annual molt. 
 
 Egg color is apparently controlled by several pairs of genes (Hays, 
 1937) , some of them acting cumulatively to produce darker shades, others 
 acting as inhibitors of color (Punnett, 1923). Crosses of dark brown by 
 white strains usually produce eggs ranging in the medium-brown shades. 
 Crosses of medium brown by white strains may produce fowls with egg 
 color ranging from creamy to light brown. Blue color is dominant to 
 white ; crosses of blue and brown produce a greenish color, as Punnett 
 (1933) has shown. 
 
 In white-egg strains, several generations of selection of both male and 
 female breeders from white-egg families will reduce the incidence of 
 creamy or tinted eggs to a very low proportion. Frequently, crossing two 
 different white-egg strains produces a considerable degree of color in the 
 egg of the offspring. Similarly, mating creamy or tinted individuals to- 
 gether may actually produce some birds laying brown eggs. All of these 
 results are explained by the fact that there is probably some complemen- 
 tary action between some of the genes producing brown color in eggshells. 
 
 Shell Quality. — The most fundamental of the factors of shell quality is 
 that of thickness. Thick shells are desired both for market eggs and for 
 hatching eggs. In the incubation of eggs, characteristic differences in 
 hatchability between matings can sometimes be traced to the amount of 
 shell present. 
 
 Data from several generations of selection for shell thickness in White 
 Leghorns at the California Station are given in table 5. In this experi- 
 ment, birds from families selected for thick shell had a significantly 
 higher percentage of shell during four generations than corresponding 
 pullets bred from thin-shell lines. In these instances, the birds from both 
 lines received the same rations and management. This evidence confirms 
 the previous finding of Taylor and Martin (1928) that shell thickness is 
 in part dependent upon hereditary factors. Munro (1938) has also found 
 similar evidence for inheritance of the amount of ash in the shells of eggs 
 from a flock of Barred Plymouth Rocks. Probably the genetic basis for 
 inheritance of an ability to produce thick-shelled eggs is complex. The 
 importance of nutritional factors in producing shells of high quality 
 must also be emphasized, since deficiencies of calcium or vitamin D in the 
 diet quickly lead to production of defective shells. 
 
Bul. 626] 
 
 Breeding for Egg Production 
 
 29 
 
 Other common types of poor shell quality are sometimes produced by 
 closely related individuals, and the inheritance of tendencies for their 
 production may be suspected (Hays, 1937). Such types include rough 
 or "sandy" shells, mottled shells, and "glassy," or very nonporous shells. 
 Hens producing these types of defective shells should be removed from 
 the breeding flock. 
 
 Another quality which seems to be inherited is the character of the 
 bloom or finish of the shell. While the Leghorn breed is said to produce a 
 chalk-white egg characteristically, many individual hens lay eggs having 
 
 TABLE 5 
 
 Proportion of Shell and Hatchability in Eggs from 
 Thick- and Thin-Shell Lines 
 
 Year 
 
 of 
 hatch 
 
 Line 
 
 Number 
 
 of 
 pullets 
 
 Average 
 
 proportion 
 
 of shell 
 
 Hatchability 
 
 of 
 fertile eggs 
 
 1934 
 
 / Thick shell 
 
 38 
 
 27 
 
 36 
 
 27 
 
 per cent 
 9.84 
 9.44 
 
 9.94 
 9.24 
 
 per cent 
 82.1 
 
 
 \ Thin shell 
 
 58.5 
 
 1935 
 
 f Thick shell 
 
 90.3 
 
 
 \ Thin shell 
 
 76.2 
 
 
 
 
 a pronounced glossy finish. Data on the California Station Leghorn flock 
 would indicate that a single gene difference is usually found, the chalky 
 finish being dominant. 
 
 Albumen Quality. — Hoist and Almquist (1931) have produced evi- 
 dence that individual hens were surprisingly constant in the proportion 
 of total albumen found to be in the firm condition. Since then, other re- 
 ports (Lorenz, Taylor, and Almquist, 1934; Knox and Godfrey, 1934) 
 have given evidence for the inheritance of this character. From further 
 breeding results, the amount of firm white now seems to be the result of 
 the expression of a considerable number of genes ; crosses between high 
 and low lines give intermediate results. Strains producing eggs with 
 characteristically large quantities of firm white can be created by breed- 
 ing. All experiments designed to modify the proportion of the firm white 
 of freshly laid eggs by nutritional or other means have failed. 
 
 Van Wagenen and Hall (1936) use a different criterion of albumen 
 quality : they make a visual comparison of the appearance of the albumen 
 with a photograph of eggs classified according to quality. This measure, 
 known as the albumen-condition score, is intimately correlated with the 
 actual height of the firm albumen observed in egg contents removed from 
 the shell and membranes. On the other hand, there is considerable evi- 
 
30 University of California — Experiment Station 
 
 dence to show that the percentage- firm-white and albumen-height meas- 
 ures are not necessarily closely correlated in eggs from certain hens. A 
 good albumen-condition score is probably dependent upon many genes 
 for its expression. 
 
 Good quality is required of a high-grade market egg. Lorenz and Alm- 
 quist (1935) found that eggs with an original high percentage of firm 
 white are not only of superior quality when fresh but also after storage 
 since the rate of liquefaction of the albumen in such eggs is lower than 
 in eggs with lower percentages of firm white. Hall and Van Wagenen 
 (1936) have also claimed higher hatchability for eggs with the best 
 albumen-condition scores. Some tests conducted at the California Sta- 
 tion have not confirmed this. There seems to be no association between 
 the percentage of firm white and hatchability. 
 
