if "K p ' vi « ' o» of Agricultural Science i \ > UNIVERSITY OF CALIFORNIA "'".HjJQ ~ — — €«fi CULLING and REPLACEMENT SYSTEMS for LAYING CAGE JPERATIONS HANS AB PLAN ALP LEON S. ROSENBLATT W. E. NEWLON :aufornia AGRICULTURAL XPERIMENT STATION BULLETIN 756 CULLII Visual culling, as practiced by expert poultrymen, can give good results, and may be preferable to complicated record keeping. Egg records as a basis for culling tend to be superior to visual appraisal, even allowing for added costs of record keeping. A combination of visual culling plus egg records may improve culling accuracy a? compared with either method alone. METHODS— HOW ACCURATE? Ihe increasing use of laying cages in California poultry enter- prises is due in part to the fact that such arrangements enable the poultry rancher to eliminate and replace unprofitable birds more easily. Each rancher, however, has his own system of culling and replacement. It was therefore desirable to attempt to establish systems of culling and replacement based on results of controlled experiments. The system of visual culling, so prevalent in commercial prac- tice, was compared with culling methods based on the egg record alone. In almost all cases tested, the egg records proved to be more accurate than the judgment of two experienced poultrymen. Com- bination of visual and record culling is, however, advisable. The culling schemes outlined in this bulletin are based on length of pausing and the rate of lay. The egg records were exam- ined daily in some cases and only periodically in others. The re- sults indicate that a system based on periodic inspection may be quite feasible. A major mistake made by many ranchers is culling too early and too heavily. This study shows that a delay in culling is advis- able because it gives pullets a chance to prove themselves, and reduces the number of replacements needed without reducing income per cage. The study also shows that replacements may be adapted to take advantage of seasonal trends whenever such trends exist. Thus, it may be possible to eliminate replacements at certain times of the year without detriment. In deciding the number of replace- ments to be made, however, judgment must be based upon the existing economic situation. It should be emphasized that, in general, conclusions drawn here pertain to the flock studied. THE AUTHORS: Hans Abplanalp is Instructor in Poultry Husbandry and Jun:or Poultry Geneticist in the Experiment Station, Davis. Leon S. Rosenblatt is Kimber Fellow in Poultry Genetics, Berkeley. W. E. Newlon is Agriculturist in Agricultural Extension, Berkeley. AUGUST, 1956 CULLING and REPLACEMENT SYSTEMS for LAYING CAGE OPERATIONS 1 Hans Abplanalp Leon S. Rosenblatt W. E. Newlon A recent development in the management of poultry flocks has been the estab- lishment of plants in which laying birds are kept in individual cages. Presumably, one advantage of this system is the fact that the operator has an opportunity to maintain a high laying rate for his flock by continuous elimination of unprofitable birds. To date, however, no systematic study has been made of the factors on which decisions should be based as to when a bird should be discarded and a replacement introduced in order to maximize the operator's net income. The current practices in the field vary widely and seem to be based on purely subjective notions of indi- vidual poultrymen. The purpose of the present study is to compare a number of possible culling procedures in order to provide at least gross indications of eco- nomically efficient schemes. General Considerations Three sources of information from which culling decisions can be made are available to the cage operator: l.Feed consumption and drop- pings. The stationary position of each hen makes it possible for the operator to note any irregularities and to elimi- nate any bird that displays them. 2. Physical appearance of the bird. Cursory inspection of the hens can be made without individual handling in the course of daily egg collection. Changes in comb size and color, marked loss of weight, appearance of neck and body molt, general unthriftiness, de- velopment of iritis, paralysis, and a large number of other manifestations may be cause for removal as soon as they be- come evident to an alert operator. Peri- odic handling of birds for detection of 1 Submitted for publication August 4, 1955. less pronounced abnormalities is also possible. 3. Egg production records. The in- dividual performances of each hen may be recorded in the course of egg collec- tion, and decision as to removal may be based on inspection of that record. Vari- ous record-keeping schemes are possible, ranging from those equivalent to daily trapnesting to those based on periodic recording at longer intervals. In general it may be assumed that the majority of cage operators, under all management schemes, remove obviously unthrifty birds. With respect to the other two types of information, the problems are: (1) Is the operation more efficient with visual culling based on periodic in- spection than it is when no culling is used? (2) How does the efficiency of various empirically derived, systematic 4] culling schemes based on records com- pare with that of other methods? Answers to these questions will not, however, provide all the information necessary for devising practical culling schemes. Since the economics of poultry production requires that housing capac- ity be efficiently utilized, the commercial operator is not interested primarily in the net return from any particular culled population, but rather in the net return from the set of cages, in which the culled birds are replaced by pullets just coming into production. Hence any culling scheme used must be put into operation in combination with a replacement pro- gram. Theoretically, it might be assumed that a cage vacated because of death or the culling of its inhabitant would be immediately put into use for housing a laying pullet. In practice, however, it is not feasible for a rancher to carry re- placement stock available daily to fill the empty cages. Instead, replacement occurs only at fixed times, when groups of pullets hatched at monthly, bimonthly or longer intervals are just coming into production. The productive value of a replacement depends upon the expected laying per- formance of the population of birds from which it is to be selected. Hence the criteria for culling should be such that birds are not likely to be culled heavily during a period for which the replacements (to be made at the end of the period) have a low expected eco- nomic return. Thus we may consider the net return for a plan of operation to be the result of two possible, interdepend- ent, management variables: continuous elimination of undesirable birds, and discontinuous replacement of culled and dead birds by younger pullets. Various factors determine the expected economic return from the particular group avail- able for replacement. The most impor- tant ones seem to be biological — sexual maturity, rate of egg production, and egg weight — and the economic factor of egg price. Other possible factors, such as meat value of culls and feed price, were not found to vary in any important way for birds hatched in different months of the year. Hence, for the purposes of the present study such factors will be considered as constant for all groups of birds within each culling scheme al- though, as will be seen later, different levels of feed cost will be taken into account. Under any system of replacements, it should be realized that the replacements will themselves be replaced. A whole series of birds, representing many re- placing populations, may therefore be in the cages at any one time. This intricate problem, together with those outlined above, makes it evident that no simple theoretical models could lead to suffi- ciently accurate answers. An empirical approach based directly on appropriate data appeared to be a more reasonable way to seek a solution. Hence the pres- ent study is based on data gathered on 12 populations of pullets from a single source, hatched at successive monthly intervals, and maintained on a single plant under as identical conditions as possible for a period of 17 months each. Admittedly, more complete information could have been obtained by replicating the material over a period of several years with stock from the same and other sources. However, the cost of such an undertaking and the facilities required could not be met. There are other questions of possible importance in actual cage operations that could not be considered here. Perhaps the most important ones deal with the possibility of varying culling criteria and replacement rates with season. Whether such refinements of the culling schemes investigated are economically worth- while cannot be established with suffi- cient precision in the absence of data extending over several years. Such prob- lems were therefore not investigated. [5] The conclusions that have been drawn from these data must be considered to be applicable to the particular strain, year of hatch, and management condi- tions described herein. (Some aspects of the reliability of the data were checked, and are discussed in Appendix A, page 37.) Nevertheless, they provide certain indications which may have more gen- eral applicability and which are subject to confirmation by subsequent experi- ments or by other cage operators. Materials A group of 110 S.C.W. Leghorn, sexed, day-old pullet chicks of commer- cial grade, originating from a breeder- hatchery, formed the basis of each popu- lation. One such group was delivered to a commercial cage plant situated in Ven- tura County, California, on or about the fifteenth of every month, starting with July, 1951, and ending with June, 1952. The chicks were raised in battery brood- ers, and any cockerels found among them were removed at three weeks of age. The remaining pullets were left in the batteries without artificial heat for another three weeks and then trans- ferred to outdoor growing cages. When the experimental animals were four weeks old they were given an intra- muscular injection of killed Newcastle virus without further booster injection. At six to seven weeks of age, when the birds were moved to outdoor batteries, an inoculation of attenuated pox vac- cine was given in the wing web, and a vaccine against laryngotracheitis was brushed into the vent. Either at 4 1 /2 months of age or when the first eggs were produced from each population, the birds were placed in in- dividual laying cages. Surplus birds (any over 100 for each population) were removed at random at five months of age. The egg laying performance of each individual was recorded daily from the age of five months to 17 months (or prior death) . Thus each population's life in laying cages lasted 12 months, and an over-all total of 23 months' observations was made. The responsibility for main- taining the birds without culling and for North Population 10 Hatched April, 1952 Aisle Population 8 Hatched Feb., 1952 Population 3 Hatched Sept., 1951 Population 1 Hatched July, 1951 Population 9 Hatched March, 1952 Population 4 Hatched Oct., 1951 Population 6 Hatched Dec, 1951 