T C W ‘A 5-1422 November 1982 i e Davis Mountains UBRARY “nil! ElSlw The Texas Agricultural Experiment Station, Neville P. Clarke, Director, The Texas A&M University System, College Station, Texas CONTENTS SUMMARY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I INTRQDUCTIQN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I MATERIALS AND METHQDS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Description of The Study Area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Herbicide Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Response of Threadleaf Groundsel and Associated Vegetation to Herbicide Treatments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 RESULTS AND DISCUSSION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Summer Treatments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Fall Treatments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Winter Treatments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Spring Treatments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Herbicide ——— Season Interaction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Forage Response . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 CONCLUSIQNS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 LITERATURE CITED . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 ACKNOWLEDGMENTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 Appendix . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 Metric Units . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 KEYWORDS: Poisonous plants/range management/threadleaf groundsel/woolly groundsel/woolly senecio/ZAD/dicamba/ picloram/tebuthiuron/pelleted herbicides. Threadleaf Groundsel and Forage Response t0 Herbicides in the Davis Mountains R. D. Iones, D.N. Ueckert, I. T. Nelson, and]. R. Cox* SUMMARY Sprays of 2,4-D + picloram (4:1), 2,4,5-T + picloram (1:1), or picloram alone at 1.1 kilogram per hectare (kg/ha) in fall 0r winter effectively controlled threadleaf groundsel on Deep Upland and Igneous Hill and Moun- tain range sites in the Davis Mountains. Tebuthiuron applied as sprays of wettable powder in water or pellets at 1.1 kg/ha during summer reduced threadleaf ground- sel densities by 99 to 100 percent for almost 20 months after treatment, but applications at other seasons were not effective. Sprays of 2,4-D, dicamba, or 2,4-D + dicamba (3:1) at 1.1 kg/ha and pelleted picloram at 1.1 kg/ha did not effectively control threadleaf groundsel. Fall or winter applications of the foliar-active herbicides were generally more effective for threadleaf groundsel control than were spring or summer applications, pre- sumably because the plants were actively growing in the fall and winter. Blue grama and associated grasses were damaged by summer applications of most herbicides, but recovered during the subsequent growing season. INTRODUCTION Threadleaf groundsel (Senecio douglasii DC. var. lon- gilobus [Benth.] L. Benson),1 also referred to as “woolly groundsel” or “woolly senecio”, causes serious livestock losses throughout the western range states, especially in the Trans Pecos resource area of Texas (Clawson, 1933; Mathews, 1933; Norris, 1951; and Vardiman, 1952). The perennial subshrub occurs throughout the western half of Texas and extends north to Nebraska and Wyoming, west to Arizona and south to the 20th parallel in Mexico (Ediger, 1970; Sperry et al., 1964). The stems and leaves of threadleaf groundsel are covered with whitish, woolly hairs. The leaves may be pinnately lobed on the lower half of the plants but the lateral and terminal leaves are linear. The principal involucral bracts, which do not exceed 9 millimeters (mm) in length, usually number 21 ‘(Correll and Johnston, 1970). The plant is herbaceous when young but becomes woody with maturity. Yellow flowers are produced from May through November (Pat- raw, 1953). Populations of threadleaf groundsel typically occur on rocky slopes and grass-dominated mesa-s at elevations from 762 to 2,134 meters (m) (Rickett, 1969). The species increases in abundance following overgraz- ing and soil disturbance (Sperry et al., 1964). Cattle are most susceptible to threadleaf groundsel toxicity, but horses, sheep, and goats may also be affect- ed (Sperry et al., 1964). Longilobine, a pyrrolizidine alkaloid, is the poisonous principle in threadleaf ground- sel. Longilobine is more concentrated in young plants and leaves of older plants (Manske, 1931). Briske and Camp (1982) reported that total alkaloid concentration in threadleaf groundsel leaves, stems, and roots increased with increasing severity of water stress. Threadleaf groundsel may be consumed by livestock at any season, but is usually eaten in greatest quantities when snow covers desirable forage or during extended dry periods. Toxicity symptoms or livestock losses are usually greatest during late spring and summer months since there may be a time lapse of several months between consumption and first visible signs of intoxica- tion (Kingsbury, 1964; Sperry et al., 1964). Consump- tion of 1 to 5 percent of an animals body weight in a single feeding or over