 Hatchability. — In our experience, the greatest differences found in 
 hatchability in commercial poultry flocks have been produced by nutri- 
 tional factors. Insufficient amounts of one or several vitamins and min- 
 eral substances in the rations of breeding birds frequently cause poor 
 hatchability under practical conditions of poultry breeding. In order to 
 obtain high hatchability as well as chicks of good quality, the breeding 
 ration must not be deficient in these essential nutrients. 7 
 
 Under good conditions of nutrition and management, variation in 
 hatchability between matings is common. Hays and Sanborn (1924) have 
 postulated a genetic explanation by which a single gene controls hatch- 
 ability : HH individuals hatch 85 per cent or more of fertile eggs, Hh 55 
 to 84 per cent, hh less than 55 per cent. Jull (1932) has produced evi- 
 dence which would indicate a more complex inheritance of hatchability. 
 His view is supported by the fact that some six or more lethal genes are 
 known to cause embryonic death in chickens. Practically, however, Hays' 
 method of selecting birds hatching 85 per cent or more is an effective 
 means of selecting inherently high-hatching stock ; at the same time, it 
 allows for a reasonable amount of embryonic deaths produced by non- 
 inherited conditions. 
 
 BREEDING SYSTEMS 
 
 The comparative value of various systems of breeding, by which improve- 
 ments of characters in poultry have been made, has long been a subject of 
 debate by poultry breeders. The principal systems which have been used 
 involve close inbreeding, line breeding, outbreeding, crossbreeding, and 
 grading. 
 
 7 Almquist, H. J., T. H. Jukes, and W. E. Newlon. Feeding chickens. California Agr. 
 Ext. Cir. 108:1-38. 1938. 
 
Bul. 626] Breeding for Egg Production 31 
 
 Close Inbreeding and Line Breeding. — Some degree of inbreeding is 
 involved when any mated pair of birds have a common ancestor. The 
 closer the common ancestor is to the individuals mated and the greater 
 the number of common ancestors, the greater the degree of inbreeding. 
 Closest inbreeding is attained by successive generations of full brother 
 to sister matings. High degrees of inbreeding can also result from mating 
 a larger number of generations of less closely related individuals, such 
 as are involved in half brother by half sister, father by daughter, mother 
 by son, and cousin by cousin matings. 
 
 Numerous reports of the detrimental effects of close inbreeding are 
 available. Four or five generations of full brother by sister matings have 
 generally given reduced hatchability, decreased viability of chicks and 
 adult stock, decreased growth rates, and poorer average egg production 
 (Cole and Halpin, 1922 ; Dunn, 1923 ; Hays, 1924 ; Dunkerly, 1930; Jull, 
 1933a) . When less-intense inbreeding, involving the mating of more dis- 
 tantly related birds, has been practiced, high degrees of inbreeding have 
 been obtained with fewer detrimental effects in some instances (Jull, 
 1933a; Waters and Lambert, 1936). The latter procedure may permit 
 more rigorous selection against undesirable genes, which often seem to 
 be recessive and lethal or semilethal in action. Under intensive inbreed- 
 ing, such genes become homozygous very rapidly in a considerable pro- 
 portion of the progeny and lead to serious detrimental effects. A more 
 gradual approach to homozygosity permits a more effective elimination 
 of them as they are discovered. Since lethal genes are apparently very 
 widely distributed in chickens, utilization of very close inbreeding, while 
 satisfactory for quickly increasing homozygosity, is often accompanied 
 by unfavorable results which make the practice undesirable. Asmundson 
 (1935) and Marsden and Knox (1937) have obtained similar results in 
 inbreeding turkeys. 
 
 Line breeding involves the repetition of certain desirable individuals 
 and their offspring in successive generations for the production of inbred 
 strains. By this method a male may be mated to his daughters and then 
 later to the female offspring resulting from the mating with his daugh- 
 ters. Similarly a female may be mated to her son and later to a male pro- 
 duced from the mating with her son. Line breeding attempts to concen- 
 trate the favorable qualities of an individual in a flock by inbreeding in 
 the above manner. 
 
 Systems involving mating of less closely related individuals have been 
 widely used and have produced good results. It is common practice to 
 line-breed for several generations and also to mate individuals not more 
 closely related than first cousins. Even in such matings, detrimental ef- 
 
32 University of California — Experiment Station 
 
 fects may be produced in some cases. But, if the stock is vigorous and lias 
 previously been selected for desirable characters associated with hatcha- 
 bility, growth, and viability, successful matings may be expected. 
 
 Outbreeding and Crossbreeding. — The opposite of inbreeding is in- 
 volved in outbreeding and crossbreeding. In outbreeding, or outcrossing, 
 birds of the same breed or variety but of nonrelated strains are mated. 
 In crossbreeding, the birds are of different breeds or varieties. One type 
 of outbreeding recently advocated (Waters, 1937) has involved topcross- 
 ing, in which two nonrelated strains are crossed and their offspring in 
 turn mated to another strain not related to the first two. Frequently the 
 strains so mated are themselves inbred to some degree, but after out- 
 breeding to nonrelated strains, the degree of inbreeding as calculated on 
 the basis of a reasonable number of generations is reduced to zero. 
 
