DOC - TA245.7 i873 13-1356 JUHQ hitebrush Response to The Texas A&l\/i University System, The Texas Agricultural Experiment Station, Neville P. Clarke, Director, College Station, Texas CONTENTS Summary . . . .i . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 1 Materials and Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Results and Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 8 Literature Cited . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 Metric-English Equivalents . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 Appendix: Scientific Names of Plants Mentioned in Text . . . . . . . . . . . . . . . . . . . . . . . . . back cover KEYWORDS: Brush management/range management/whitebrush/tebuthiuron/picloram/pelleted herbicides. Whitebrush Response to Tebuthiuron and Picloram Pellets C. I. Scifres, D. L. Embry, and]. L. Mutz* SUMMARY Ground or aerial applications of 20 percent active ingredient tebuthiuron pellets at 1.1 or 2.2 kilograms per hectare (kg/ha) dur- ing the winter controlled white- brush on the South Texas Plains more effectively than either the 5 or 1O percent formulations of pic- loram pellets. Whitebrush control varied little whether the tebuthiuron pellets were applied in the fall, winter or spring. Tebuthiuron applied at 0.3 or 0.6 whitebrush. Applications of pic- loram pellets (5 or 1O percent ac- tive ingredient) at 1 to 1.3 kg/ha in the summer did not control whitebrush. Shredding white- brush immediately prior to appli- cation of tebuthiuron or picloram pellets usually did not improve whitebrush control, but shredding did improve accessibility to the rangeland by eliminating standing woody debris which typically per- sists for several years following herbicide application. kg/ha did satisfactorily control Whitebrush (Aloysia lycioides Cham.), also called ”beebrush” and ”vara dulce,” is a serious range management problem on the South Texas Plains and Ed- wards Plateau vegetation regions of Texas. It also occurs in New Mexico, Arizona, Mexico, and South America. This difficult-to- control member of the Verbenaceae family occurs on approximately 2.4 million hectares of South Texas rangeland of which almost 250 thousand hectares support infesta- tions of 2O percent or greater (canopy cover (Smith and Rechen- thin 1964). Whitebrush may occur in mixed stands with more than 15 other species of woody plants or in almost pure, dense-thickets of in- dividuals 2 metersi (m) tall. Moder- ate to dense infestations of white- b- 48h seriously decrease range forage production and efficiency itilization, and infestation oc- INTRODUCTION curs in greatest densities on sites having relatively high production potential (Scifres 1980). Although its flowers are sought out by bees and other insects, whitebrush is of little value as browse for cattle or whitetailed deer (Odocoileus vir- ginianus). Whitebrush will defoliate almost completely when soil water is in- adequate, but rapid refoliation ac- companied by profuse flowering may readily occur after significant rainfall any time during the grow- ing season (March-November) (Meyer and Bovey 1980). White- brush inflorescences are dense, many-flowered spikes or racemes having white or violet-tinged corollas (Correll and Johnston 1970). The fruit is a small, dry schizocarp composed of two small berries, and the seeds are without endosperm. The leaves are entire and generally narrow but width is somewhat variable. Stems and branches are greyish-white, brit- tle; and the underlying wood is yellowish. j Herbicides commonly used for range improvement, such as 2,4-D ([2,4-dichlorophenoxy]acetic acid), 2,4,5-T ([2,4,5-trichlorophen- oxy]acetic acid), and dicamba (3,5- dichloro-0-anisic acid) at applica- tion rates of 1.1 kilograms per hec- tare (kg/ha) generally do not effec- tively control whitebrush (McCul- ly 1955; Bovey et al. 1965; Meyer et al. 1969). For several years, a sug- gested treatment for whitebrush control was 1.4 kg of MCPA ([(4- chloro-0-tolyl)-oxy] acetic acid) amine salt in 9.4 liters of diesel oil and enough water to make 75 li- ters per hectare (L/ha) of spray sol- ution. This treatment is most effec- tive when whitebrush is in full bloom (Hoffman and Ragsdale 1967). Although MCPA effectively ‘Respectively, professor and research assistant, Texas Agricultural Experiment Station (Department of Range Science); and assistant research scientist, Texas Agricultural Experiment Station, Corpus Christi. kills whitebrush topgrowth, it kills the roots 0f very few plants. Root kill from MCPA at rates between 1.1 and 9.0 kg/ha generally av- eraged less than 25% (Meyer 1966). If the whitebrush roots are not killed, the canopy is rapidly replaced by resprouting from basal stem buds. Picloram (4-amino-3,5,6-tri- chloropicolinic acid) is one of the few herbicides evaluated for rangeland use which has effective- ly controlled whitebrush. Picloram sprays applied at 1.1 kg/ha in the spring killed 8O percent of the whitebrush plants (Meyer et al. 1969). Higher picloram rates killed more whitebrush plants but also injured some of the desirable grasses. Aerial sprays of picloram applied in the fall at 3.4 or 4.5 kg/ha killed more than 9O percent of the whitebrush (Gibson and Grumbles 1970). Picloram applica- tions in the spring were usually less effective than those i.n the fall. Herbicide pellets or graunules have also shown promise for whitebrush