Population 1 1 Hatched May, 1952 Population 12 Hatched June, 1952 Population 7 Hatched Jan., 1952 Population 5 Hatched Nov., 1951 Population 2 Hatched Aug., 1951 South [6] keeping the records was assumed by the highly experienced and efficient owner- manager of the plant. All management procedures (except for lack of culling) were uniform with those on the remain- der of the ranch, which represents one of the more successful commercial op- erations in southern California. Com- mercial 17.5 per cent protein mash was fed, supplemented with whole barley. Feeding was done mechanically. A con- stant day length of 15 hours was pro- vided by artificial lighting. One continu- ous shelter containing two double-tier rows of 300 cages (8" x 16" x 16") each was assigned to the experiment. The exposure was east-west, with wind- breaks against the prevailing winds. The individual populations were randomized in contiguous blocks as shown in the chart on page 6. The only restriction imposed on the randomizing procedure was that three of the first six hatches, and hence three of the last six, were placed on each side of the aisle. It was hoped by this means to minimize the main environmental ef- fects resulting from location within the shelter. For purely managemental rea- sons it was not possible to randomize all individuals, irrespective of date of hatch, throughout the space available. The analysis of egg production of unculled populations has subsequently shown that the four populations at the south end of the house had the lowest production. Randomization of the populations in- sured that such location effects would not be interpreted as seasonal trends. In addition to the complete daily rec- ords of production for a duration of 12 months for each population, a number of other observations were recorded. For example, all the eggs laid by each popu- lation on the first day of every month were weighed in bulk. This procedure provided an estimate of average egg weight for each population every month (12x12, or 144 weighings in all). At the end of the testing period of each Table 1. Monthly Average Egg Prices Used in Study* Month Price for egg-class t Large Medium Small and peewee January cents per dozen 48.0 41.7 41.9 43.4 44.3 45.3 52.2 55.9 59.3 60.4 60.7 55.9 45.7 39.3 39.5 40.4 41.2 41.2 49.2 50.5 52.0 51.1 51.1 51.5 41.5 33.4 32.3 32.7 31.7 30.1 33.1 33.2 32.7 34.5 40.9 44.8 February , March April May June July August September October November December * Average for 1948-1952 of San Francisco wholesale minus 5 cents (weekly) . Centered on fifteenth day of each month. f Large includes eggs heavier than 24 ounces per dozen; medium, larger than 21 ounces but smaller than or equal to 24 ounces per dozen; small, smaller than or equal to 21 ounces per dozen. [7 population, surviving hens were ap- praised with respect to their expected commercial meat value at that time. In order to evaluate visual criteria of culling as well as those based on a com- bination of visual and egg-record sys- tems in commercial practice, "paper" culling was also undertaken. This did not lead to the actual elimination of any birds. Notations were simply made on the back of each egg-record card, or on a separate form, that a given bird should, in the judgment of the observer, have been culled on the date noted. Three ob- servers, independent of each other, con- tributed to this phase of gathering the material. Their methods and procedures will be described later. The economic variables used in esti- mating net returns from different culling procedures were based on quotations of the San Francisco wholesale market. Ex- pected egg prices were derived from cor- responding weekly intervals of the four years 1948 to 1952. Monthly average egg prices centered on the fifteenth day of the month were then obtained by in- terpolating average weekly prices by a free-hand curve. In order to be repre- sentative of egg prices obtained at the farm these values were lowered by 5 cents. Table 1 gives average monthly egg prices for three size classes of eggs, as specified by market quotations. Cost estimates for day-old chicks and for raising pullets to five months of age, for feed, labor, and capital investments, were considered constant regardless of season since no consistent trends could be demonstrated from past feed price quotations. In order to bracket a wide range of possible price-cost relation- ships, three different cost levels were ex- plored in the analysis, corresponding to feed costs of $3, $4, and $5 per hundred- weight. Table 2 gives a more detailed account of the assumptions underlying the cost estimates. Methods of Analysis Calculation of egg value. The ex- pected income from eggs depends on seasonal egg prices and on the distribu- tion of available eggs into three recog- nized size classes. In this study the com- position of egg samples had to be esti- mated from the known average egg weight of each monthly sample. In the absence of direct information about the distribution of egg size within samples it was assumed that the underlying fre- quency distribution for each month of each population was normal about some mean. The actual average egg size of the sample was not used as an estimate of this mean since, upon inspection of average egg size in consecutive samples from the same population, it appeared that the bulk weight determinations had been subject to large fluctuations other than those expected from systematic ef- fects of age. Instead, estimates were ob- tained by hand-fitting a smooth curve to the consecutive monthly means of each population. Furthermore, a constant variance of 4.20 ounces per dozen eggs was postulated for all distributions on -w the basis of collateral information on egg weight in the University of Cali- fornia S.C.W. Leghorn flock and from the literature. Given these assumptions, the composition of an egg sample pro- duced by a population in a particular month of its laying test can be estimated by the proportions of eggs from a nor- mal distribution with the estimated mean, and with variance equal to 4.20 ounces per dozen, which would be ex- pected to fall outside or between speci- fied size limits, 21 and 24 ounces per *? dozen eggs. The size classes of eggs are denned as: small and peewee (smaller [8] Table 2. Costs for Raising Pullets to Five Months of Age, and Weekly Production Costs per Laying Hen at Three Levels of Feed Price Factor Price of feed per cwt. $5.00 $4.00 $3.00 Cost of a five-months-old pullet: Chick $0.43 1.09 0.10 0.04 0.40 $0.43 0.83 0.10 0.04 0.40 $0.43 0.60 0.10 0.04 0.40 Feed, 20 lbs. Brooder fuel, vaccine, etc Interest and depreciation Labor Total cost of pullet $2.06 $ 0.088 0.030 0.011 0.011 $1.80 $ 0.070 0.030 0.011 0.011 $1.57 $ 0.053 0.030 0.011 0.011 Weekly cost per hen : Feed, 1.75 lbs Labor Interest and depreciation Miscellaneous Total weekly cost pe i hen Value of cull hens $0,140 $0.60 $0,122 $0.60 $0,105 $0.60 than, or equal to 21 ounces per dozen) ; medium (larger than 21 ounces, but smaller than, or equal to 24 ounces per dozen) ; large (heavier than 24 ounces per dozen). Thus, if the egg prices as- signed to each of the three size classes for a given month are weighted by the corresponding proportions of eggs esti- mated to fall in each of those classes in that month, we have, for each population and each month of the experiment, an estimate of the egg value. All hens of a particular population were assumed to have the same egg value in any one month. Since eggs laid very early in the pro- duction cycle tend to be small, the value of all eggs laid by a hen before her popu- lation entered the cages was arbitrarily assessed as 90 per cent of the egg value of that population in the first month of the test. Calculation of net income from individual hens. In accordance with the primary objective of this study the relative merits of various culling and re- placement procedures were evaluated in terms of monetary returns. Net income per hen and per population had to be determined as a basis for such compari- sons. For a particular hen, the net income under any culling scheme is measured The value of all eggs laid in the cage up to the time of her exclusion by death or culling. plus plus The value of all eggs laid be- fore her introduction into the cage. The meat value of the hen: if she died, and 60 cents for all culled hens. L9] minus The cost of raising the pullet up to the time of her intro- duction into the cage. Three prices for feed determined the three cost levels used (table 2). minus Total cost of keeping the hen in the cage up to the time of death or culling. Again three prices for feed determined the three cost levels used (table 2). Under the adopted procedure, the net income from individual hens was calcu- lated for each culling procedure at three different cost levels. In summary it may be noted that gross income from sales of eggs was assumed to depend in a com- plex manner on the month in which they were produced and on the age of the populations from which they came. Thus, a given number of eggs produced by the same population at diiferent times of the year does not imply equal income at those times; neither does equal pro- duction by different populations in the same month imply equal income from those two populations. The use of net income figures in totals for all the hens of each population will be further de- veloped in the section beginning on page 19. The calculation of net income as de- scribed here implies several simplifica- tions of actual situations — a method which may not be entirely realistic, but which would seem admissible under the circumstances investigated. First it was assumed that variable costs would not accumulate beyond the date of culling of a hen. Such an assumption is not en- tirely realistic as it must be assumed that certain costs would arise from an empty cage. Also, in practice it would not al- ways be possible to cull hens every day and to sell them immediately, as as- sumed here. Another oversimplification of actual facts is represented by the assumption that hens would consume feed at a con- stant rate independent of their state of production; it is well known that non- laying hens need less feed than do lay- ing birds. Finally, there was consider- able variation in the residual meat value of culled hens according to estimates by the manager; again it was considered reasonable to disregard such possible differences, since no day-to-day evalua- tions of meat values were made. Properties of Unculled Populations Among the biological variables which may affect net income, those of sexual maturity, rate of lay, mortality, the tendency to molt, and egg weight de- serve detailed evaluation. Each of these characteristics is or may be affected by the season during which a group of birds is reared. A further and possibly even more important source of variation is the age of populations. In addition, there may be interaction among these primary effects in the sense that response to sea- sonal factors depends on the age at which birds are exposed to those factors. Sexual maturity. This was meas- ured by the age at first egg, and was de- termined for individual hens. The aver- age and standard deviation for each population are given in table 3. Both the mean and the standard deviation of the trait seem to have been subject to changes. In general, fall- and winter- hatched populations (Populations 3-7) apparently matured somewhat earlier than did others. A minimum average age of 150 days at first egg was attained by the December-hatched population, whereas the maximum of 189 days was [io j 3 Q. O Q. "C J) 3 u C 3 **■ O o 1- G> Q. JQ * C)lflHHtX)iOf!CMOMO CM w X! o oq^oqqoqeoo5wtNi>50fl) co bo w ri N 6 ri H rl ri ri ri ri ri rl tA 2x» cd 1 1 1 1 1 1 1 1 1 1 1 1 V «a es3 O a * 03 NHiO(CfMt>(fiHMHM 00 o 3 O ta (SN^N(OHt»tOO^^tf CO .s o 1 OrHooodooodoo d 5x) X) II III 1 V I| 5 # CO . o iMOt-OtMWOtMi-IOCOiH CO *s (OOtDiQioo^ooMaqc^o cq o — x) CD doHridridciricido © .., c *» ? o «.