 Outbreeding may be resorted to when some desirable characters are 
 lacking in the strains bred. Its proper use involves the introduction of 
 necessary genes into the strains to be improved. Care should be taken to 
 make test matings to measure the results of the outcross before new stock 
 is indiscriminately mated with known strains. The combination of char- 
 acters from the crossing of genetically different strains at times gives 
 rise to very favorable performance in the offspring (Warren, 1930), in 
 which case the two strains are said to "nick," and at other times they may 
 give an inferior type of offspring. Many strains of birds have been ruined 
 by the unwise introduction of new stock by matings involving the whole 
 flock, or the greater part of the best birds in the flock, without previous 
 test of the progeny of such matings. On the introduction of new breeding 
 strains into the flock, a number of test matings should be made and the 
 progeny given a chance to prove their qualities before extensive matings 
 between the two strains are attempted. 
 
 Successful crossbreeding requires the maintenance of two or more 
 breeds or varieties with such characteristics that, when they are crossed, 
 the first-generation offspring are superior to the parental stock (Warren, 
 1930). The product of the cross is said to show hybrid vigor and is of 
 superior commercial value. These crossbred birds are not satisfactory 
 breeders as a rule, and therefore the cross must be repeated when more 
 crossbred stock is desired. The average poultry breeder has sufficient 
 difficulties in breeding one strain of poultry without multiplying his 
 work with more breeds or varieties. Only in cases where a large produc- 
 tion of crossbred chicks is possible, does a sufficient emphasis on good 
 breeding of the parental stock become warranted economically. Some 
 breeds and strains are not suitable for crossing because they transmit un- 
 desirable characters to their offspring, and hybrid vigor does not result. 
 
Bul, 626] Breeding for Egg Production 33 
 
 Grading. — When a flock is found to be deficient in many respects, a 
 poultryman may resort to a system of grading. In this system, males are 
 introduced each year from a superior strain or breed and are mated to 
 successive generations of selected breeding hens until the flock comes 
 to resemble more and more the quality of the superior birds purchased. 
 Lippincott (1920) has shown that flocks of mongrel fowls can be rapidly 
 improved by this system of breeding. It is also widely used by multipliers 
 of commercial egg-production stock, who return each year to the same 
 poultry breeder for breeding males. 
 
 A slight degree of inbreeding may be involved in the practice of grad- 
 ing, the extent depending on the relationship between the males pur- 
 chased in different years. Such relationship is usually not sufficiently 
 close to produce any detrimental effects and there is no reason to change 
 to another strain of breeding males if satisfactory results have been ob- 
 tained. 
 
 Value of Breeding Systems. — The purpose of inbreeding is to increase 
 the uniformity of stock by increasing the number of pairs of genes in 
 the homozygous condition. Outbreeding and crossbreeding, on the other 
 hand, definitely increase heterozygosity and lead to less uniformity in 
 breeding results in subsequent generations. If the poultry breeder desires 
 to obtain a flock which will breed reasonably true for desired characters 
 for a number of generations, then some degree of inbreeding must be in- 
 volved. If the breeder desires only one generation of stock possessing a 
 maximum of qualities for a specific purpose, then outbreeding or cross- 
 breeding may produce the type of stock desired. 
 
 The success of various systems of poultry breeding rests not so much 
 in the method chosen as in the use of records for the elimination of un- 
 desirable characters and selection of superior birds for mating. Blind 
 adherence to any method is apt to lead to failure. The value of adequate 
 records rests in the information made available to the breeder, who then 
 can intelligently choose the system most likely to produce good results. 
 The system, of course, may vary from year to year, with the quality of 
 the stock available. 
 
 RELATION OF GENETICS TO POULTRY-BREEDING 
 
 METHODS 
 
 Pedigree Breeding. — Knowledge of genetics can be applied to best ad- 
 vantage in the improvement of strains of poultry only with the aid of 
 pedigree breeding. Information on the characters exhibited by individ- 
 ual birds must be supplemented by knowledge regarding the same char- 
 acters in the ancestors of the mated birds in order to determine the 
 
34 University of California — Experiment Station 
 
 degree of probable homozygosity for desirable genes. The best test of a 
 breeding bird's value is found in the performance of its progeny, the 
 determination of which demands complete pedigree records. Other meas- 
 ures of the probable value of a breeding bird rely on the performance of 
 the family of which the bird was a member. Here, too, pedigree records 
 are required to establish relationship. 
 
 With adequate records of pedigree and of individual characteristics 
 and with a knowledge of the principal genes concerned with egg produc- 
 tion in birds, results to be expected in the offspring from a given mating 
 may be predicted with a fair degree of accuracy. Once a mating is ade- 
 quately tested and known to give good progeny, it may then be repeated 
 as long as the original breeders give viable offspring with the knowl- 
 edge that the future offspring will perform reasonably like their older 
 brothers and sisters. 
 
 Pedigree breeding, coupled with a knowledge of genetics and progeny- 
 test data, eliminates many of the hit-or-miss results obtained by other 
 methods of mating in which the relationship of birds is not accurately 
 known. It also makes possible more rapid improvement in specific heredi- 
 tary characters which are controlled by several pairs of genes than can 
 be obtained by simply selecting individuals showing the desired char- 
 acters. Today, practically all of the leading breeders of egg-production 
 strains of poultry pedigree their stock. 
 