control (Meyer and Riley 1969; Scifres et al. 1979). Ap- plications of dry formulations have several advantages com- pared to herbicide sprays. Drift potential is greatly reduced if dust associated with dry formulations is minimized. In addition, sprays must normally be applied during short periods in the spring or fall when the brush foliage is fully de- veloped and the woody plants are actively growing. In contrast, pel- lets or granules may be applied over a much broader time span (Scifres 1980). Generally, 3.4 or 4.5 kg/ha of pelleted picloram have been necessary to control whitebrush effectively. Meyer (1966) found that pelleted picloram was almost as effective as equal rates of sprays when applied during October, Februaryfior April. Picloram granules applied at 4.5 kg/ha killed 8O percent or more of the white- brush plants in another study. Ap- plications of picloram granules were most effective when applied during cooler months and fol- lowed shortly thereafter by enough rainfall to move the her- 2 bicide into the soil (Meyer and Riley 1969). Tebuthiuron (N—[5-(1,1—di- methylethyl)-1,3,5-thiadiazol-2- yl]-N,N'-dimethylurea) is a pro- mising new herbicide for range improvement (Bovey et al. 1975; Scifres 1979). Application rates of 1.1 to 6.7 kg/ha have selectively controlled a variety of woody plant species on rangeland. Appli- cations of tebuthiuron pellets in the spring at 1.1 kg/ha effectively controlled whitebrush, spiny hackberry,1 and Berlandier wolf- berry (McNeill et al. 1977; Scifres et al. 1979; Meyer and Bovey 1980). At 2.2 kg/ha, tebuthiuron pellets applied in the spring effec- tively controlled post oak, black- jack oak, winged elm, yaupon, water oak, woollybucket bumelia, guajillo, downy hawthorn, Texas colubrina, blackbrush acacia, and willow baccharis (Bovey et al. 1975; Meyer et al. 1978; Mutz et al. 1979; Scifres et al. 1979, 1981). Lotebrush, guayacan, cenizo, twisted acacia and javelinabush are controlled by 2.2 to 3.4 kg/ha of the herbicide. The highest rate evaluated by Scifres et al. (1979), 4.5 kg/ha, controlled huisache, but was only partially effective against honey mesquite. Lime pricklyash, Texas persimmon, pricklypear, eastern redcedar, saw greenbrier, American beautyberry, pepper- vine, and tasajillo tolerated 4.5 kg/ha of tebuthiuron (Scifres et al. 1979, 1981). Macartney rose is ap- parently fairly tolerant of tebu- thiuron since 2.2 to 4.5 kg/ha killed only 10 to 3O percent of the plants (Meyer and Bovey 1979). Shredding reduces the overall stature and stem density of some woody plant species but generally results in a net increase in stem density of plants that resprout from basal buds (Dodd and Holtz 1972; Scifres 1980). However, shredding followed by herbicide applications has shown promise for control of some problem range species. For example, 1.1 kg/ha of picloram pellets applied im- lScientific names of plants mentioned in text are given in the appendix. mediately after shredding effec- tively controlled spiny aster (Mill; et al. 1979). Neither shredding pellet applications effectively con- ‘ trolled undisturbed stands. Shred- ding mixed brush communities prior to pelleted picloram applica- tions did not improve control com- pared to use of picloram pellets only (Kitchen et al. 1980). The objectives of this study were to (1) determine the influ- ence of date of application of tebuthiuron application on white- brush control and (2) evaluate the interrelationship of top removal I \ shredding with control from tebuthiuron and picloram pellets. MATERIALS AND METHODS Description of the Study Area and Sites This research was conducted on the 74 Ranch located 80 kilometers (km) south of San Antonio on the South Texas Plains. Annual pre- cipitation ranges from 52 to 115 centimeters (cm) with wettest periods generally occurring in May, June, and September. Ex- tremely high summer tempera- tures and mild winters are com- mon (Gould 1975). Chemical and physical analyses of soils from the immediate study areas were conducted on quadrup- licate samples recovered at 15—cm increments to 60-cm deep and from 60 to 90-cm deep. Soil anal- yses included texture by the hy- drometer method (Milford 1975), organic matter by acid digestion and titration (Mortensen 1965), and pH of a 1:1 soil-water slurry (Peech 1965). The 1975 experiment was estab- . lished on a Claypan Prairie range site typified by Laparita loam. Laparita soils occur on nearly level to gently sloping uplands usually in close proximity to small drainageways. Slopes are oft I less than 1 percent but may range to 3 percent. The solum thickr I ranges from 100 to 159 cm. On the immediate study area, soils of the Claypan Prairie range site were sandy clay loams over- Laving a clay 10am subsoil which dually increased in clay content with increasing depth (Table 1). A claypan was prominent at the 15 or 30 cm depth, where the clay content increased from 36 to 54 percent. Surface organic matter content was approximately 2.6 percent and gradually decreased with depth. Claypan Prairie soils were acidic or slightly acidic. Experiments established in 1976 or later were located on a gently sloping, upland, Tight Sandy ' Qam range site characterized by fine-loams of the Imogene series (Typic Natrustalfs). These deep, moderately ‘well drained, slowly permeable soils have a solum thickness of 55 to 9O cm. Soils of the immediate study