=> hfi.S r/ ,S'0 tOOHOHTf^HOHHM C5 go &>g$ t»lOOCl-M!Ot»ffi»t-t- f 0) 3 hO a S i-lT-ltNiN*-li-li-li-li-lfHi-li-l tH >fl»h § *l~a« s § ^ c8 ■ J3 CD qeqt>i>TH©cqiq«q-^iqa J ^ 1! 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An inspection of the 12 sets of monthly production averages suggests that all populations followed the same basic pattern of changes in rate of production. It appears that, within the time interval set for the experimental observation, each population underwent a state of rapidly increasing produc- tivity beginning generally during the sixth month of life, reaching maximum rate of lay during the seventh or eighth month and thereafter decreasing gradu- ally. These changes may be attributed in part to increasing numbers of hens reaching sexual maturity during the first few months. They also represent the fact that individual hens tend to increase the rate of lay for some time after sexual maturity has been attained. Similarly the gradual reduction in egg production which is apparent after the seventh or eighth month may be attributed either to complete cessation of lay for some hens (caused, for instance, by molt or disease) or to a generally lowered rate at which hens lay successive eggs. Although the general pattern of egg production was comparable for succes- sive populations, the latter did vary in average output of eggs. Table 3 shows the average number of eggs laid by each group during its year as well as per cent hen-day production based on the ratio of eggs produced to hen-days lived. These figures show that egg production was highest for the populations hatched in September and October. Table 5 gives a monthly breakdown of per cent hen- day production. Mortality. Monthly mortality figures are presented in table 6. They indicate that no major disease outbreaks occur- red in the populations under observa- tion. Table 3 summarizes incidence of mortality by populations. The data pre- [ sented show consistently high mortality (over 14 per cent) among winter- hatched populations. Total mortality was 149 birds, or 12.4 per cent. Molting. The appearance of a first molt, as diagnosed by the manager of the flock, was used as one of the culling criteria. A monthly summary of this characteristic is given in table 7. A total of 599 (or almost 50 per cent) of the hens underwent a visible, discernible molt. Hens began to molt as early as five to six months of age and an ex- pected high incidence of molting occur- red during October-November. Popula- tions differed during the February- March period of 1953. Data on egg pro- duction seem to reflect this onset of pausing. Also, it appears from table 6 that this high incidence of molting was preceded by fairly high mortality. A mild outbreak of a respiratory disease may have taken place to which these ex- ceptional trends in egg production, molt, and mortality could be attributed. Egg weight. Monthly average egg weights as determined by bulk weighing of one day's egg production are given in table 8. Although the figures show considerable nonsystematic variation within populations, they nevertheless demonstrate the usual increase of egg weight with aging of hens. Table 3, page 11, summarizes the average egg weights for the individual populations at the ages of 7%, IOV2, and 13% months — a period covering the time of most pro- nounced increases in egg size. Popula- tions hatched from October to January show consistently low egg weight at 7% months, while summer-hatched popula- tions from May to September seem to have had relatively high early egg weights. The populations with late ma- turity tended to have high initial egg weights. The estimated values per dozen eggs produced by different populations in successive months are given in table 9. 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G) U> UJ o b 4) > < _>* X *■ c 00 X a h -a a 3. o la 3 a o Ph IN o .£5 O a 3 ►■9 O rH o>^coiocNi>(>iocoioq r-"coco©iriididididido6 CNCNCNCNCNCNCNCNCNCNC>I 4 a o B « 2 "o w u «a a) ■s » I CO a "I -3 « 5 W efl V a 0) 03 r* 09 e a* bo bo W rH >> s ooo lO CNCNCNCNCNCNCNCN . 00 o • t-j q oq «o «o ! *-*> 1> ci eo C7J t> : CN CN CN CN CN CN CN . . . . E» a •"9 CN | E- Oi CO CO N NNNN t-j oq q co oq q id id id id co id CN CN CN CN CN CN ', (0 O Q OS Ci CO CN CO 00 l> '. t-^ d th c4 co co -^ r-l CN CN CN CN CN CN CN • id id id co co CN CN CN CN CN . W o S5 . • • -^ t-i o co a co oo co ■ Ci © tA CN iH CO \rt lO . . . . tH CN cn cm cn cm CN cn q t> iq t> co id id co CN CN CN CN ^ u O • • ••'tfCOCOOCNqcDCNCO :<35(NT-i(NCO(NCO"^lO . . . rH # O CO CN :aidcocNcoidt>ididid . . rH CN CN CN CN CN CN CN CN CN CO r-J • co co : CN CN 'crJdcN'^coco^coirJcoirj .r-ICNCNCNCNcNCNCNCNCNCN t> ::::::::: : CN rH >> 3 lOHOH^^OWONOOO iHCN^COCD^lOlO^CDlOCD CNCNCNCNCNCNCNCNCNCNCNCN •3 O •s & a o •■d o 5 ft Sr?fcg«j;gbh;s -Q ■D v 3 ■c It} r4 Ui Ci ©©(N ^< ^i o o ^OOOOOOHOOHIOCOOOCJ ■^•cococo'^''*^ioioir3io^ The data show the superimposed effects of seasonal changes in egg prices and in egg size as dependent on age of hens, on the final value of eggs. No consistent effects of the hatching date of popula- tions are discernible from the data. The combined effects of egg value, egg production, and feed costs were meas- ured in net income, as given in the last three columns of table 3 (p. 11). High- est net income was obtained from Sep- tember- and October-hatched popula- tions, while summer hatches seemed to perform consistently poorly. It must be realized that effects of season and of rearing conditions of pullets cannot be separated here. Hence, even without sea- sonal trends there would be considerable fluctuation in the performance of popu- lations hatched in successive months. Comparison of Schemes for Culling and Replacement The relative merits of different plans of operation must be evaluated on a per cage, not a per bird basis. All the cages are to be filled by birds from the same population at the beginning of the first year; some birds may remain in their cages under a given culling scheme until the end of the year when they are re- placed by birds of the same population, exactly one year younger; others may be culled during the year, and their cages remain empty until the first re- placement subsequent to their removal. At the same time, any replacement may either remain in her cage until the end of her laying year (when she reaches 17 months of age) or may be culled and re- placed herself. If we assume: l.that culling frequencies during any fixed periods, as observed in the present experiments, would have been identical year after year, for the same population; 2. that all hens remaining in cages until the end of their seventeenth month of age would be culled at that time and replaced immediately (according to the first assumption the incoming pullets would then be expected to perform identically with those they replaced) ; 3. that replacements were made from a defined set of 12 or less populations; and 4. that each year the replacements [19] from a given population would be placed in cages at exactly the same date, then it appears that after some years a state of equilibrium should be reached so that every year thereafter, any particular population will enter a specific propor- tion of the total cages as replacements and will be culled during each fixed in- terval between replacements with the same relative frequency as was the orig- inal population. Thus under any culling scheme it is possible to compute the pro- portion of the 100 cages that will be entered each year by each population. The net income of each of the popula- tions weighted by this proportion, and summed over all populations represents the total net return for 100 cages per year for the given culling scheme. (It should be noted that "net income" al- ways refers to the contributions of sep- arate populations, while "net return" always refers to the contribution of a whole set of populations at equilibrium.) Division by the number of cages in the operation (100 in this study) gives a net return per cage. The replacement rate per year is, of course, the total of the equilibrium entry proportions. Other averages, such as the number of days of occupancy per cage and the number of eggs per cage, are obtained in like manner. Appendix B gives the method for finding these equilibrium entry propor- tions. First, a culling scheme and a fixed number of replacements (thus, of popu- lations) must be chosen. Equilibrium entry values can then be derived in terms of the culling proportions determined for each population by the particular culling scheme. An example of a culling scheme applied with three replacement times during the year, and illustrating the procedure described above, is also given in Appendix B. Whenever possible a replacement sys- tem was investigated by using all 12 available populations. If replacements were made on a monthly basis, the 12 populations would form one set; in case of bimonthly replacements, two inde- pendent sets of six populations each would be formed by incorporation of all even-numbered populations into one set and all odd-numbered populations into the other. Similarly, under a system of quarterly replacements, three sets could be formed, et cetera. Figures pre- Table 10. Properties of Different Culling Methods Culling system no. Criterion for culling Approximate age of hens at first inspection Number of annual replacements No culling months 5 7 5-6 5 6 6 6 6 6-8 7 6 6-8 6 7 7 5 5 7 1 6 1 No culliag, with replacement of dead birds 2 Bimonthly visual (Observer B) a 3 System 2 with delayed inspection . . a 4 5 Irregular visual, 6-11 times per year (Observer C) First 5-day pause accompanied by molt 6 6 6 First 3-day pause 6 6 a 7 First 3-day pause (production records 1 week each month) 8* Second 3-day pause 9 First 6-day pause 6 a 10 First 6-day pause, begin after 98 per cent of birds are in lay (m + 2s) 11 First 6-day pause with delayed inspection. . . . Q 12 First 9-day pause 6 a 13 First 9-day pause, begin after 98 per cent of birds are in lay (m + 2s) 14 First 12-day pause a 15 Less than 3 eggs per week (production records 1 week each month) . 6 16f First 6-day pause ; omitting May- August hatched replacements 4 17 18f 19 Daily inspection of birds and records (Observer A) System 17 ; omitting May- August hatched replacements Systems 3 and 11 combined 6 4 6 t ?