 Pedigree Records. — Trapnesting, pedigreeing, and record keeping are 
 futile if an analysis of records is not made in such a way as to reveal not 
 only the best individual performances but also the best means of repro- 
 ducing superior offspring. Record-keeping systems may be simple or 
 complex according to the number of characters for which selection is 
 practiced. Any system which demands so much work in its keeping that 
 adequate analysis of records is prevented is a detriment to, rather than an 
 aid in, breeding. The rule should be to simplify record taking to the point 
 where further reduction in scope would eliminate essential information 
 on characters of importance in the selections made. 
 
 We have often been asked to recommend record forms for pedigree 
 breeders of poultry. In our opinion no one system of records is best for 
 all breeding purposes. Pedigree breeders should be encouraged to de- 
 velop their own record forms to meet their own needs. In any case, 
 some form should be provided for each of the following : (1) daily egg- 
 production record of individual birds, (2) hatching and chick-banding 
 records, (3) mating records, and (4) family summaries. The latter 
 should summarize the performance of individual members of each family 
 with respect to the characters affecting production and viability. Wher- 
 
Bul. 626] Breeding for Egg Production 35 
 
 ever possible, records of mortality and autopsy findings, as well as of 
 quality characteristics of the birds and of the eggs produced, should be 
 taken and summarized. 
 
 Sib and Progeny Testing. — Recent developments in breeding methods 
 which have come into popular use involve the analysis of records of fam- 
 ily performance. Males and females from families in which the sisters 
 have made known records of performance are called "sister-tested." 
 When records of growth, viability, feathering, and other characters are 
 also available for the brothers of such individuals, the term "sib-tested" 
 is used. The term "sib" here refers to the family of full brothers and 
 sisters. Similarly the sires and dams which produced the family are 
 called progeny-tested. The sib and progeny tests may reveal the family 
 to be superior, average, or poor. These tests enable the selection of breed- 
 ing stock from the superior individuals belonging to superior families. 
 The genetic composition of breeding birds is more easily revealed by a 
 study of family records than in cases where no sib or progeny data are 
 available. 
 
 The value of sib, sister, and progeny tests depends partly on the num- 
 ber of offspring tested for each mating and partly on taking unselected 
 groups of individuals for the tests. From matings which give a high de- 
 gree of uniformity in production and viability of the offspring, as few 
 as five pullet offspring may suffice for a reasonably accurate progeny 
 test, especially when the hatching season is limited. More offspring are 
 usually desired for matings giving a wider segregation of qualities ; in 
 these cases probably not less than ten should be tested. Any means of 
 selection which tends to eliminate certain types of offspring from the test 
 leads to a false measurement of the breeding value of the mated birds. 
 Thus an accurate progeny, sister, or sib test demands an unselected and 
 unculled group. 
 
 When data from accurate progeny tests are available, the breeder has 
 eliminated a considerable factor of chance in results to be expected from 
 repetition of matings. The egg production of the groups of offspring re- 
 sulting from the same mating may vary in noninherited characters from 
 year to year; also in cases where environmental conditions are known to 
 interact with genes controlling inherited characters, variation between 
 offspring produced in different years or seasons may occur. However, 
 the greater part of the genetic control of production characters will be 
 expressed alike in groups of pullets hatched in different years. 
 
 Table 6 gives some data on results obtained from the California Station 
 flock on repeated matings. Despite considerable differences in environ- 
 mental conditions during the years in which these data were taken, the 
 
36 
 
 University of California — Experiment Station 
 
 relative order of superiority of the matings was maintained. Unfortun- 
 ately, data here presented are limited both in the number of repeated 
 matings and in the number of daughters per mating. Frequently, pro- 
 duction indexes of the daughters of a mated pair will vary in different 
 years by 20 to 30 eggs or more. Rarely, however, will a mating that has 
 proved to be superior in one year give inferior progeny in another year, 
 or vice versa. Jull (1933&) has reported even closer agreement between 
 groups of offspring produced by the same matings. 
 
 TABLE 6 
 
 Production Indexes of Progeny Produced in 
 from the Same Matings 
 
 Different Years 
 
 Class 
 
 by first year of 
 
 mating 
 
 Number 
 
 of 
 matings 
 
 Year of mating 
 
 Number 
 
 of 
 daughters 
 
 Production 
 index 
 
 100.1-125.0 
 
 2 
 
 2 
 
 
 5 
 
 3 
 
 f First 
 
 16 
 15 
 
 21 
 17 
 
 62 
 37 
 
 24 
 20 
 
 108.6 
 114.7 
 
 125.1-150.0 
 
 f First 
 
 149.6 
 170.9 
 
 150.1-175.0 
 
 
 
 175.1-200.0 
 
 ( First 
 
 187.4 
 174.9 
 
 200.1-225.0 
 
 f First 
 
 207.9 
 185.6 
 
 
 
 
 The relative value of sister tests in breeding for egg production can be 
 illustrated by data on the same flock involving a greater number of birds 
 (table 7) . While the individual egg-production record of the dam showed 
 no correlation with the production index of the daughters, the produc- 
 tion index of the family of full sisters including the dam bore a direct 
 relation to the production index of the daughters. For this group of 
 birds, the production of the dam's family of full sisters is certainly a 
 better criterion of breeding value than the individual record of the dam. 
 These results indicate the desirability of selecting good families from 
 which good individuals may be chosen as breeders. However, Jull (1933fr) 
 has been able to find no correlation between the average production of 
 full sisters of the dam and that of the daughters in his work. An expla- 
 nation for the differences between Jull's results and ours in this respect 
 is not available. 
 