area con- tained considerably more sand than those of the Claypan Prairie site (Table 1). Organic matter con- tent varied from 2.1 percent to 1.4 percent, decreasing with depth, and pH was neutral or near neu- tral. Moderate to dense stands of whitebrush dominated the sites, with scattered honey mesquite, Texas colubrina, Texas persim- mon, spiny hackberry, agarito, guayacan, guajillo, blackbrush acacia, and twisted acacia compris- ing the remainder of the woody vegetation. Major grasses were buffalograss, common curly mes- quite, plains bristlegrass, hooded windmillgrass, Texas wintergrass, and several threeawns. Forbs in- clude frostweed, western and common ragweeds, yellow thistle, and common lantana as well as members of the Acanthaceae, Euphorbiaceae, Malvaceae, and Solanaceae families. The study area was fenced to prevent live- stock grazing, and rain gauges were installed prior to herbicide application. Herbicide Applications Picloram (5 to 10 percent active ingredient [a.i.]) or tebuthiuron (20 percent a.i.) pellets, depending. on the specific experiment, were applied by hand, with a tractor- mounted spreader or with fixed- wing aircraft. Herbicide was hand- applied with a crank-type spread- er normally used for dispensing fertilizer or seeds. Herbicide was applied to some experiments, usu- ally in 6.2—m-wide swaths, with a fertilizer spreader on a small trac- tor. Where liquid formulations were included for comparison to pellets, sprays were applied in 82 L/ha of water with a tractor- mounted sprayer equipped with a .6.2-m-wide boom. Tebuthiuron was aerially applied with a fertiliz- er spreader-seeder attachment in swaths 11 m wide in one experi- ment. On ]uly 14, 1975, picloram pel- lets (5 percent a.i.) were applied at 0.7, 1, or 2 kg/ha with the tractor- mounted spreader to plots 15 >< 45 m in size, arranged in a ran- domized complete block design with six replications. The white- brush stand had been shredded 18 months previously, and the re- growth was about 1 m tall on the uplands and 1.5 to 2 m tall on the TABLE 1. GENERALIZED SOIL CHARACTERISTICS OF RANGE SITES UTILIZED FOR EVAL- UATION OF PICLORAM AND TEBUTHIURON PELLETS FOR WHITEBRUSH CONTROL ON THE SOUTH TEXAS PLAINS NEAR CAMPBELLTON Depth Textural components (%) Organic matter pH (cm) (%) (1 :1) Sand Silt Clay 0-15 2.6 I 5.5 41 23 36 15-30 2.2 6.0 25 21 54 ' 15 2.1 . 6.5 20 20 55 .~ Tight Sandy Loam .5 2.1 7.0 71 15 14 15-30 I 1.6 6.9 63 15 22 30-45 1.4 7.1 51 13 36 lowlands. At the time of herbicide application, about 75 percent of the whitebrush foliage was fully developed, and floral structures were 25 percent formed. Light rains for two days previously had wet the surface cm of soil. On ]uly 15, 1975, the tractor-mounted spreader was used to apply the picloram pellets to stands shred- ded 30 days previously on which the regrowth averaged 15 cm. Pic- loram pellets were applied at 0.5, 1 or 2 kg/ha to each of six, 15 >< 45 m plots arranged as randomized complete blocks. On June 9, 1976, the tractor ap- plicator was used in an experiment to evaluate herbicides for control of mature and recently shredded whitebrush. Treatments included picloram pellets, 5 or 10 percent a.i., applied at 1 or 1.3 kg/ha, re- spectively, to undisturbed white- brush, and at 1.1 kg/ha to shredded whitebrush. Sprays of the potassium salt of picloram were applied at 1.1 kg/ha. Tebuthiuron pellets (20 percent a.i.) were applied at 0.7 and 1.7 kg/ha to undisturbed whitebrush, and at 0.9 and 1.7 kg/ha to shredded whitebrush. Each treatment was replicated three times using plots 24 >< 46 m in size, in a whitebrush stand of plants 1-2 m tall which represent- ed mature growth (last shredded in October 1973), and in a stand shredded 30 days prior to the her- bicide application with regrowth 2-15 cm tall. The experiment was designed as a randomized com- plete block design arranged in split plots with shredding treat- ment as the main plot effect and herbicide rate as the subplot effect. On May 25, 1977, tebuthiuron pellets (20 percent a.i.) were aeri- ally applied at 1.5 or 2 kg/ha in swaths 11 m wide to plots 100 m wide and 275 m long. Treatments were duplicated in a randomized complete block design. Tebuthiuron and picloram pel- lets were applied to undisturbed whitebrush stands and to areas which were shredded immediately prior to herbicide application at various dates in 1976 and 1977. Pelleted tebuthiuron (20 percent 3 '7‘ “£1 a.i.) was applied broadcast with a hand-operated spreader to shredded and undisturbed white- brush stands in the fall (October 12, 1976), winter (February 25, 1977), or spring (Iune 1, 1977). Ex- perimental design was a ran- domized complete block with three replications and arranged as a split-split plot. Date of treatment constituted main plot effects, and shredding treatment contributed subplot variation. Sub-subplots, 15 >< 23 m, were treated with 0, 0.3, 0.6, 1.1, or 2.2 kg/ha of tebuthiuron. Picloram pellets, 5 and 10 percent a.i., were applied at 1.1 and 2.2 kg/ha during the winter of 1977 to shredded and to unshredded whitebrush im- mediately adjacent to the experi- ment with tebuthiuron. Main plots were shredded on February 25, 1977, and herbicide was applied the same day to 15 >< 30-m sub- plots using the hand-operated broadcast spreader. Because pic- loram was applied only during the winter, data were analyzed as a separate experiment