£/«,♦ ^ f e ° S13 ? P?P ulatl °ns was used ; in all cases investigated the set of odd populations gave somewhat ' t -t. W A « * P°P ulatlons each were used; one comprising populations Nos. 3, 5, 7, 9, the other, populations t The three feed costs are, respectively, $3, $4, and $5 per hundredweight. [20] sented in the tables are averages over all possible sets for a given replacement scheme unless specifically described as otherwise. Although net income of particular populations is undoubtedly influenced by such major factors as sexual maturity, price of eggs, or seasonal rate of lay, the interplay among populations as a consequence of replacement makes it impossible to predict which will be the best culling and replacement schemes. Under the circumstances, the only feas- ible method is to seek such schemes through a process of trial and error. The various systems of culling and re- placement tested were selected as prac- ticable for commercial poultrymen. In fact, many are actually being used. All results obtained have been incor- porated into table 10, which summarizes all culling schemes investigated. Of pri- mary importance are the net returns per cage per year (at the three cost levels) expected under equilibrium conditions. In addition, the effects of each culling Inder Systems of Continued Replacement at Equilibrium Annual eplacement Hen-days per cage per year Average age at culling Hen-day production Average number of eggs Net returns per cage per year at: rate (hens per 100 cages) per cage per year per hen low feed costf medium feed costj high feed cost| 342 362 331 335 339 350 300 330 319 330 332 329 332 333 335 336 303 334 313 325 days 494 500 388 409 428 442 277 360 323 346 366 373 370 384 380 379 379 394 412 368 per cent 52.4 52.8 58.8 58.1 56.4 56.1 62.4 61.4 63.0 62.2 62.2 61.8 61.4 61.3 60.8 60.7 64.8 61.2 63.9 61.8 178.7 190.9 194.4 194.4 191.2 196.5 187.0 202.3 200.9 205.3 206.8 203.3 204.1 204.3 203.9 204.3 196.1 204.8 200.1 201.2 178.7 183.3 138.4 148.5 155.1 162.2 76.6 127.2 107.4 123.7 134.3 135.4 132.9 141.5 137.9 137.2 146.1 147.8 165.3 133.5 dollars 100 104 140 131 123 121 244 159 187 166 153 148 154 144 148 149 134 139 121 151 0.83 0.99 1.19 1.23 1,18 1.32 0.19 1.28 1.07 1.35 1.45 1.45 1.39 1.49 1.41 1.37 1.71 1.56 1.87 1.40 -0.38 -0.28 -0.07 -0.04 -0.07 0.04 -1.23 -0.02 -0.27 0.03 0.15 0.17 0.09 0.20 0.11 0.07 0.53 0.29 0.70 0.13 -1.62 -1.59 -1.38 -1.34 -1.36 -1.27 -2.72 -1.38 -1.67 -1.35 -1.21 -1.15 -1.26 -1.13 -1.23 -1.28 -0.68 -1.03 -0.51 -1.20 tter returns than the even set. ds.4,6,8,10. [21 1 scheme on annual replacement rate, hen- days per cage, average age of hens at culling, rate of lay, and average number of eggs per cage and per hen are also given. Use of this table allows rapid com- parison of the various culling schemes studied. Visual culling. The efficiency of visual culling was tested by having three men participate as culling experts. Each had several opportunities to inspect all hens of the test populations and to pass judgment on individual birds. As a re- sult, it happened that some hens were excluded on more than one evaluation, but only the first one was considered in this analysis. The three culling experts did not fol- low strictly comparable working meth- ods. Observer A, as owner-manager of the test flock, had daily access to the experimental animals. He therefore made culling notations day by day on the basis of his usual practice in managing the flock, utilizing his daily inspection of the birds in combination with the actual egg records. Although he noted decisions based upon visual appraisal separately from those based on record culling, both types of decision were considered simul- taneously in order to represent the best possible culling procedure. Observer B, a county Farm Advisor, inspected all the birds of each popula- tion once a month, noted hens which should be culled, and stated his reasons for culling without reference to the egg- laying records. His first inspection of a population was made when the new pul- lets were to be placed in cages, at five months of age. At that time he graded the birds into three quality classes ac- cording to how well their appearance ful- filled his ideal of promising laying hens. He then culled all birds in the lowest class. Outer signs of late sexual maturity and obvious defects played a major role in the observer's decision at this stage. It is assumed that he culled birds of the lowest class on the first day of the test. His later decisions for culling laying hens were based on beak, earlobe, shank and eye color, comb condition, and color and type of droppings. Observer C, an extension poultryman, paid irregularly spaced visits to the plant in the course of which he inspected each population on some six to 11 occasions and examined all birds alive at the time, without reference to their egg-laying records. Although he judged all popula- tions present at the time of inspection, some populations were examined as many as 11 times while others under- went only six inspections. Reasons for culling were noted in each instance. In- tervals between subsequent visits ranged from 18 to 83 days. It must be assumed that Observer A had at least three advantages over his colleagues in so far as judging future laying capacity of hens was concerned: First, he was in closer contact with the test flock, a fact which permitted him to adjust his decisions in line with gen- eral fluctuations in the performance of populations. Second, because he was able to cull hens at any time, he presumably was able to cull visibly inferior hens at an early stage. Finally, he had the ad- vantage — if such advantage exists — of using accurate records of past egg pro- duction. In view of these circumstances, Observer A's culling performance will be discussed in conjunction with those other procedures which depend on records. Of the two experts who practiced purely visual culling, Observer B must have had two advantages over Observer C in that he was able to inspect the test flock more frequently and regularly. The net returns at equilibrium for the visual culling under a system of bi- monthly replacements are given in table 10. The results from unculled populations are given under culling system No. 1 for which the net returns for the 12 un- culled populations were averaged, while [22] those of Observers B and C are given under systems No. 2 and No. 4, respec- tively. The results indicate net returns of 99 cents for unculled populations at the low level of feed costs. In compari- son, both culling experts B and C had been able to raise the net returns by about 20 per cent, to a total of SI. 19 and $1.18, respectively. Visual culling thus appears to provide a useful basis for improving the efficiency of a cage operation, provided expert evaluation of hens is practiced. However, the differ- ence between net returns achieved by Observer B and those of Observer C are so small as to be almost nonexistent in spite of B's presumed advantage of more frequent and more regular inspections. Although closely comparable in terms of net returns, the two culling experts achieved their results in quite different ways. Observer B seemed to favor some- what greater replacement rates than did C. Under the conditions of this analysis, he would have required as many as 140 pullets per year in order to keep 100 cages filled. Observer C, on the other hand, could have managed with only 123 birds. His less intense culling resulted in a higher average age of hens at cull- ing, somewhat higher egg production per hen, but lower hen-day production per cage. Also, as shown in figure 1, it may be seen that early exclusion of hens was practiced more frequently by Ob- server B than by C. In general, then, it may be said that the results presented here demonstrate the effectiveness of visual culling by experienced poultry- men. Length of pause as culling cri- terion. The question under considera- tion in this section is whether culling on the basis of egg records alone can be practiced to advantage. Undoubtedly there are a number of rules which may be useful for deciding when the past egg record of a hen warrants her exclu- sion from the laying flock. However, several qualifications will have to be met by such a procedure before it can be con- sidered useful for application under con- ditions of a commercial cage operation. First, the precision of prediction of potentially and actually unprofitable hens on the basis of egg records must be greater than that obtainable from visual inspection alone. Practical application of the new methods will be warranted only if costs of recording daily egg pro- duction are exceeded by the higher in- come from improved culling precision. In particular, the applicability of a given record culling method will depend on both its own inherent merit as well as on the skill in visual culling of the man- ager who is considering its application. A second practical limitation in the usefulness of a proposed culling method is the need for simplicity of application. It must be possible for a cage operator to make culling evaluations without much effort and loss of time, particu- larly if the proposed culling method calls for continuous decisions. For purposes of a preliminary inves- tigation it was considered suitable to measure a hen's state of productivity by the number of consecutive misses in egg production preceding a given day of in- spection. The longer such pauses, the more likely it is that a pullet has lost her general ability to lay eggs. Procedures were designed to evaluate the possible use of this measure of productivity as a means of deciding when individual hens pass from a state of profitable high egg production into a state of low egg pro- duction from which recovery is impos- sible or unlikely. The usefulness of consecutive misses as a measure on which to base culling decisions depends in particular on the possibility of finding a decision point at which : 1. The chance of classifying an actu- ally good prospective layer as un- productive is reasonably low. 2. The chance of passing a poor pros- pective layer as a good hen is low. [23] 90 40 30 20 10 40 30 z 20 LU O S 10 UNCULLED OBSERVER "A" I 2 3 4 5 6 7 8 9 10 II 12 CO 40 30 20 10 I 2 3 4 5 6 7 8 9 10 II 12 OBSERVER B OBSERVER "C" 2 3 4 5 6 7 8 9 10 II 12 6-DAY PAUSE BEGIN AT 6 MONTHS I 2 3 4 5 6 7 8 9 10 II 12 6-DAY PAUSE BEGIN WHEN 98% OF HENS ARE IN LAY I 2 3 4 5 6 7 8 9 10 II 12 MONTHS OF TEST I 2 3 4 5 6 7 8 9 10 II 12 MONTHS OF TEST Fig 1. Distribution of culling, by months, of the pooled populations. Culling in the unculled populations is by death until the last month. For all groups, the culling shown in the last month represents mainly the number of birds which were culled automatically at the end of the 12- month laying period. [24] 3. The number of consecutive misses which are needed to arrive at the decision is low. If much time were needed for evaluating a hen's worth, the measurement of productivity would become expensive since poor layers would be allowed to remain in cages to the end of the set obser- vation interval. Economic considerations will impose the practical limits on all three of the above criteria. An empirical solution of the problem stated was sought by choosing pauses of increasing length as decision points at which action