 Whether or not the individual record of the dam bears any important 
 relation to the production ability of the daughters has been strongly de- 
 
Bul. 626] 
 
 Breeding for Egg Production 
 
 37 
 
 bated. Jull (19335) has found little or no correlation between dam's and 
 daughters' egg-production records. In this respect, his results are similar 
 to ours given in table 7. Goodale (1935), on the other hand, has found 
 some correlation between the production of the dam and that of the 
 daughters. These two conflicting reports and also the data given above 
 can perhaps be reconciled to a certain degree. In Jull's data and in ours, 
 the range of production of the dams was relatively narrow (between 200 
 and 300 eggs), while in Goodale's data a number of groups of dams had 
 
 TABLE 7 
 
 Belation of Production Indexes of 262 White Leghorn 
 
 Pullets to the Average Record of Dam and to 
 
 the Average Production Index of the 
 
 Dam and Her Full Sisters 
 
 (Each group represents approximately one-quarter of 
 
 the total number of pullets) 
 
 Classification by dam's record 
 
 Classification by family records 
 
 Average 
 record of 
 
 dams 
 
 Production 
 index of 
 daughters 
 
 Average 
 
 production 
 
 index of 
 
 dams and their 
 
 full sisters 
 
 Production 
 index of 
 daughters 
 
 eggs 
 287.6 
 270.5 
 250.2 
 226.6 
 
 183.4 
 214.4 
 191.0 
 185.3 
 
 230.3 
 201.0 
 191.6 
 108.5 
 
 215.1 
 213.0 
 205.0 
 139.2 
 
 produced less than 200 eggs and others had produced over 300 eggs. It 
 still is possible to conclude that when the production of the dams is be- 
 tween 200 and 300 eggs, close correlation with daughters' records is often 
 not obtained. 
 
 Another valuable use for progeny tests is found in connection with the 
 introduction of new and unrelated breeding stock into a strain, as pre- 
 viously discussed under "Outbreeding and Crossbreeding" (p. 32). 
 
 Progeny testing is also of value in experimental matings. The number 
 of such matings possible under commercial poultry-breeding conditions 
 usually must be small. However, in the breeding of any flock, conditions 
 will often arise which cannot be answered from previous experience. In 
 these cases, the poultry breeder is justified in making matings designed 
 to reveal new facts which will guide him in future breeding procedure. 
 
 Multiplication of Superior Stock. — The multiplier may buy superior 
 stock from the pedigree breeder and reproduce it by generations of non- 
 pedigree matings. His job is to increase the number of superior birds 
 
38 
 
 University of California — Experiment Station 
 
 available for commercial poultry keeping with as little loss of quality in 
 the strain as possible. He may sell his products as pullets or adult stock 
 to commercial poultrymen or as eggs to the hatcheries, which in turn sell 
 baby chicks to the commercial poultrymen. In either case, the multiplier 
 serves a very useful purpose : he brings the improvements gained by 
 pedigree breeding within the means of the commercial poultryman, who 
 often cannot afford to buy pedigreed stock for the production of market 
 eggs, even if unlimited quantities were available. The multiplier who, by 
 lack of an effective breeding program, makes no effort to improve the 
 quality of his stock, is of doubtful service to the poultry industry. 
 
 TABLE 8 
 
 Production and Mortality with Respect to Quality of White 
 
 Leghorn Pullets as Determined at Five Months of Age 
 
 Grade 
 
 at 5 months 
 
 of age 
 
 Total 
 
 number of 
 
 birds 
 
 Per cent 
 
 of 
 
 flock 
 
 Production 
 index 
 
 Per cent 
 pathology 
 
 1 
 
 2 
 
 3 
 
 263 
 783 
 31 
 
 24.4 
 72.7 
 2.9 
 
 100.0 
 
 182.2 
 150.9 
 
 62.8 
 
 156.0 
 
 40.3 
 
 48.5 
 77.4 
 
 
 
 All grades 
 
 1,077 
 
 47.3 
 
 The males used in multipliers' flocks should be from pedigreed ances- 
 try. The most rapid dissemination of good qualities can only be obtained 
 by mating superior males with selected hens. The females can be culled 
 frequently or trapnested for short periods of time in order to select the 
 superior individuals. In flocks where the pullets will later be used for 
 breeders, a constant selection of stock should be practiced from hatching 
 time until the matings are made. Birds showing retarded development 
 should be eliminated throughout the growing period. At 5 to 6 months of 
 age, the pullets should be graded and segregated according to their de- 
 velopment. First-grade birds will, on the average, prove to be the best 
 stock. Second-grade birds often are profitable commercially but do not 
 average as well as the first-grade stock. Third-grade birds are under- 
 developed and should be culled, because they rarely prove profitable even 
 under favorable commercial conditions. 
 
 Table 8 gives production and mortality data on three grades of birds 
 from the California Station flock. This type of results has been repeatedly 
 obtained in grading birds over a period of years. Profitable levels of pro- 
 duction were reached only in the first and second grade. Pathological 
 conditions in the first and second grades were lower than in the third 
 grade. 
 