designed as a randomized complete block hav- ing three replications and ar- ranged in a split plot. Shredding constituted main plot effects while subplot variation was contributed by picloram rate or formulation. Whitebrush Response to Herbicide Treatments Whitebrush response was eval- uated periodically following treat- ment, generally at 6- to 12-month intervals, in all experiments treated with ground equipment except those installed at the vari- ous seasons from the fall 1976 to spring 1977. Ocular estimates of canopy reduction, usually by three workers, were averaged for pre- sentation. At the same times, live whitebrush stems were counted in six to ten 1 m2 plots, located on a line diagonally across each plot. In some cases, height of whitebrush regrowth stems was measured in each quadrat. Line transects were established diagonally across each experimen- tal unit (subplot of picloram- treated areas or sub-subplot on 4 tebuthiuron-treated areas) in the experiment evaluating date of her- bicide application. Percentage whitebrush foliar cover and aver- age height of canopy were recorded prior to the installation of each treatment and again in Octo- ber 1977 and May 1978. During these evaluations, six 1 m2 quad- rats were also sampled at regular intervals to determine densities of live whitebrush stems. Numbers of new growth and original live stems were recorded separately. Foliar cover and live stem density are reported as percentages of pre- treatment values. On September 5, 1980, canopy cover in each plot was estimated by three workers. Canopy reduction of white- brush in plots aerially treated with tebuthiuron was visually es- timated at 6, 16 and 38 months after herbicide application. Per- centage of whitebrush plants ap- parently killed (plants complemhf defoliated, branches dead, nc growth) were recorded in but 1 transects, 2-m wide, down the center of each plot. RESULTS AND DISCUSSION Picloram pellets applied in ]uly 1975 did not control undisturbed whitebrush (Table 2, see footnote). Although only about 9 cm of rain- fall occurred on the study site in June, more than 23 cm were re- ceived in May (Table 3). That rain- fall combined with about 1 cm I tal precipitation the week before herbicide application had wet the surface thoroughly, and the whitebrush was fully foliated. At 11 months after application, the pelleted picloram applied at 0.5 or 1 kg/ha was only slightly more ef- fective when applied to- stands TABLE 2. PERCENTAGE CANOPY REDUCTION AND APPARENT STEM KILL AT VARIOUS TIMES AFTER APPLICATION OF PICLORAM PELLETS WITH GROUND EQUIPMENT IN JULY 1975 TO WHITEBRUSH SHREDDED 30 DAYS PREVIOUSLY ON THE SOUTH TEXAS PLAINS NEAR CAMPBELLTONI 4 Months after treatment 29 44 Picloram 11 rate Canopy Dead Canopy Dead Canopy Dead (kg/ha) reduction stems reduction stems reduction stems 0 0 a 0 a 0 a 0 a 0 a 0 a 0.5 20 c 5 ab 0 a 0 a 0 a 0 a 1.0 20 c 5 ab 0 a 0 a 0 a 0 a 2.0 8O e 31 d 15 bc 6 ab 10 b 5 ab ‘Means followed by the same letter are not significantly different at the 95% level according to Duncan's multiple range test. Mean canopy reduction of undisturbed whitebrush treated at the same time with 0, 0.7, 1 or 2 kg/ha of picloram pellets was 0 a, regardless of time after treatment. TABLE 3. MONTHLY RAINFALL ON THE STUDY AREA DURING THE PERIOD IN WHICH PELLETED HERBICIDES WERE BEING EVALUATED FOR WHITEBRUSH CONTROL ON THE SOUTH TEXAS PLAINS NEAR CAMPBELLTON Rainfall (cm) by year Month 1975 1976 1977 t 1978 1979 1980 January 0.1 1.5 6.0 1.8 5.5 1.1 February 4.1 0 2.0 0.8 3.0 2.0 March 0.9 0.9 2.3 8 3.0 0.7 April 8.2 I 8.5 15.5 1.8 4.4 0.1 May 23.1 15.3 5.7 7.5 4.9 June 8.8 9.1 1.4 9.1 10.2 July 5.6 11.1 5.5 8.2 4.9 August 10.1 6.3 0 5.5 1.9 September 5.4 7.7 0 4.7 3.3 October 10.2 20.6 10.0 1.8 0.2 November T‘ 7.9 4.5 2.2 T December _2.7 9.9 1.0 2.3 2.5 Annual total 79.2 I 98.9 53.9 45.7 43.8 ‘T=trace, less than 0.05 cm. shredded 30 days earlier than when applied to undisturbed nds (Table 2). From the time of herbicide application until the first evaluation almost a year later, about 69 cm of rainfall occurred, most during late summer to early fall after treatment and the follow- ing April and May (Table 3). This amount of rainfall was considered adequate to leach the herbicide in- to the root zone of the whitebrush. However, canopy reduction on shredded areas was only 20 per- cent where 0.5 or 1 kg/ha of pic- Tram had been applied (Table 2). Canopy reduction was 8O percent 11 months after shredded white- brush stands were treated with 2 kg/ha of the picloram, and 31 per- cent ofthe stems appeared dead. By 29 months after application of the picloram pellets at 0.5 or 1 kg/ha, the whitebrush had com- pletely recovered on the shredded areas, and the canopies were re- duced by only 15 percent where 2 kg/ha were applied. Whitebrush control at 44 months after applica- tion of the pellets was no different from that after 29 months. Applications of picloram pellets in the winter were generally more effective than those applied in the summer. At 8 months after appli- cation of the 5 or 1O percent pic- loram pellets at 1.1 kg/ha (a.i.) to undisturbed stands in February 1977, whitebrush canopies were reduced by 85 to 87 percent (Table 4). Shredding did not significantly increase the defoliation of white- brush by the picloram pellets, and defoliation from 2.2 kg/ha of the herbicide was only slightly greater where