would be taken, and by evaluating expected net returns for each method. For most of the methods it was assumed that daily egg records would be taken between the hen's sixth and seventeenth months of age, and that de- cisions for culling could be made contin- uously. (Note that hens which died were automatically excluded.) In particular, the following decision points were chosen: System 6: Cull if three consecutive misses occur for the first time after six months of age. System 8: Cull if three consecutive misses occur for the second time (either separated by a laying period or not) after six months of age. System 9: Cull if six consecutive misses occur for the first time after six months of age. System 12: Cull if nine consecutive misses occur for the first time after six months of age. System 14: Cull if 12 consecutive misses occur for the first time after six months of age. The results for all five procedures are summarized in table 10 (p. 20). Figures for net returns at low feed costs show that culling hens after their first three- day pause results in an income of only 19 cents as compared with $1.07 if exclusions are delayed until the second three-day pause has occurred. Even bet- ter predictions are possible if decisions are restricted to pauses anywhere from six to nine to 12 days, giving net incomes of $1.35, $1.39, and $1.41, respectively, for these procedures. The low efficiency of culling after three-day pauses must be a result of several independent causes. First, the procedure gives low average egg production per cage per year, pre- sumably because hens are excluded be- fore they have had a chance to complete a period of low rate of lay which they inevitably undergo when relatively im- mature. A second and equally important feature of the procedure is represented by an estimated replacement intensity of 244 hens per 100 cages per year. With bird requirements as high as that it must be virtually impossible to obtain satis- factory net returns, mainly because fixed costs for raising 2.44 hens per cage would require considerably higher egg production per cage than even highly screened hen populations could achieve. Since there is an upper limit of laying intensity which is not likely to be ex- ceeded under any culling system in any population of hens now available, there must also be an upper limit of replace- ment intensity above which additional costs for replacements are not balanced by increased intensity of egg production. The results obtained under culling System 8, with a criterion of two three- day pauses, take, as expected, an inter- mediate position between those obtained under a three-day exclusion rule and those from exclusions after six-day pauses. Two independent sequences of three misses represent, then, a culling criterion which is less rigorous (and thus, on the average, implies later exclu- sion) than a three-day pause but is more rigorous (implying earlier exclusion) than a six-day pause. As expected, the replacement turnover due to culling is intermediate between that of three- and [25] six-day culling. Egg production per cage per year similarly reaches an intermedi- ate level. It appears that an extension of pause length beyond six days, as an exclusion criterion, improves culling efficiency. The net returns expected on the basis of nine- and 12-day pauses exceed those obtained under six-day pauses by 4 and 6 cents, respectively, although replace- ment intensity and hen-day production become somewhat lower as pause lengths are increased. The use of pauses of in- creasing length undoubtedly results in a lower chance of excluding good hens, but at the same time more errors are committed by letting hens stay for some time after they surpass their maximum usefulness. The present results suggest that changes in average egg production due to either of the two errors are not of sufficient magnitude to admit conclu- sions. It can only be said that somewhat lower egg production obtained under a culling system of 12-day pauses seems to be balanced by a substantial reduction in replacement requirements. From a practical standpoint, such in- sensitivity of net returns to a wide varia- tion in culling criteria is very desirable. If the populations used in this study are close to being representative of present- day farm flocks, then an application of pauses as culling criteria would seem feasible. It is very likely that consider- able freedom is permissible in the precise length of pauses used or in combinations of those found to be useful here. Thus it is possible that a sequence of one three-day miss and a six-day miss would produce results closely comparable with those of either six-day or nine-day pauses. Culling by length of pause or rate of lay with only periodic inspection of records. The applicability of record culling depends to some extent on the amount of work it takes in order to ar- rive at a relatively good decision about a hen's laying capacity. So far, for all pausing criteria investigated, the daily egg record of a hen was used in order to appraise her productivity. Since paus- ing — at certain rates — proved to be a relatively effective means for detecting poor layers, the question arose as to whether the keeping of egg records could be reduced materially without much loss of accuracy of culling decisions. Accord- ingly, two methods were devised which called for exclusions of hens on the basis of pausing or low rate of lay, while utiliz- ing only partial egg records. The first' method (represented by Sys- tem 7 in table 10, p. 20) specified the keeping of egg records for one week out of every four, beginning with the fourth week after pullets reached six months of age. Hens would be culled if they paused for three days within one of the weekly observation periods. All information from possible previous recording periods was disregarded. Although the chosen method of record inspection would have permitted the evaluation of four separate sets of periodic seven-day inspections, only the one specified above was sub- jected to analysis. Thus, observation se- quences beginning with the first, second, and third week after six months of age were not considered. The results of this first sampling pro- cedure indicate that a reduction in re- cording work is feasible without involv- ing serious loss of culling accuracy. With net returns of $1.28, this system is about intermediate between visual culling and culling on the basis of 12-day pauses (systems 2 and 4, and 14, respectively). In the second sampling method, the keeping of egg records was similarly re- duced to one fourth of a full-time inspec- tion. Recording of eggs was restricted to one week out of four in the sense that observations would begin every fourth week and last for seven days. Failure of hens to lay three eggs or more during any one inspection period would be cause for exclusion. This second proce- dure differed from the first one in that its [26] culling criterion was less severe. It was actually chosen after a preliminary eval- uation of similar weekly inspections had shown that culling of hens with less than four eggs per week was much too rigor- ous to be profitable. The pertinent information is given under System 15 in table 10 (p. 20). Expected net returns, egg production and cage occupancy assume values closely comparable with those from pausing procedures with full-time egg records. The fact that net returns obtained under System 15 are only 4 cents below those of System 14 (12-day pausing) does not represent the full advantages of part-time inspections, since costs due to record keeping were not included in estimates of net returns. If costs for arriving at culling decisions were included in the final answer, the net returns from part- time inspection would be higher than those of the full-time procedures. (The costs of full-time record keeping were estimated to reach about 10 to 20 cents or more per cage per year, depending on the system used.) Within the framework of this study, all evidence supports the view that re- stricted record keeping is feasible with- out much reduction in net returns. Whether this conclusion is a generally valid one is an open question. Effects of delay in culling inspec- tion. With respect to any particular population, it is important for the poul- tryman to know how old the birds need be before being subjected to any evalua- tion, particularly if the culling scheme does not rely upon visual inspection of the birds. The poultryman may make two kinds of errors — culling too early and culling too late. If he culls too early, he may exclude late-maturing birds that have not yet reached their period of maximum return. If he culls too late — beyond the time when nearly all of the birds should have begun to lay — he may fail to exclude birds that matured early but have poor capacity for production. An answer to the problem was sought empirically by devising a series of cull- ing procedures which varied in the time at which inspection of each new popu- lation began. The time at which the pul- lets of any population could be expected to have reached sexual maturity may be assumed to be in close relation to the optimum time at which culling inspec- tion ought to start. Accordingly, the onset of culling for a particular popula- tion was defined in terms of average age at first egg increased by a certain number of standard deviations of that trait. If m stands for the average age at first egg for any population and 5 repre- Table 1 1 . Age of Populations, in Days, at the Beginning of Culling Inspection Under Various Procedures Population number Age of populations (m +s) (m + 2s) (m + 3s) (m + 4s) (m + 5s) 1 182.8 172.6 173.6 169.9 186.1 203.8 192.3 182.6 184.5 186.1 198.9 218.4 201.7 192.5 195.4 202.4 211.8 233.1 211.1 202.4 206.2 218.6 224.6 247.7 220.6 212.3 217.1 234.9 237.4 262.4 3 5 7 9 11 Averages 181.4 193.8 206.2 218.4 230.8 [27] 4a o *a co i> t- co co J3 O N rj r| rj rj M rj boO £ *"S 1 1 1 1 1 1 1 c9 a) to 1 1 1 1 1 1 1 5 e3 fap CD >» M a) °co p. V bo S to 3 O CO H CO iH i-H t- CO © t- 4) H X) i a ,d w cotj Jg rlNNH i-l t-H l-i M "o 6 6 6a d d d M 0) Pi B CO T3 CO Pi to IO H 63- 5 CD U 73 co "£ •* fc CO 1 ° t»HHlO t*i tN lO w- tji iq io ■«* ^ ^ ^t* crT .2t3 r! H rl r d H ri <©• O ^ > * c 01 # *> ** to *> !3 t> 00 t> "^ r-J T)J 1ft ^ 6 CO o o o o O O O O * U HI N^M H tN H C5 o , o t» CO CO CO w CO CO ►jHftP. S *■ u co M i Ul "2 H"^ — 4> CD !£ 00 ^ H O! CO iH 00 CO ci S 43 +S to 08 5 O i8 fl o CD rH 1(5 W U5 ^ ^ CD ^ la 1 M P. 3 CO * 5 bo ,o * •'■§ h co ^3 »»« w r-i *# CO 00 H M tJ< H *?s s & 00 C5 O tH CO 00 iH d T3 12 s ^ T-i th » P. co bo >> ee >. 