Bul. 626] Breeding for Egg Production 39 
 
 Segregation into grades as here used was based on the comparative de- 
 velopment of the birds. In the first grade, only birds of good size and 
 weight, without coarseness, with good pigmentation of shanks and beaks 
 (in case of yellow-skinned breeds), and showing freedom from any appar- 
 ent disease, were included. The second grade consisted of birds not so well 
 developed as those in the first grade but also apparently free from dis- 
 ease. The third grade included all small birds, as well as those with poor 
 pigmentation and those showing signs of the onset of pathological condi- 
 tions. One of the most frequent of these was the beginning of blindness 
 produced by an iritis. 
 
 While many birds belonging to the second grade will make good 
 production records, the character of their development should cause hesi- 
 tation about their promiscuous use as breeders. A strain of birds char- 
 acterized by rapid and sturdy growth is desired in commercial poultry 
 production. Under the same flock conditions, the first-grade pullets de- 
 velop in a superior manner and, provided they meet all other standards, 
 are to be preferred as breeders. Third-grade birds should be marketed as 
 poultry meat or destroyed. 
 
 Where circumstances prevent the trapping of potential breeding birds 
 for a complete year of production, trapping for a period of two to three 
 months is a valuable aid in selection of stock. When combined with the 
 use of culling methods, part-time trapnesting can be used to determine 
 with good accuracy the productive ability of a bird. It is particularly 
 valuable for determining the ability of the bird to lay at a rapid rate, 
 which cannot be established by culling methods alone. However, culling 
 methods can be used to determine with reasonable accuracy the length of 
 time a bird lays if applied during usual periods in which the birds are 
 maturing, pausing, or molting. 
 
 A Breeding Program for Multipliers. — While it is not considered ad- 
 visable to present a rigid program of breeding procedure for pedigree 
 breeders, because of the variability of conditions encountered in different 
 flocks, a general breeding program for multipliers may be advanced. 
 Such a program can be presented in the form of a summary of the pre- 
 ceding discussion. The application of the following specific recommen- 
 dations should be limited to birds hatched in the spring season, from 
 February to April : 
 
 1. Remove all poorly developed birds at any stage of development. 
 
 2. Segregate the pullets at five to six months of age and discard all 
 birds falling into the third grade. 
 
 3. If part-time trapnesting can be used for birds aged six to eight 
 months, eliminate pullets laying at less than 65 per cent net rate. This 
 
40 University of California — Experiment Station 
 
 procedure will also enable elimination of individuals producing unde- 
 sirable types of eggs. 
 
 4. Remove late-maturing or pullet-molting birds from the breeding 
 flock in November or December of the first laying year. 
 
 5. Remove all birds which become broody in the course of the year. 
 
 6. Cull the flock in the summer or early fall to eliminate early molters 
 and poor producers, as determined by the usual methods of judging past 
 production. 
 
 7. Remove all diseased birds and pullorum reactors. 
 
 8. Mate selected females to sib- or sister-tested pedigreed males from 
 high-production stock which also exhibits high viability under commer- 
 cial flock conditions. 
 
 9. Do not set small, overly large, misshapen, poor-shelled, or off-colored 
 eggs. 
 
 Age of Breeding Stock. — The comparative value of young and old 
 birds as breeders is frequently debated. If natural selection operates to 
 remove the weaker birds and if these birds were weak owing to genetic 
 causes, then older birds should unquestionably be superior to the young 
 birds of the same flock as breeders. However, the individuality of birds 
 frequently does not reveal their complete genetic constitution. Even in 
 families with low average viability, there are often individuals which 
 live to an advanced age. In numerous instances in the California Station 
 flock, these birds have given offspring with poor viability. Age, in itself, 
 is no guarantee of the ability of an individual to transmit high viability 
 to its offspring. On the other hand, a flock of old birds should have been 
 reduced by culling and natural selection to the point where its offspring 
 should excel offspring from the same flock before the poorer and weaker 
 individuals had been eliminated. So far as is known, the same bird pos- 
 sesses the same ability to transmit desirable inherited characters to its 
 offspring when it is a pullet as at any later time in its life (Hays, 1928) , 
 although a change in the incidence of certain diseases may greatly affect 
 the amount of mortality in the offspring from different years of mating. 
 
 In commercial stock produced without the aid of pedigree records, 
 breeding from older hens and cocks is advisable unless younger stock 
 from strains characterized by superior viability are available. Where 
 pedigrees are known, family characteristics and proved breeding ability 
 rather than age of the breeding bird should be the decisive factors. 
 
Bul. 626] Breeding for Egg Production 41 
 
 WHAT BREEDING CAN ACCOMPLISH 
 
 As previously stated, many characters of production and viability in 
 poultry appear to result from the interaction of genetic and environ- 
 mental factors. Improvement in any flock may represent actual increase 
 in number of desirable genes or it may be merely the reflection of im- 
 proved nutrition, disease-control methods, or management. Munro (1937) 
 has given strong evidence that much of the improvement in average egg 
 production attributed to breeding selection is not genetic in origin. Cer- 
 tainly no one can dispute the fact that improvements in knowledge of 
 poultry nutrition during the last two decades have made possible a bet- 
 ter average winter egg production than was previously attainable. 
 
 To present concrete evidence for an undiluted effect of breeding selec- 
 tion, all controllable nongenetic influences must be kept constant as far 
 as is possible ; or, if they do vary, their effect must be measured independ- 
 ently from the effect of the genes. Such conditions have prevailed for a 
 number of years in the Single Comb White Leghorn breeding flock of the 
 University, and it is possible to state that no environmental changes 
 associated with nutrition or management can account for the results ob- 
 tained, with the exception of an improvement in early growth and sexual 
 maturity obtained by nutritional means. The results presented in table 9 
 thus give a reasonably accurate measure of the effect of the breeding 
 selections practiced for all except the above-mentioned characters. 
 