a period of several days will induce symptoms (Dollahite, 1972). Sperry et al. (1964) recom- mended supplemental feeding for reducing livestock losses to threadleaf groundsel poisoning. Dollahite (1972) reported that abundance of threadleaf groundsel may be reduced by grazing infested rangeland with goats or sheep. The present recommendation for controlling thread- leaf groundsel in West Texas is application of 2,4-D2 sprays at 1.1 kg acid equivalent (ae)/ha during the spring (Sperry et al., 1964). However, this practice has resulted in erratic control, and is usually applied too late to *Respectively, graduate research assistant, Range Animal Science Department, Sul Ross State University, Alpine; professor, Texas Agricultural Experiment Station, San Angelo; assistant professor, Range Animal Science Depart- ment, Sul Ross SftHtG University, Alpine; and range scientist, U.S. Department of Agriculture, Agricultural Research Service, Tucson,‘ Arizona. R. D. Jones’ current address is Box 158, Keams Canyon, Arizona 86034. sadleaf groundsel has long been referred to as Senecio longilobus Benth. until the taxonomic change by Barkley (1978). zChemical names of herbicides mentioned in text are given in. Table 2. prevent livestock toxicity problems. Norris (1951) re- ported 2,4-D to be more effective than 2,4,5-T for control of threadleaf groundsel. Application of low volatile esters of 2,4-D at 0.406 kg per liter (kg/l) of water during January or May completely controlled threadleaf groundsel, whereas applications in February when soils were dry did not effectively control the weed. This study was initiated in 1978 to evaluate several herbicides applied at different seasons for consistent and extended control of threadleaf groundsel in western Texas; and to determine the response of associated for- age species following control of threadleaf groundsel. MATERIALS AND METHODS Description of the Study Area The study was conducted on the Billy and Tommy Weston Ranch, 6.4 kilometers (km) southeast of Fort Davis, Texas in Ieff Davis County. Elevation of the study area is 1,524 m and average annual precipitation is 40.6 centimeters (cm). About two-thirds of the annual pre- cipitation falls from June through September. The ranch was grazed with cattle, sheep, and goats prior to 1971 and has been grazed yearlong with cattle stocked at about 1 animal unit (AU) per 10 ha since 1971. Experiments were installed on Deep Upland and Igneous Hill and Mountain range sites typified by soils within the Musquiz Association (Aridic Argiustolls) and very similar to the Brewster Association (Lithic Haplus- tolls). These soils were generally shallow and rocky throughout with about 80 percent of the surface covered with igneous gravel and 1 percent igneous cobbles. The A horizon is 8 to 10-cm deep, typically reddish brown in color, gravelly loam in texture, and moderate medium subangular blocky in structure. The B horizon averages 10 to 38 cm in depth and a B2 horizon may or may not be present. Igneous cobbles and gravel com- prise about 50 percent (volume basis) of the A and B horizons, and these are coated with caliche in the B horizon, which rests above masive caliche or igneous rock. Chemical and physical analyses of soils from the study area were conducted from six bulk samples taken from 0 to 15-cm and 15 to 30-cm deep. Soil analyses included texture by the hydrometer method (Day, 1965), organic matter by the Schollenherger method (Allison, 1965), and pH measured in 0.1 M CaClz (Peech, 1965). Soils of the study site were slightly acidic loams over- laying loam subsoil (Table 1). Clay contents averaged ‘n. and 24 percent in the 0 to 15-cm and 15 to 30-cm dep respectively. Soil organic matter content averaged 2. percent in the upper 15 cm and increased with depth to 2.73 percent (Table 1). The major plants on the study area were blue grama, black grama, hairy grama, hook threeawn, babywhite aster, and spiney goldenweed. The 4-lI1a study site was fenced to exclude livestock and pronghorn antelope throughout the study period. \ 3 Herbicide Applications Herbicides were applied during summer (August 9) and fall (November 15) of 1978 and during the winter (March 15) and spring (May 15) of 1979 to 10-m by 20-m plots arranged in a completely randomized design with three replications (Table 2). Herbicide liquids were ap- plied in 140 l/ha of a 1:14 (vzv) diesel fuel-water emulsion containing 0.1 percent (v/v) emulsifier (alkylaryl poly [ethyleneoxy] ethanol, ethylhexyl butanedioate, petro- leurn distillates) with a tractor-mounted, small-plot sprayer equipped with a 6-m boom. The wettable pow- der formulation of tebuthiuron was applied in 140 l/ha of water containing 0.1 percent (v/v) surfactant (alkylaryl- polyoxyethylene glycols, free fatty acids, isopropanol). Pelleted herbicides were applied with a hand spreader. Each herbicide treatment was applied at 1.1 kg/ha ai or ae in all experiments. Soil temperature, relative humidity, air temperature, wind speed, and direction and cloud cover were recorded during treatment applications. Soil water con- tents in the surface 15 cm were