8 J .2 -s ca '■CSS a> tj 2g -O « .SI d«_ 66 co o — fi g O ft CD o a bo** a s CO '55 c3 *H tM .a ' — » a> w CO M CO Ifl iH sents the standard deviation of the same trait in the same population then on the assumption that age at first egg is nor- mally distributed about its mean, m, it would be expected that about 84 per cent of hens ought to be laying at the age of {m + s) and 98 per cent at the age (m + 2s) . The investigation was ex- tended to onset of culling at (ra + 3s), (m + 45), and (m + 5s) at which most populations had reached ages between seven and nine months (table 11). The effects of delay in culling inspec- tion are presented in table 12. Given a culling rule of six-day misses in egg pro- duction, a postponement of culling by two or three standard deviations beyond the mean age at first egg was shown to be desirable at all cost levels. Further delays up to five standard deviations could apparently be made be- yond average age at first egg without serious drops in expected net returns. For all populations in this analysis the optimal time for starting to cull lies somewhere between six and nine months of age. Roughly speaking, it should not matter where, between the ages of (m + 2s) and (m + 5s) , culling inspection was started. Simply knowing the age limits would enable cage operators to set the time for onset of culling (between six and nine months) without committing grave errors. This would allow consider- able flexibility in the use of the opera- tor's labor resources. The adaptation of onset of culling to the sexual maturity of hens seems to be a desirable practice, as shown by a com- parison between culling with onset at (m + Is) and culling with a uniform de- lay of one month. The latter gave rela- tively low net returns although, on the average, the delay in culling was about the same as in the adapted procedure. The discrepancy in net returns can mainly be attributed to too rigorous cull- ing of populations with late sexual ma- turity, under the rigid procedure. [28] A more detailed analysis of the vari- ables underlying net returns reveals that delays in culling result in lower replace- ment requirements. Average egg produc- tion per cage per year shows a maximum for delay in culling of two standard de- viations beyond the average sexual ma- turity of populations. The results also show that a rigid onset of culling inspec- tion at the age of six months raises bird requirements as compared with the ma- turity-adapted culling. It should be kept in mind that the results presented are limited to a particu- lar culling criterion, the six-day pause. If birds were evaluated by means of some other measure, the present conclusions would have to be applied with caution. On purely theoretical grounds it might be predicted, for instance, that culling on the basis of three-day misses would require more delay in the onset of in- spection, because early culling would be- come much more likely under such rigorous rules than was observed for six-day misses. In general, it seems very unlikely that a delay of culling inspec- tion until hens have reached seven to nine months of age could seriously handi- cap efficient appraisal of hens for culling. Even where evaluation of laying ca- pacity is done by means of visual inspec- tion it is doubtful whether early exclusion of hens is a sound practice. This par- ticular point was subjected to test within the framework of this study. Of the three participating culling experts Observer B had been conspicuous by a relatively rigorous screening of pullets at the time when they were to enter cages. His pro- cedure resulted in the culling of a fair number of birds when they were five months of age. (See figure 1.) In accord- ance with the conclusions reached above, a waiting period of two months was im- posed on Observer B. No culling was allowed until the birds were seven months old. This was achieved by simply disre- garding any of his culling decisions that would otherwise have led to the exclu- sion of hens under seven months of age. All later decisions of Observer B were applied as indicated by him. The results of this procedure are given under System 3 in table 10 (p. 20). They confirm the general conclusion that delaying culling inspection well beyond average age of sexual maturity is a safe practice. In the present case the net returns under de- layed culling were slightly better than those obtained by B originally ($1.23 as compared with Si. 19 at the low cost level ) . Under the new culling system bird requirements were reduced from 140 to 131 birds per 100 cages per year. This shows more clearly the effect of the more conservative approach. One would ex- pect that the present findings might have been more drastic if a novice had done the early culling instead of an experi- enced poultryman. In summarizing the evidence pre- sented, the following practical conclu- sions can be drawn: 1. If culling is to be practiced on the basis of egg records, inspection of birds ought to be delayed at least until 95 per cent of the birds have come into pro- duction. 2. Delay of a few weeks beyond that point has no serious consequences. 3. In the absence of adequate estimates of the average age at which at least 95 per cent of hens in a population will reach sexual maturity, it should be safe to begin culling inspection when the birds are from six to nine months of age, whatever be the month of hatch of the population. However, such a rigidly set beginning of culling may be less desir- able than one adjusted to the sexual ma- turity of each population. The combined results on optimum time for culling and best pause length as exclusion criteria suggest that a com- bination of two promising alternatives would lead to higher net returns than those obtained under previously investi- gated systems. Such optimal culling pro- cedures would result, for instance, if hens [29] * LO 00 Tt< OS rH U5 lO O O lO 00 CO 00 00 *tf cm co lo cm io CO OS iH O o (N t- t> OS CO bo o 1 o o d d o d o o d d o O O tH o d d d d d OOH i-J d q q q q q u a> "d o i i i i © p. as ■d i i • i i CO «H U> a E 1 o *■ * (A >s © CO "^ OS «*» O 00 t- IO OS OS rH os os io CO 00 00 t> CM 00 CO * 8 o CM rH rH r-j 00 CO CO CN 1- ho © k. ho & OS ^ c4 -^ 00 co eo os 00 co rH © OS 00 CO T-H O OS N N H 00 i> r-4 O OS 00 t- £ 3, s © © CM rH T-i i-l rH NNHHrl t-H tH < C ° P.P. t/j u © T5 « o ^ C 3 7 « p fl g-d © *: *: ° 00 00 00 00 00 rH CN -» ^ CO rH o t- "^ CO tH OS CO ^ CO H o !> lO CO CM O 00 +■ P h in © a> © CO CO CO CO CM CO CO CO (N - £_ * CO CO CM CO "^ CO CN iH * CO CM t-I 69- LU • 5 rt p. 6 © » "© p— bo .s bo S3 g 1 © Pi © 3 w o ej ® 2 » 2 * *- •a et3 CO dt -4-3 s a c3 CO CD ■§ ^ ©" 3 CO p (h O >» d s» * CO "h © o eg be ct3 bo fl ft M ^ '*H "G V o CO S j_ OS 2'f 2 -a J * "2 ^ M g 8 § s * £ S A •d 0) © © © M © H * Spa 35 6 !&* O to CM os LO rH were culled after first pausing for nine days but not before about 98 per cent of the hens had started to lay. The out- come of such procedure is given under System 13 in table 10 (p. 20). As ex- pected, the net returns of this system ($1.49 at low feed cost) exceeded those from System 12 ($1.39) in which the onset of observation was set at six months. Effect of frequency of replace- ments. It has been pointed out that the efficiency of a cage culling operation is partly dependent on the frequency and on the regularity with which empty cages are refilled by new pullets. The present section is devoted to an empirical inves- tigation of this feature, but will be re- stricted to the case where replacements were made at equally spaced time inter- vals, and where all populations were used as possible members of the defined re- placement sets. The effects of decreasing rates of replacements were thus evalu- ated in several culling systems by inves- tigating the effect of replacing pullets at intervals of one, two, three, four, and six months. Table 13 gives the results of variable replacements for four different culling procedures. The pertinent data on bird requirements, egg production, and net return are given for each series of re- placement procedures. Consistent results were obtained under all culling proce- dures, hence those obtained for six-day pauses will be used to discuss the major effects of varying replacements. Expected egg production per cage per year showed drastic reductions as a re- sult of lowered replacement rates. In- creasing by one month the time interval between successive replacements caused egg production to drop by about 10 eggs. Thus under a schedule of monthly re- placements 214 eggs would have been expected, while biannual rearing of pul- lets would have permitted an average egg production of only 165 eggs per cage. A doubling of replacement frequency from three to six replacements annually produced greater changes in expected egg production than did a similar inten- sification from six to 12 replacements. An economic evaluation of the replace- ment systems shows that net returns of the present cage operation could be in- creased by rapid refilling of cages only if costs of feed were low. Under economic conditions which made for both high feed costs and expensive stock, it was not profitable to replace often. This rela- tion is clearly demonstrated by compari- son of columns 8 and 10 of table 13. Under conditions of low feed costs, a reduction of replacement frequency simi- larly reduced net returns; however, where feed was assumed to be expensive a low rate of replacement was more ad- vantageous. Such results are expected to hold gen- erally since an increase in replacement frequency may be expected to affect the rate at which hens are placed in cages, but not the average net returns per hen; hence the more frequently replacements are made, the more pronounced become gains or losses from a given culling scheme. The results further indicate that re- placement rate is not a factor which might bring about important changes in the relative efficiency of different culling procedures. Thus the ranking in net re- turns and egg production of the four culling schemes investigated remains al- most the same under all conditions, for a given cost level. The general consequences of changes in periodic replacements might be stated as follows: 1. Lengthening the interval between successive replacements reduced the number of hens needed for keeping cages filled. 