 Egg Production. — As can be seen, a rather unusual improvement in 
 the average productive level of the flock was obtained. Associated with 
 the improvement in number of eggs produced were earlier sexual ma- 
 turity, higher rate, higher persistency as measured by age at last egg, 
 and a reduced pathology. No material improvement with respect to 
 broodiness was obtained except in the last two years of mating. While 
 the amount of winter pausing was reduced, improvement was not con- 
 sistently obtained. The amount of spring and summer pausing, however, 
 was greatly reduced. The improved production was not associated with 
 any sacrifice of desirable egg size or egg color. 
 
 During the years represented by the data in table 9, improvement was 
 obtained in family lines originally represented in the first year of the 
 study, as well as in new families originating from purchased stock 
 crossed with the foundation lines. Whether the stock originated from 
 families present in the original flock or from crosses with other strains, 
 the progeny produced was submitted to an analysis of production char- 
 acters and family performance before any further use as breeders was 
 made of the parents. Of five males from other strains which were mated 
 
42 
 
 University of California — Experiment Station 
 
 with selected hens of the Station flock in the course of these years, two 
 were proved satisfactory by the progeny test for continuation in the 
 breeding flock. In the case of the three unsatisfactory males, only a few 
 progeny were saved for breeding. Since matings headed by males not 
 
 TABLE 9 
 
 Production and Pathology Characteristics of the California Station Flock 
 
 of Production-Bred Single Comb White Leghorns 
 
 
 1933 
 
 1934 
 
 1935 
 
 1936 
 
 Number of pullets banded 
 
 535 
 
 704 
 
 485 
 
 403 
 
 Average weight at banding time 
 
 2.59 lbs. 
 
 2.65 lbs. 
 
 2.98 lbs. 
 
 2.90 lbs. 
 
 
 Nov. 12 
 
 Oct. 13 
 
 Oct. 4 
 
 Sept. 16 
 
 Average age at first egg 
 
 211 days 
 
 195 days 
 
 187 days 
 
 174 days 
 
 Per cent of birds maturing under 200 days of 
 
 
 
 
 
 
 43 p.ct. 
 
 64 p.ct. 
 
 71 p.ct. 
 
 84 p.ct, 
 
 Per cent of birds with high rate* 
 
 
 52 p.ct. 
 
 76 p.ct. 
 
 73 p.ct. 
 
 Per cent of birds with winter pause, 7 days 
 
 
 
 
 
 
 67 p.ct. 
 
 65 p.ct. 
 
 
 52 p.ct. 
 
 Per cent of birds with spring or summer 
 
 
 
 
 
 
 
 37 p.ct. 
 
 37 p.ct. 
 
 34 p.ct, 
 
 
 15 p.ct. 
 
 18 p.ct. 
 
 13 p.ct. 
 
 9 p.ct. 
 
 Average date of last eggf 
 
 Oct. 13 
 
 Oct. 4 
 
 Oct. 8 
 
 Oct. 17 
 
 
 
 
 557 days 
 
 566 davs 
 
 Per cent of birds laying October 1 at end of 
 
 
 
 
 
 
 50 p.ct. 
 
 67 p.ct. 
 
 
 69 p.ct. 
 
 Per cent of birds attaining 2-oz. egg size 
 
 72 p.ct. 
 
 74 p.ct. 
 
 76 p.ct. 
 
 79 p.ct. 
 
 
 2 p.ct. 
 
 3 p.ct. 
 
 3 p.ct. 
 
 2 p.ct. 
 
 Per cent pathology to September 1J 
 
 
 
 
 
 
 38.3 p.ct. 
 
 32.7 p.ct, 
 
 27.8 p.ct. 
 
 33.8 p.ct, 
 
 Per cent pathology to October It 
 
 
 
 
 
 
 55.0 p.ct. 
 
 38.1 p.ct. 
 
 36.8 p.ct. 
 
 36.5 p.ct. 
 
 Per cent reproductive pathology for year§. . . 
 
 22.0 p.ct. 
 
 14.3 p.ct, 
 
 14.2 p.ct. 
 
 18.6 p.ct. 
 
 Per cent neoplastic pathology for year§ 
 
 10.6 p.ct. 
 
 9.1 p.ct. 
 
 8.9 p.ct. 
 
 6.7 p.ct. 
 
 Per cent digestive pathology for year§ 
 
 19.2 p.ct. 
 
 7.1 p.ct. 
 
 6.8 p.ct. 
 
 4.7 p.ct, 
 
 Per cent functional pathology for year§^ 
 
 16.2 p.ct. 
 
 10.6 p.ct. 
 
 10.3 p.ct, 
 
 10.7 p.ct. 
 
 Average 365-day egg record of survivors 
 
 217 eggs 
 
 234 eggs 
 
 239 eggs 
 
 243 eggs 
 
 
 175.6 eggs 
 
 206.6 eggs 
 
 241.9 eggs 
 
 249.7 eggs 
 
 
 126.3 
 
 170.0 
 
 186.0 
 
 196.0 
 
 Production index to October 1 (average age 
 
 
 
 
 
 
 125.6 
 
 171.8 
 
 186.5 
 
 201.6 
 
 
 
 * See figure 4 for definition of high rate. 
 
 t Records closed on January 15. 
 