the lower rate was applied, regardless of shredding treatment. Canopy reduction percentages at 14 months after application of the picloram pellets differed little from those at 8 months after treatment of the whitebrush. Moreover, there was little difference attribut- able to the percentage active ingre- dient of pellets applied or to shredding treatment. By 42 months after application of the picloram pellets in winter of 1977, effects of the shredding treatment were not discernible. There was a tendency for the herbicide applica- tions to be less effective on the plots which had been shredded prior to picloram application. Al- though the 5 percent formulation more effectively reduced white- brush canopy where 1.1 kg/ha of the herbicide was applied, there was no difference in response to 2.2 kg/ha. By 14 months after application of the picloram pellets in the win- ter of 1977, there tended to be few- er stems killed by the herbicide in shredded than in unshredded areas (Table 4). A greater percent- age of stems were killed by the higher rate than by 1.1 kg/ha but there was little difference in stem kill between formulations within an application rate applied to un- disturbed stands. Foliar cover of the untreated whitebrush stand did not change appreciably from the time of tebuthiuron application to evalua- tions of plots in May 1978 (Table 5). Applications of 0.3 kg/ha of tebuthiuron in the fall or winter did not significantly decrease the whitebrush foliar cover, but appli- cation in the spring reduced the canopies by 40 percent. New sprouts on undisturbed plantsin- creased, on the average, by 18 per- cent from initiation of the study to evaluations almost 19 months later (Table 6). However, the number of live whitebrush stems was signifi- cantly reduced by the lower rate of tebuthiuron only after application in the winter 1977. Based on these data, 0.3 kg/ha of tebuthiuron was considered ineffective for white- brush control. TABLE 4. PERCENTAGE REDUCTION IN FOLIAR COVER AND LIVE STEM DENSITIES OF WHITEBRUSH 8, 14 AND 42 MONTHS AFTER APPLICATION OF PELLETED PICLORAM TO UNDISTURBED STANDS AND IMMEDIATELY FOLLOWING SHREDDING IN THE WINTER 1977 ON THE SOUTH TEXAS PLAINS NEAR CAMPBELLTON Months after treatment‘ 8 14 42 Rate Formulation . (kg/ha) (%) Undisturbed Shredded Undisturbed Shredded Undisturbed Shredded Foliar cover reduction 0 — 16 a 61 b 4 a 35 b 0 a 0 a 1.1 5 85 c 90 c 91 d 85 cd 57 bc 41 b 1.1 10 87c 90c 76c 87 cd 14a 19a 2.2 5 96 c 94 c 95 c 89 d 89 e 65 cd 2.2 10 94 c 94 c 90 d 85 cd 85 de 67 cd i. Reduction in live stern densities 0 " 0 0 a 0 a O a 0 a —2 — A 1.1 5 87 cd 65 b 67 e 2O b — — 1.1 10 88 cd 61 b 61 e 30 bc — — x 2.2 5 99 d 78 bc 83 f 55 de — — 2.2 10 98 d 89 cd 86 f 39 cd — — ‘Means within an attribute and time of evaluation are not significantly different at the 95% level according to Duncan's multiple range test. Values in parentheses represent actual increases relative to original densities. ZDensities not recorded. Vi“ ‘*3; Tebuthiuron applied at 0.6 kg/ha had significantly reduced the foliar cover 0f the unshredded whitebrush by May 1978, but there was no difference in defolition at- tributable t0 season of application _ (Table 5). The 0.6 kg/ha rate re- duced the live stem density by 29 to 54 percent, and fall applications tended to be more effective than treatments in the spring or sum- mer (Table 6). Scifres et al. (1979) indicated the possibility of 0.6 i kg/ha being an effective rate of tebuthiuron for whitebrush con- trol. However, in this study, at least 1.1 kg/ha of the herbicide was required to reduce the canopies of undisturbed whitebrush by 78 per- cent or more on these range sites, regardless of application date, by May 1978. Whitebrush control on un- shredded areas was no different within a season of application whether 1.1 or 2.2 kg/ha of tebuthiuron were applied (Tables 5 and 6). However, there was a trend for applications in the fall or winter to reduce live stem density and foliar canopy of whitebrush more effectively than when tebuthiuron was applied in the spring. From 34 to 52 percent of the whitebrush foliar cover on areas which were shredded but not treated with herbicides had been replaced by May 1978 (Table 5). In general, shredding followed im- mediately by application of 0.3 kg/ha of tebuthiuron tended to be . only slightly more effective for re- ducing the foliar cover of white- brush than was shredding alone. Based on canopy reduction from the same rate of herbicide applied to undisturbed whitebrush, the combined effects of tebuthiuron at 0.3 kg/ha with shredding were simply additive. There was a general tendency for 0.6 kg/ha of tebuthiuron and shredding to reduce the white- brush canopies more effectively than herbicide application alone (Table 5). However, there was no difference in stem kill whether 0.6 kg/ha of the herbicide was applied to undisturbed stands or to shredded areas, except when the 6 herbicide was applied in the fall (Table 6). There was no difference in foliar cover reduction of whitebrush from 1.1 or 2.2 kg/ha of tebuthiuron whether the herbicide was applied to shredded or undis- turbed stands, regardless of appli- cation date (Table 5). Based on evaluations in May 1978, percent- age of stem kill was increased when shredding was used as pre- treatment in the spring but not in the fall or winter (Table 6). Regrowth of unshredded