2. Replacement frequency mainly af- fected the intensity of cage usage. High replacement rates are thus ad- vantageous only under favorable economic conditions. [31] 3 a o Q. 3 or O a 3 or UJ c u O u ■u- © pi o H I o o. CO hi & •a -50.49 -11.96 1.63 -24.83 -19.31 -42.85 00 t> 1 Oi m CN CN i-H 1 -10.16 16.30 15.40 -3.86 -4.34 -11.27 t> C CN C5 00 © to O CO iH W O CO ^ l> CO -* CN lO t> CO ^ M* © tA 00 1 m m co ih co ct> CD CO tJJ CO i-J l> OS "<^ l" rA o> ^ ' 1 CN lO id 03 M Pi o o si §§■ • J2 o "S to a CD > CD a © CO 3 "o t> O 00 CN t- o a ih t- CN lO lO iH CN U3 r f r r CD O CO CN 00 CO CO iH o th t- co th a lO tH CO CO CO o f r r r ^ CO lO O t- CN CO rH t> IO "^ IO U3 O b- tH O O lO r r Is a> co '3© S-o CDrH ■e.2 n © CO C CD © U3 CN W 00 l£i *H LO 00 O CO to io t> k> CN T* CD L> lO "^ ^ CO CO CO CO OS id lO CN to CN co ^ o a co co O CN CS CO "^ »o oo cn i> d c> oi CO CO CN CN CN tH lO 00 lO o co CO m CD CO O IO W iH IO CO CN CO "^ CN to oq to cn oo oo © to id IO CO CO CO CO CO CO CN CO CN ^ t« IO t- 00 H OS CO CO 00 tH to O id 05 CN CN t> "^ CN CN tH CN iH CN co 00 tH d e .2 I o CD CO C CD > CD CN "^ CO 00 O CN CN -^ CO 00 O CN 1-4 t-I © CO •d ■a o »H CO IO t- Ci iH iH CO lO t- C75 tH 00 1 u "o a o 'B CD •c o CO IQ fVw co >>-m bo all i to CO to XI o ■H o CO bo cS l-l CO > < CO m r^-^ bo -2 bD e *-' co '3 2 JC C-5og o fa CO CO to o ■M o CO bo «3 M CO > .5 © £ >> U to d a to CN iH 3. The absolute rates of replacement obtained in this study do not apply to actual farm conditions unless the latter entail a culling procedure comparable with the one used here, where all hens surviving to 17 months of age are culled at that time. The consistent adherence of a cage operation to some set culling standard necessarily meets with some practical dif- ficulties. This fact is well illustrated in table 14, where replacement requirements are given for each bimonthly replace- ment, under typical culling systems. The number of pullets required to replace culled hens appears to be extremely vari- able, ranging anywhere from 13 to 41 per 100 cages in the case of exclusions by nine-day pauses and bimonthly re- placements. Unless these variations in replacement requirements can be pre- dicted, it will be impossible for a cage operator to plan the exact replacement needs in time to raise appropriate num- bers of chicks. Evidently a number of factors, such as disease, and changes in weather and management, tend to intro- duce unpredictable responses of laying flocks and hence of replacement needs for a given culling system. From the practical standpoint it seems difficult to anticipate even some of the possible re- current seasonal fluctuations in culling intensity. Therefore it would appear reasonable for a cage operator to work on the basis of the average number of expected replacements, using a slightly variable culling intensity to adjust the available empty cage space to the con- stant number of replacements. Such adaptations in culling intensity could most probably be incorporated into a rigid culling procedure by varying the intensity of culling in the oldest age groups of birds, excluding many of them when replacements were in excess of reg- ularly available cages, and holding them somewhat longer if young replacements were insufficient. Visual appraisal of old [ hens would most likely be a good aid. Omission of replacements in a given season. Both culling practices and replacement schedules can be adapted to fit seasonal trends of eco- nomic variables or production charac- teristics in such a way as to maximize income under equilibrium conditions. Known trends in egg production or egg prices might theoretically be exploited, provided their recurrence were depend- able and regular. As for egg prices, the existence of a yearly cycle was at one time established. Unless drastic changes in the economy of egg marketing take place, prices could be expected to reach a high point in the late fall and a mini- mum during the spring months. Another, but less well established effect of season appears to be a reduction of productivity in summer-hatched populations. Some evidence in this study (table 3, page 11) tends to support such an assumption. On the basis of these presumed rela- tionships, a replacement procedure was tested in which no pullets would be raised during the months of May, June, July, and August. Accordingly, no replace- ments of culled birds were available dur- ing October, November, December, and January. The new replacement system may best be visualized as derived from a scheme with regular bimonthly replace- ments from which two summer-hatched populations were omitted. The present system was investigated on two inde- pendent sets of four populations, each corresponding to those in bimonthly re- placement systems. Accordingly set (1) was made up of populations 3, 5, 7, and 9, while set (2) comprised populations 4, 6, 8, and 10. Culling was practiced on the basis of six-day misses, beginning when pullets were six months of age; equilibrium estimates from the two sets were averaged to provide final values of population properties. The net returns obtained under such a season-adjusted replacement procedure 33] were 22 cents higher than those of the best procedure with regular replacements (System 13), reaching $1.71 at low feed costs. These results are presented under System 16 in table 10, along with esti- mates of egg production and cage occu- pancy. Compared with the equivalent regular bimonthly replacement system (No. 9, table 10), the adjusted one re- quired 32 hens fewer per year. Feed days per cage per year were reduced from 330 to 303, and hen-day production in- creased from 62.2 to 64.8 per cent. Conclusions based on the evidence presented on this point must be drawn with caution. It should be realized that the properties of summer-hatched popu- lations as defined in this study may have represented a particular set of conditions quite independent of the known general trends. The design of this study does not provide experimental checks on such pos- sible errors. In particular it should be noted that, in the present study, the poorly performing summer populations were immediately followed by popula- tions which, when unculled, produced the highest net incomes. Pullets from these September- and October-hatched populations, however, accounted for a major part of replacements under equi- librium conditions. In the event that the cost of summer- hatched chicks is less than that of spring and winter chicks, the difference between the two schemes will not be so great. Also, it is entirely possible that differ- ences in climatic conditions might strongly affect the efficiency of a season- adjusted replacement procedure. How- ever, such would also be the fate of any other scheme which attempted to exploit seasonal trends. Just how much confidence can be placed in the present results must be de- termined by future experience. Until evidence to the contrary is adduced, these results may be taken as evidence that adjustment of replacement to economic and/or seasonal trends may be an effec- tive means of increasing the returns of a cage operation. The following potential advantages of season-adjusted proce- dures are indicated: 1. Net returns may be increased by omitting summer populations from replacement procedures. 2. Even without the advantage of higher income an adjusted schedule, as presented here, might be desirable during certain seasons because of the convenience of reduced labor. 3. Replacement systems such as the one presented would tend to render a cage operation flexible, both eco- nomically and managementally, be- cause of its relatively low average cage occupancy. Comparison between record cull- ing and visual culling. Before the ap- plication of precisely defined culling procedures based on egg records could be advocated, the possible advantages would have to be weighed carefully against higher costs incurred by record- ing work. Visual culling, as practiced by expert poultrymen, might be the pref- erable method even though its net returns as defined in this study did turn out to be somewhat lower than those from record culling. Other reasons may exist which would tend to favor visual culling in spite of its possible lower efficiency. Some poultrymen simply might prefer to avoid the complications in manage- ment required by record keeping. Also, it might be desirable to keep two or more hens per cage under a system of close visual culling. These qualifications will be assumed in the following comparison of visual culling with record work. The results obtained when hens were culled after six-, nine-, or 12-day pauses, respectively, compare favorably with those from visual methods. At the low cost level, they yielded net returns up to 30 cents in excess of those obtained by either observer B or C. If it is as- sumed that the record keeping amounted to expenses of 10 to 20 cents per cage 134] per year, the record methods would ap- pear superior to visual culling. Good results would also be obtained under methods of restricted record keeping, as illustrated by System 15, table 10. On the basis of evidence from this study, the conclusion seems justified that keep- ing hens in individual cages affords a chance for poultrymen to better their income by efficient use of egg records. There is nothing to indicate that records should be used without visual inspection of hens. Most probably several culling systems could be devised which would use record culling as a supplement to visual inspections or vice versa. Observer A, whose results are shown under system 17, table 10, was given a chance to proceed on a basis of com- bined appraisals. As outlined earlier, he used egg records to supplement his visual decisions about a hen's laying condition. Under these circumstances, Observer A was able to achieve net returns higher than those of any other system with regular replacements. His results were further improved when adjusted replace- ments were used as described for System 16. The corresponding results which combine Observer A's culling and ad- justed replacements (System 18) show a net return of $1.87 at the low cost level. Observer B's culling procedure was also modified in order to explore pos- sible improvements that he might have made by supplementing his decisions with information from egg records (Sys- tem 19, table 10) . Thus, by enforcing a delay of Observer B's culling decisions to the time when pullets were seven months old and by using pauses of six days as a supplementary culling crite- rion, his net income was increased by 21 cents per cage per year at the low cost level. However, culling on the basis of six-day pauses with delayed inspec- tion (System 11) would have been even better. This result does not necessarily mean, however, that Observer B could not have improved the culling decisions of regular six-day pauses (System 11) had he been permitted to weigh the latter against his own visual decisions at the time when he inspected the flock. General Conclusions and Recommendations It must be kept in mind that the cull- ing procedures were evaluated in this study on the basis of estimated returns under equilibrium conditions. These esti- mates necessarily involve a number of assumptions as to method and consist- ency of bird replacements as well as to the recurrence of events as observed in the 12 populations studied. Net returns obtained under these procedures were considered the most adequate estimates of relative merit of culling systems. The results obtained for various cull- ing procedures under a system of con- tinued cage operation with a set number of yearly bird replacements suggested that at least three factors are instrumen- tal in determining the relative merit of a culling-replacement procedure, each of which is presumably completely or partly under the control of the cage operator. l.The accuracy of culling deci- sions. Presumably there is a time, for each hen, at which her exclusion by cull- ing would be of maximum benefit to the operation. This point is dependent on the defined rules of replacements as well as on the performance of other hens in the same and in other populations. High- est accuracy in culling would be achieved if every hen could be inspected at that point in time, and if the resulting deci- sion would exclude her from the opera- tion. In some cases this could be before a hen was placed in a cage since some hens never pay for the cost of rearing. [35] The accuracy of culling decisions can- not be evaluated directly, since nothing is known about the actual points for optimal exclusions of individual hens under various replacement systems. How- ever, a comparison of culling procedures with identical replacement rates gives an indication as to which ones were superior in culling precision. On this basis it may be concluded that: a) Delay of culling inspection to the point where about 95 per cent of hens has reached sexual maturity increases culling accuracy as compared with meth- ods where culling begins earlier. A delay of first culling up to eight or nine months of age seems desirable. b) Pausing is a useful criterion by which culling accuracy can be improved over that of purely visual inspection. The utilization of pauses of not less than six days is advisable if full egg records are used. Under the same circumstances, pauses up to 12 days or more in length can be utilized with an equal degree of culling precision. Pauses of three days have been found to be valid culling criteria if egg records are inspected in only one week out of every four. c) Rate of lay observed during weekly intervals about once a month gives good culling accuracy if hens that produce less than three eggs per week are excluded. d) Methods based on optimum use of the above principles (based on egg rec- ords) tend to be superior in culling ac- curacy to visual appraisal, even if added costs for record keeping are taken into account. e) The use of records in combination with visual culling may improve culling accuracy as compared with visual ap- praisal. 