 % Records based on original number of pullets banded at five months of age. The difference between 
 pathology to September 1 and that to October 1 largely represents the proportion of the flock culled in 
 September which showed pathological lesions. 
 
 § Includes mortality and birds disposed of at the end of the year which showed pathological lesions. 
 The figures for different types of pathology are nonadditive, since many duplications exist. 
 
 t Largely degeneration of the kidney and liver. 
 
 bred by us accounted for less than 20 per cent of the total matings in the 
 years cited, the whole improvement in the flock cannot be attributed to 
 outbreeding with birds of other strains. However, a valuable hereditary 
 contribution was made by the two introduced males which proved to be 
 satisfactory. 
 
Bul. 626] Breeding for Egg Production 43 
 
 Viability. — During this period the flock has been free from all known 
 infectious diseases except coccidiosis which, however, caused the death of 
 a very few birds and was a minor pathological factor. The preceding 
 statement does not refer to neuro- and visceral lymphomatosis, the in- 
 fectiousness of which has not been demonstrated for this flock (Beach, 
 1938). Yet the proportion of the flock showing pathological conditions 
 has not been low. 
 
 An examination of the types of pathology present in this flock empha- 
 sizes the importance of loss from diseases or functional disturbances of 
 unknown origin and for which no means of control has been devised 
 
 TABLE 10 
 
 Occurrence of Neoplastic Diseases in the Offspring Produced 
 
 from matings of blrds from families showing 
 
 Frequent Occurrence of Neoplasms 
 
 Year 
 
 Number 
 of birds 
 
 Per cent 
 neoplasms 
 
 1933-34 
 
 52 
 129 
 152 
 134 
 
 13.5 
 
 1934-35 
 
 25.5 
 
 1935-36 
 
 27.6 
 
 1936-37 
 
 23.9 
 
 
 
 (Lubbehusen and Beach, 1938). Reproductive and functional types of 
 mortality have not been specifically selected against. Losses from pathol- 
 ogy associated with the digestive system in the 1933 flock were un- 
 usually high and were found predominantly in the families from one 
 male and his son. After elimination of these families, losses from dis- 
 orders of the alimentary system dropped sharply for the succeeding 
 years. 
 
 Neoplastic diseases (including neurolymphomatosis, lymphoid infil- 
 tration of organs, iritis, tumors, etc.) have declined slowly during the 
 four years reported. At the same time, deliberate efforts were made to 
 breed a strain of birds susceptible to these neoplasms. Data in table 10 
 indicate that deliberate breeding of birds from families showing a fre- 
 quent occurrence of neoplasms may result in increasing losses from 
 pathology of this kind. Birds produced from these matings received the 
 same rations and management as those from the production-bred line 
 in which a decrease in incidence of neoplasms has occurred (table 9) . In 
 fact, birds from high- and low-mortality selections were hatched in the 
 same incubators, reared in the same brooders, and kept in the same laying 
 houses for their first year of production. Yet while, in the production- 
 bred flock, a decrease occurred in the proportion of birds exhibiting neo- 
 
44 University of California — Experiment Station 
 
 plasms, the families shown in table 10, after a marked first-year increase, 
 maintained approximately the same higher level of neoplastic pathology. 
 The evidence from these matings indicates a possible genetic basis for 
 susceptibility or resistance to certain neoplastic diseases. Similar evi- 
 dence has been obtained by Williams, Gildow, and Lampman (1938) and 
 Wilcke, Lee, and Murray (1938). Probably either the hereditary basis 
 for resistance is complex or it involves complicated interactions with 
 nongenetic influences. 
 
 Breeding for disease resistance is slow and costly. Where adequate 
 means are available to control diseases by means of agglutination tests, 
 vaccines, specific drugs, or sanitation, trying to breed for resistance is 
 uneconomic. Where no means of control are available and where there 
 are evidences of different responses by birds of different strains or fam- 
 ilies, genetic selection is advisable, even though improvement of the stock 
 comes slowly. 
 
 Some poultrymen are prone to suggest that, if birds are properly 
 selected and mated, strains will be created with such inherent vigor 
 that they can withstand adverse conditions commonly found on poultry 
 ranches. But this seems more than can logically be expected to result 
 from good breeding. Inadequate or deficient rations, insanitary condi- 
 tions, and generally poor management cannot be overcome by better 
 breeding of poultry. 
 
 Importance of Other Conditions. — In presenting the results obtained 
 with the California Station flock, we have attempted to show how iden- 
 tification of and selection for characters of production and viability have 
 resulted in improved average egg production in successive generations. 
 Poultry breeding is but one means for improvement of conditions in 
 poultry production. As such, its contribution must be added to the con- 
 tributions from other sciences as applied to poultry for the solution of 
 the problems which the poultryman faces in his business. Breeding can 
 create stock with improved potentialities for profitable production, but 
 such potentialities will not be realized unless the birds are well fed, 
 housed, and managed. Good breeding probably will not help the ineffi- 
 cient poultryman; it can expand the possibilities for success on the part 
 of the poultryman whose methods of feeding and flock management are 
 sound. 
 
Bul. 626] Breeding for Egg Production 45 
 
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46 University of California — Experiment Station 
 
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Bul. 626] Breeding for Egg Production 47 
 
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