white- brush stands treated with 0.6, or 2.2 kg/ha of tebuthiuron l not attained the height of un- treated stands by May 1978, re- gardless of season of herbicide ap- plication (Table 7). There was no significant difference in the height of untreated whitebrush and stands treated with 0.3 kg/ha of tebuthiuron. Stands of whitebrush which were shredded only were significantly shorter than undis- turbed stands, having replaced '\ " TABLE 5. PERCENTAGEREDUCTION IN FOLIAR COVER OF WHITEBRUSH IN MAY 1978 ON SHREDDED AND UNSHREDDED RANGELAND FOLLOWING FALL 1976, WINTER 1977 OR SPRING 1977 APPLICATIONS OF PELLETED TEBUTHIURON NEAR CAMPBELLTON, TEXAS - Season of treatment‘ Rate (kg/ha) Fall 1976 Winter 1977 Spring 1977 Unshredded w» 0 I 9 ab 0 a (+13) a 0.3 18 b 11 ab 41 C 0.6 59 c-f 53 cde 50 cd 1.1 95 i 81 f-i ’ 78 e-i 2.2 92 hi 90 ghi 7a e-i ' Shreddedz _ 0 I _ _ 68 d-h 48 cd 66 c-g 0.3 5s c-f 69 d-h s3 ghi 0.6 79 e-i 84 ghi 89 ghi 1.1 ' 96 i 89 ghi 89 ghi 2.2 98 i 96 i 96 i ‘Means followed by the same letter are not significantly different at the 95% level according to Duncan's multiple range test. ZShredded immediately prior to herbicide application. TABLE 6. PERCENTAGE REDUCTION OF WHITEBRUSH STEM DENSITY IN MAY 1978 ON SHREDDED AND UNSHREDDED AREAS FOLLOWING FALL 1976, WINTER 1977 OR SPRING 1977 APPLICATIONS OF PELLETED TEBUTHIURON NEAR CAMPBELLTON, TEXAS Season of treatment‘ Rate . (kg/ha) Fall 1976 Winter 1977 Spring 1977 Unshredded 0 g _ 0 a 0 a 0 a 0.3 0 a 12 abc 4 a 0.6 I 54 fgh 29 bcd 42 def 1.1 96 k 71 hij 35 cde 2.2 83 ijk 8O ijk 55 fgh Shreddedz 0 O a O a 34 cde 0.3 3 a 5 ab 3 a 0.6 - 3O cd 14 abc 42 def 1.1 71 hij- 61 ghi 71 hi] 2.2 97 k 82 ijk 92 jk ‘Means followed by the same letter areynot significantly different at the 95% level according to Duncan's multiple range test. ZShredded immediately prior to herbicide application. 7;. l“ from 27 t0 53 percent of the aver- . age height of untreated plants by 1y 1978. Within shredded wands, there was no difference in the height of whitebrush regrowth 0n areas treated with 0.3 or 0.6 kg/ha of tebuthiuron and those shredded only, regardless of date of herbicide application. Although regrowth height was variable, re- growth on shredded areas treated with 1.1 or 2.2 kg/ha of tebuthiuron generally was shorter than where stands were shredded _\ only, and shorter than regrowth where the herbicide was applied to undisturbed stands. In September 1980, approxi- mately 4 years after application of I the tebuthiuron pellets in 1976, whitebrush canopies on plots re- ceiving 0.6 kg/ha or more of the herbicide were reduced by more than 9O percent (Table 8). Canopy reduction of whitebrush treated in the spring of 1977 was the same as where the herbicide was applied in the fall of 1976 at 0.6, 1.1 or 2.2 IABLE 7. AVERAGE HEIGHT (CM) OF LIVE WHITEBRUSH IN MAY 1978 FOLLOWING APPLICATION OF TEBUTHIURON IN THE FALL 1976, WINTER 1977 OR SPRING 1977 TO SHREDDED AND UNSHREDDED STANDS NEAR CAMPBELLTON Season of treatment‘ Rate (kg/ha) Fall 1976 Winter 1977 Spring 1977 Unshredded 0 91 I 106 I 113 I 0.3 94 I 86 kl 97 I 0.6 56 f-i 60 g-j 73 jk 1.1 63 hij 52 e-i 46 d-h 2.2 - 64 ij 58 g-j 38 cde Shreddedz 0 40 def 56 f-i 31 a-d 0.3 48 d-i 49 d-i ' 24 a-c 0.6 44 d-g 33 b-e 24 a-c 1.1 21 abc 24 abc 17 ab 2.2 18 ab 15 a 14 a lMeans followed by the same letter are not significantly different at the 95% level according to Duncan's multiple range test. ZShredded immediately prior to herbicide application. TABLE 8. PERCENTAGE CANOPY REDUCTION OF WHITEBRUSH IN SEPTEMBER 1980 FOLLOWING APPLICATION OF TEBUTHIURON IN THE FALL 1976, WINTER 1977 OR SPRING 1977 TO SHREDDED AND UNSHREDDED STANDS NEAR CAMPBELLTON Season of treatment‘ Rate (kg/ha) Fall1976 Winter 1977 Spring 1977 Unshredded 0 0 a 0 a 0 a 0.3 16 ab 33 bc 39 c 0.6 92 de 39 c 91 de 1.1 96 de 84 d 92 de 2.2 96 de 73 d 98 e _ Shredded2 0 0 a 0 a 0 a 0.3 17 ab 36 bc 37 c 0.6 84 d 36 bc 91 de 1.1 96 de 94 de 89 de T 2.2 99 e 98 e 95 de means followed by the same letter are not significantly different at the 95% level according to Duncan's multiple range test. ZShredded immediately prior to herbicide application. kg/ha. Control from winter 1977 applications tended to be less than from fall or spring applications, except where 0.3 kg/ha of tebuthiuron was applied. The per- centage of canopy reductions of whitebrush was similar within an application rate and date whether originally applied to shredded or unshredded plots. Picloram sprays applied at 1.1 kg/ha on ]une 9, 1976 more effec- tively reduced the canopy of un- disturbed whitebrush than did picloram pellets applied at 1 or 1.3 kg/ha (Table 9). Based on canopy reduction at 18 months after appli- cation, picloram sprays were more effective than dry herbicide formu- lations for controlling undisturbed whitebrush. However, 28 months after treatment, tebuthiuron at 1.7 kg/ha had completely defoliated the whitebrush and had appar- ently killed more than 90 percent of the stems. In comparison, the-- whitebrush had recovered com- pletely where the picloram pellets were applied, and canopy reduc- tion was only 15 percent where the lower rate of tebuthiuron was ap- plied. As in the previous experi- ment, whitebrush control with less than 1 kg/ha of tebuthiuron was not considered satisfactory. Also, as reported by Mutz et al. (1979), tebuthiuron tends to act somewhat more slowly than pic- loram pellets or