2. The rate at which regularly spaced replacements can be made. This has been shown to be of impor- tance for the economy of a laying opera- tion. It has been concluded that the main effect of high replacement frequency is one of intensifying the rate of operation in the sense that more hens are being processed through a given number of cages. Depending on culling procedures and the economics of an operation, this intensification may result in an increase of already positive returns, or con- versely, in further aggravated losses when net returns are negative. 3. The extent to which replace- ment frequency can be adapted to seasonal trends or special properties of populations has been found to be a po- tentially efficient means of improving net returns from a laying flock. By omit- ting replacements during summer months it was possible, in this experiment, to: a) increase net returns per cage; b) de- crease operating intensity; c) simplify the actual operation of rearing pullets and recording eggs. It must be pointed out that, in a planned improvement of a cage opera- tion, the major emphasis must be on the initiation of better culling procedures which, in turn, will determine an average rate at which birds have to be replaced. Recommendations as to optimum rates of replacement are therefore meaningless unless proposed in connection with a well-defined procedure of culling which promises to be repeatable over a wide range of managemental and biological conditions. Viewed in this light, the present study has provided information which may be helpful and encouraging to those interested in expanding or re- fining their culling standards. [36] Appendix A In order to establish the consistency of the relative merit of various culling sys- tems, as estimated by their net returns under equilibrium conditions, the equilibrium calculations were replicated within five culling schemes. This was done by dividing each population of 100 hens into two subgroups of 50 hens each and by assigning each of the two samples to one of two replacement sets of six subpopulations. The same subsamples of hens and sets of subpopulations were used in all culling sys- tems thus investigated; they are designated as A and B, respectively, within each of the monthly populations. Net returns expected under equilibrium conditions for five culling systems with bimonthly replacement are shown in appendix table 1. As in the previous estimates of net returns, an arithmetic average was taken of the results obtained for even- and for odd-numbered sets of monthly subpopulations. In general, there appears to be good agreement in the relative merit of culling procedures between the results obtained from A and from B samples. Thus the difference in net returns from systems 7 and 12 varies between 10 and 12 cents for both of the samples. Similar consistency of results obtains for comparisons of other culling procedures. From these results, the conclusion seems justified that samples of 100 hens are adequate for comparing the relative merit of different culling systems under equilib- rium conditions, so long as the same samples of hens are used in estimating net returns from alternate procedures. Appendix Table 1. Variations in Net Returns per Due to Limited Population Size (Samples of 50 Hens) Cage per Year Culling system no. Net returns per cage per year Sample A at: Sample B at: low feed cost* medium feed cost* high feed cost* low feed cost* medium feed cost* high feed cost* 6| dollars dollars -0.11 1.21 1.31 1.42 1.29 -1.54 -0.10 0.02 0.14 -0.01 -3.05 -1.46 -1.32 -1.19 -1.36 0.04 1.35 1.47 1.55 1.44 -1.40 0.05 0.16 0.27 0.14 -2.92 -1.31 -1.21 -1.06 -1.21 7 12 13 15 * The three feed costs are, respectively, $3, $4, and $5 per hundredweight, t Only even-numbered populations are represented by this culling. [37] Appendix B Procedure for Finding Equilibrium Entry Proportions Whatever culling scheme is used, the year is divided into n periods of time (of the same or of differing lengths). The n may be 1 (for no replacement until the end of the year), 2, 3, 4, 6, and 12 (for monthly replacement). At the beginning of each period, a particular population enters the cages. The proportion culled from any population during a given period in the year's life of that population is repre- sented by pi). For example, let n = 3. There will then be a set of 3 x 3 of these propor- tions: p 115 p 12 , p 13 , for Population 1, through p 31 , p 32 , p 33 for Population 3. The cages are filled with Population 1 at the beginning of period 1, and during that period the proportion p xl of them is culled by the application of a chosen culling scheme. Thus, at the beginning of period 2, birds from Population 2 must be added to p lx of the cages. The proportions of Populations 1 and 2 in the cages are now 1— Pn and p 1T , respectively. During period 2, p 12 of the Population 1 birds and p 21 of the Population 2 birds are culled so that at the beginning of period 3, (l—p lt — p 12 ) and p n (l-p 21 ) are the proportions of cages occupied by Populations 1 and 2, respectively; then 1— (1-Pn— P12) +Pn( i- P2i) i s tne proportion of cages to be filled with Population 3 birds. At the end of the third period (the end of the year) , for any population, all birds not previously culled are assumed to be removed from the cages and replaced with birds one year younger. The assumption is necessary because these data encompass only the first 17 months of each bird's life; however, they do not appear to be unrealistic. Now let e x denote the proportion of cages to be occu- pied by the first population to enter the cages and, in general, e^ the proportion of cages to be refilled by the i th population when it is ready to enter the cages. The culling proportions for the first population, p 11? p 12 , p 13 , will then be applied to e 1 each in turn, and the culling proportions pi x , Pi 2 , pi 3 will be applied to e\. It is clear that a table can be constructed, giving the proportion of cages occupied by each of the three populations at the beginning of any period of consecutive years, like the following incomplete one for three populations: Year Period Population 1 2 3 4 1 1 \ = e l 2 1-Pu Pu = e 2 3 1-Pll-Pl2 Pis e 2 (l-p 2 i) P12 + P11P21 = e 3 2 1 MI-P21-P22) P23 e 3 (l-p 3 i) l-e 2 p 28 -e 3 (l-p 31 ) =e\ 2 e 3 (l-p3i-p 32 ) e\U-Pu) p 38 3 e'i(l-Pu-Pi 2 ) etc. [33 Obviously, what is desired is to carry such a table on until an equilibrium is reached, after which the proportion of cages to be refilled by any particular population is the same every year. That is, the set of values e 19 e 2 e n remains unchanged from year to year. Each e can be obtained as a function of the three previous ones by application of the formulas : e' 1 =p 1 3e 1 + / p 22 e 2 + p sl e 3 e'3=P33 e 3 + Pi2e'i + p 21 e' 2 or, in general, for n periods : P2' «— 1^2 ~"~ e 2 = p 2n e 2 + p 3 6n = Pnn&n "+" Pi ,e + L«l+ • 1 Pn-i? 2 e «-i + P>n e » • + Pri2 e n + p xx e x • 1 Pn-2? 2^«~2 ~^~ P n -U 1^» -1 The entry proportions at equilibrium can be evaluated directly by formulas ob- tained for their limits in the case of two populations: e 1 - P12P22 Pn 1 - P12P22 or or 1 -p 22 1 - p 12 p 22 1 - p 12 p 2 . When n is larger than 2, even for n = 3, the formulas for these limits become much too complicated to be useful since the equilibrium proportions can be obtained successively as shown above. As a numerical example consider three populations with successive culling pro- portions of 0.50, 0.25, 0.25 for the first population, 0.40, 0.35, 0.25 for the second, and 0.45, 0.35, 0.20 for the third. Then, starting with Population 1 in occupancy of all the cages, the calculation below characterizes the procedure over six years. The entries represent the proportion of cages filled by birds of the population desig- Calculation of Entry Proportions at Equilibrium Year Period Population 12 3 12 3 12 3 12 3 12 3 12 3 1 1.00 .50 .50 .25 .30 .45 2 .125 .2475 .0900 .6275 .3138 .5962 .1569 .3577 .4854 3 .5840 .6109 .4874 4 .5791 .6129 .4874 5 .5786 .6131 .4874 6 .2681 .0975 .5786 .2893 .6132 .1447 .3679 .4874 7 .1533 [39] nated in the corresponding column at the commencement of the period designated in the corresponding row. Only the entering proportions appear in the table except at the beginning and end since they alone are required for the equilibrium pro- portions. These are reached in the sixth year, after which Population 1, as it be- comes available (reaches five months of age), each year fills 57.86 per cent of the cages, while Population 2 always starts by filling 61.32 per cent of the cages, and Population 3, 48.74 per cent of the cages. Acknowledgments It is a pleasure to acknowledge the generous assistance of Mr. Mayo Argabrite, on whose ranch the experimental populations were raised, who was responsible for supervising the management and the recording of eggs in addition to cooperat- ing as culling expert. Without his aid the present study would not have been possible. We also wish to thank Mr. Herbert Hogsett for his valuable cooperation and en- couragement. We are greatly indebted to Mrs. Dorothy C. Lowry for her help in the statistical treatment of the data — particularly for preparation of Appendix B and for review of the manuscript. We express our thanks to Professor I. M. Lerner for initiating this study, and for his many helpful suggestions throughout the investigation. The information on economic variables was prepared by Mr. A. Doyle Reed of the Department of Agricultural Economics in Davis. Finally, thanks are due Mr. R. Brendler, Farm Advisor of Ventura County, who assisted as a culling expert throughout the experiment. This work was supported in part by donations made by the Kimber Farms, Inc. Niles, California, and by the Donsing Breeding Farm & Hatchery at Rio Linda, California. 10m-8,'56(B7616)L.L. [40]