sprays and does not usually manifest potential brush control levels until the sec- ond or subsequent growing sea- sons following application. Whitebrush which had been shredded 30 days prior to chemical treatment was neither controlled by picloram sprays nor either for- mulation of the picloram pellets (Table 9). However, in contrast to results from previous experi- ments, whitebrush control from both rates of tebuthiuron were im- proved when applied to the shredded stands, compared to the undisturbed brush. The degree of improvement in control was most pronounced where 0.9 kg/ha of tebuthiuron was applied. Whitebrush control from aeri- ally applied tebuthiuron was simi- lar to results from ground applica- 7 tions of the herbicide. At 6 months after aerial application of 1.5 0r 2 kg/ha, canopy reduction averaged about 8O percent, but adequate time had not lapsed to allow stem mortality (Table 10). By 18 months _ after treatment, the whitebrush canopies were eliminated, and 8O to 92 percent of the original stems were showing no signs of life. At 40 months after aerial application of tebuthiuron, canopy reduction was greater than 95 percent, and “ the stems killed by the herbicide had deteriorated. CONCLUSIONS Tebuthiuron 20 percent (a.i.) pellets at 1 kg/ha control white- brush more effectively than pic- TABLE 9. PERCENTAGE CANOPY REDUCTION AND MORTALITY OF SHREDDED OR AS AGIM UNIVERSITY . a loram or than other concentrations of tebuthiuron.’ Season of tebuthiuron application was not as important as rate of application. Shredding immediately prior to tebuthiuron application does not affect the level of whitebrush con- trol substantially. Tebuthiuron acts more slowly than picloram and usually requires at least one full growing season before max- imum defoliation of the brush oc- curs. g Although shredding as a pre- treatment does not significantly increase whitebrush control with herbicide pellets, it immediately reduces the physical obstructions caused by the brush. Since white- brush regrows rapidly following UNSHREDDED WHITEBRUSH PLANTS TREATED WITH PICLORAM OR TEBUTHIURON ON JUNE 9, 1976 ON THE SOUTH TEXAS PLAINS NEAR CAMPBELLTON‘ Months after treatment 18 28 y Rate Canopy Dead Canopy Dead Herbicide Formulation (kg/ha) reduction plants reduction plants Unshredded None — 0 0 a 0 a 0 a 0 a Picloram Liquid 1.1 95 fg 5O d 30 bc 6 abc Picloram 5% pellets 1.0 34 c 15 c 0 a 0 a Picloram 10% pellets 1.3 29 bc 7 abc 0 a 0 a Tebuthiuron 20% pellets 0.7 4O c 13 bc 15 ab 0 a Tebuthiuron 20% pellets 1.7 73 de 65 e 100 g 90f ShreddedZ None — 0 33 bc 0 a 0 a 0 a Picloram Liquid 1.1 0 a 0 a 0 a 0 a Picloram 5% pellets 1.1 0 a 0 a 0 a 0 a Picloram 10% pellets 1.1 4 a 3 ab 5 a 0 a Tebuthiuron 20% pellets 0.9 58 d 53 de 80 ef 50 d Tebuthiuron 20% pellets 1.7 93 f 86 f 95 fg 95 f ‘Means followed by the same letter within an evaluation criterion are not significantly different at the 95% level according to Duncan's multiple range test. ZShredded approximately 30 days prior to herbicide application. TABLE 10. PERCENTAGE CANOPY REDUCTION AND DEAD WHITEBRUSH PLANTS AT 6, 16, AND 40 MONTHS AFTER AERIAL APPLICATION OF TEBUTHIURON ON MAY 25, 1977 NEAR CAMPBELLTON Months after herbicide application‘ q _ 6 1s 40 Rate I‘ Canopy Dead Canopy Dead Canopy (kg/ha) reduction plants reduction plants reduction 0 0a 0a 0a 0a 0a 1.5 83 b O a 100 b 92 b 95 b 2.0. 78 b O a 100 b 8O b 99 b ‘Means within a column followed by the same letter are not significantly different at the 95% level according to Duncan's multiple range test. 8 shredding, application of tebuthiuron pellets prevents red - - velopment of the brush staa ‘ Thus, the combination of tre... ments has merit for immediately improving ease of management of whitebrush-infested rangeland followed by long-term rangeim- provement from the herbicide ap- plication. Application of picloram pellets containing 5 or 10 percent a.i. in the summer (June-July) at 1 to 1.3 kg/ha does not effectively control whitebrush on the South Texas Plains. Although rainfall followir ‘ treatment in the summer may be adequate to dissolve the herbicide and move it into the root zone of the whitebrush, effects of 1.3 kg/ha of the herbicide are usually not evident by the second growing season after application. Iuly ap- plications of 2 kg/ha of the.-1-0 per- cent picloram pellets to shredded whitebrush initially suppresses the brush stands, but the white- brush can recover by the third growing season after treatment. Applications of picloram pellets in the winter are more effective for whitebrush control than are sum- mer applications. There is no ad- vantage to shredding the white- brush prior to application of pic- loram pellets and no difference in whitebrush control between the 5 and 10 percent formulations, re- gardless of season of application. ,~ 1 AX‘ 1y, R. W., F. S. Davis, M. G. Merkle, .E. Meyer, H. L. Morton, and L. F. Bouse. 1965. Defoliation and control of brush. Proc. South. Weed Sci. Soc. 18:228-292. Bovey, R. W., R. E. Meyer, and I. R. Baur. 1975. Evaluation of tebuthiuron for woody plant control. Ann. Mtg. Weed Sci. Soc. Amer. Abst: 22. Correll, D. S., and M. C. Johnston. 1970. Manual of the Vascular Plants of Texas. Texas Res. Found., Renner, Texas. 1881 pp. Dodd, I. D., and S. T. Holtz. 1972. Integra- tion of burning with mechanical manipu- -ation of South Texas grassland. I. Range Manage. 25:130-135. Gibson, I. W., and I. B. Grumbles. 1970. Aerial application of herbicides for con- trol of whitebrush and associated species. Down to Earth 26 (2):1-4. Gould, F. W. 1975. Texas Plants — a check- list and ecological summary. Texas Agr. Exp. Sta. Misc. Pub. 585: 121 p. Hoffman, G. O., and B. I. Ragsdale. 1967. Chemical brush control. Texas Agr. Ext. Serv. L-415: 1 p. Kitchen, L. M., C. I. Scifres, and I. L. Mutz. 1980. Susceptibility of selected woody plants to pelleted picloram. I. Range Manage. 33:349-353. McCully, W. G. 1955. Chemical control of whitebrush. Proc. South. Weed Control Conf. 8:260—264. 7:“ '" k; LITERATURE CITED McNeill, K. E., I. C. Banks, R. S. Bjer- regaard, I. A. Keaton, and L. C. Warner. 1977. Tebuthiuron for rangeland brush control. Proc. South. Weed Sci. Soc. 30:229-230. Meyer, R. E. 1966. Control of whitebrush (Aloysia lycioides Cham.) with picloram. Ann. Mtg. Weed Sci. Soc. Amer. p. 24. Meyer, R.E., and R. W. Bovey. 1979. Con- trol of honey mesquite (Prosopis juliflora var. glandulosa) and Macartney rose (Rosa bracteata) with soil-applied herbicides. Weed Sci. 27:280-284. - Meyer, R. E. and R. W. Bovey. 1980. Con- trol of whitebrush (Aloysia lycioides) and associated species with soil-applied her- bicides. Weed Sci. 28:204-212. Meyer, R. E., and T. E. Riley. 1969. Influ- ence of picloram granules and sprays on whitebrush. Weed Sci. 17:293-295. Meyer, R. E., T. E. Riley, H. L. Morton, and M. G. Merkle. 1969. Control of whitebrush and associated species with herbicides in Texas. Texas Agr. Exp. Sta. MP-390: 18 p. Meyer, R. E., R. W. Bovey, and I. R. Baur. 1978. Control of an oak complex with herbicide granules. Weed Sci. 26:444-453. Milford, M. H. 1975. Introduction to soils and soil science. Laboratory exercises. (2nd Ed.) Kendall and Hunt Pub. Co., Dubuque, Iowa. p. 11-18. Mortensen, I. L. 1965. Partial extraction of organic matter. In C. A. Black (Ed) Meth- ods of soil analysis. (Part II). Amer. Soc. Agron., Madison, Wis. p. 1401-1407. Mutz, I. L., C. I. Scifres, W. C. Mohr, and D. L. Drawe. 1979. Control of willow bac- charis and spiny aster with pelleted her- bicides. Texas Agr. Exp. Sta. Bull. 1194: 12 p. Peech, M. 1965. Hydrogen-ion activity. In C. A. Black (Ed) Methods of soil analysis. (Part II). Amer. Soc. Agron., Madison, Wis. p. 920-921. Scifres, C. I. 1972. Sand shinnery oak re- sponse to dicamba granules and picloram pellets. I. Range Manage. 25:386-389. Scifres, C. I. 1980. Brush Management: Principles and Practices for Texas and the Southwest. Texas A&M Univ. Press, Col- lege Station, Texas. 360 pp. Scifres, C. I., and I. L. Mutz. 1978. Her- baceous vegetation changes following applications of tebuthiuron for brush control. I. Range Manage. 31:375-378. Scifres, C. I., I. L. Mutz, and W. T. Hamil- ton. 1979. Control of mixed brush with tebuthiuron. I. Range Manage. 32:155- 158. Scifres, C. I., I. W. Stuth, and R. W. Bovey. 1981. Control of oaks and associated woody species with tebuthiuron. Weed Sci. 29: (In press). Smith, H. N., C. A. Rechenthin. 1964. Grassland restoration: The Texas brush problem. Unnumbered Bull., U.S. Dep. Agr., Soil Consv. Serv. Temple, Texas. 17 p. Metric Units — English Equivalents Metric English Unit Equivalent Centimeter 0.394 inch Hectare 2.47 acres Kilogram 2.205 pounds Kilogram per hectare 0.983 pound per acre Kilometer 0.62 statute mile Kilometer per hour 0.62 mile per hour Liter 0.264 gallon Meter 3.28 feet Square meter (Degrees centigrade >< 1.8 + 32) 10.758 square feet Degrees fahrenheit T" ‘KL APPENDIX Scientific Names of Plants Mentioned in Text Common name American beautyberry Agarito Berlandier wolfberry Blackbrush acacia Blackjack oak Buffalograss Cenizo Common ragweed Common curlymesquite Common lantana Downy hawthorn Eastern redcedar Frostweed Guajillo Guayacan Honey mesquite Hooded windmillgrass Huisache Iavelinabrush Lotebush Lime pricklyash Macartney rose Peppervine Post oak Pricklypear Plains bristlegrass Saw greenbrier Spiny aster Spiny hackberry Tasajillo Texas colubrina Texas persimmon Texas wintergrass Threeawns Twisted acacia Water oak Western ragweed Willow baccharis Winged elm Woollybucket bumelia Yaupon p Yellow thistle Scientific name Callicarpa americana Berberis trifoliolata Lycium berlandieri Acacia ridigula Quercus marilandica Buchloe dactyloides Leuchophyllum frutescens Ambrosia artemisiifolia Hilaria berlangeri Lantana horrida Crataegas mollis [uniperas virginiana Verbesina microptera Acacia berlandieri Porlieria angastifolia Prosopis glandalosa var. glandulosa Chloris cacullata Acacia farnesiana Microrhamnus ericoides Ziziphus obtusifolia Zanthoxylum fagara Rosa bracteata Ampelopsis arborea Quercus stellata Opantia sp. Setaria macrostachya Smilax bona-nox Aster spinosus Celtis pallida Opuntia leptocaulis Colabrina texensis Diospyros texana Stipa leucotricha Aristida sp. Acacia tortaosa Qaercus nigra Ambrosia psilostachya Baccharis salicina LIlmus alata Bumelia lanaginosa Ilex vomitoria ‘Cirsium horridulum Mention of a trademark or a proprietary product does not constitute a guarantee or a warranty of the product by The Texas Agricultural Experiment Station and does not imply its approval to the exclusion of other products that also may be suitable. All programs and information of The Texas Agricultural Experiment Station are available to everyone without regard to race, ethnic origin, religion, sex, or age. 1.5M-—6-81