determined by the gravimetric method (Gardner, 1965) at the time of treat- ment from 25 randomly selected samples from the ex- perimental area. Precipitation was recorded after each occurrence during the study. Response of Threadleaf Groundsel and Associated Vegetation to Herbicide Treatments Densities of threadleaf groundsel plants were deter- mined before and at selected intervals after treatment by counting live plants within a permanently marked, 18.3- m by 1.2-m belt transect on a diagonal across each plot. Post-treatment densities were determined at 98, 218, 271, 365, and 583 days after treatment in the August 9, 1978 experiment; at 120, 172, 212, 365, and 485 days TABLE 1. GENERALIZED SOIL CHARACTERISTICS OF RANGE SITES UTILIZED FOR EVALUATION OF VARIOUS HERBICIDES FOR THREADLEAF CROUNDSEL CONTROL IN THE DAVIS MOUNTAINS, TEXASl Textu ral components (%) Depth Organic matter Textural (cm) (%) pH Sand Silt Clay Class 0-15 2.38 i 0.15 6.5 i 0.2 48 i 2 29 i 4 23 i 2 loam 15-30 2.73 i 0.57 6.6 i 0.03 45 i 3 31 i 3 25 i 0.04 loam ‘Values following means are the standard errors. v sScientific names of plants mentioned in text are given in the Appendix. 2 . i r r treatment in the November 15, 1978 experiment; at I l". 91, 213, and 365 days after treatment in the March ‘1979 experiment; and at 101, 222, and 312 days after i Ttment in the May 15, 1979 experiment. Herbicide :1‘ 'veness was based on percentage reduction in num- of live threadleaf groundsel plants in the belt tran- I . Standing crops of associated grasses were har- . ed in 1O randomly located, 30- by 30-cm quadrats in _~ h plot at the end of the 1978 and 1979 growing r ns. Herbage was separated into grasses and forbs, dried for 48 hours (hr) and weighed. Z alysis of variance was applied to standing herbage - a ~ Threadleaf groundsel density data were subjected Iganalyses of covariance, using pretreatment densities ' oovariate Duncan’s Multiple Range Test was ,1 to determine differences (P€0.05) among treat- t means where appropriate. RESULTS AND DISCUSSION ummer Treatments nditions were favorable for growth of threadleaf lg ndsel when herbicides were applied on August 9, . About 2.5 cm of rain were received within 6O days b to herbicide application and more than 2O cm fell in 6O days after treatments were applied (Table 3). I water content to 15 cm deep averaged 12.2 percent. threadleaf groundsel population was composed al- p entirely of mature plants. Relative humidity was 74 t, air temperature was 2O degrees centigrade (°C), I" "temperature at 2.5 cm deep was 17°C, and cloud was 95 percent at time of herbicide application. cloram applied at 1.1 kg/ha as sprays in August ced threadleaf groundsel density by 96 percent after ays compared to adjacent untreated rangeland e 4), but did not control the weed by 271 days after _ ment. Pelleted picloram reduced threadleaf H »= dsel densities 87 to 88 percent by 98 days after tion in August and by 97 to 99 percent 218 days TABLE 3. MONTHLY RAINFALL DURING THE PERIOD IN WHICH VARIOUS HERBICIDES WERE BEING EVALUATED FOR THREADLEAF GROUNDSEL CONTROL ON THE WESTON RANCH NEAR FORT DAVIS, TEXAS Rainfall by year (cm) 55-year Month 1978 1979 1980 average January - 0.5 0.6 1.9 February — 1.2 0.3 1.0 March — 0.5 0.3 0.9 April - 0.2 - 1.2 May - 0.7 - 3.0 lune - 1.4 — 6.6 Iuly — 4.5 — 6.9 August 8.9 9.9 - 6.4 September 11.4 0.2 — 5.7 October 5.2 0.3 - 3.4 November 1.2 0.3 — 1.3 December 1.0 1.2 - 1.3 Annual Total — 20.9 - 39.6 after application. Effectiveness of picloram pellets also decreased by 271 days after treatment (Table 4). At 98 days after application in August, pelleted tebuthiuron reduced the density of live threadleaf groundsel 85 to 9O percent compared to untreated rangeland. Tebuthiuron sprays reduced threadleaf groundsel by 39 percent after 98 days and by 93 percent at 218 days after treatment. Dicamba sprays reduced threadleaf groundsel densities by 72 percent after 98 days, but did not effectively control the weeds at 218 days after treatment. Sprays of 2,4-D reduced threadleaf groundsel densities by 43 per- cent after 98 days and by 6O percent at 218 days after treatment (Table 4). Tebuthiuron sprays or pellets reduced threadleaf groundsel densities by 87 to 1OO percent after 1 year, compared to untreated rangeland (Table 4). All other herbicides reduced threadleaf groundsel densities by 36 E2. HERBICIDAL TREATMENTS EVALUATED FOR THREADLEAF GROUNDSEL CONTROL on names(s) Chemical name(s) Ratesz Formulation(s)' (kg/ha ae or ai) (2,4-dichIorophenoxy)acetic acid dicamba(3:1) dichloro-Q-anisic acid I + picloram (4:1) 3,6-dichloro-Q-anisic acid iS-T + picloram :1) thiuron N-[5-(1,1-dimethyIethyl)-1,3,4- thiadiazol-Z-yll-NJNfldimethylurea (2,4-dichIorophenoxy)acetic acid + 3,6- (2,4-dichl0rophenoxy)acetic acid + 4-amino-3,5,6-trichloropicolinic acid 4-amino-3,5,6-trichloropicolinic acid (2,4,5-trichl0rophenoxy)acetic acid + 4-amino-3,5,6-trichloropicolinic acid 2-ethylhexyl ester 1.1 dimethylamine salts 1.1 triisopropanolamine salts 1.1 dimethylamine salt 1.1 potassium salt 1.1 10% ae pellet 1.1 5% ae pellet 1.1 triethylamine salt + 1.1 diethylamine salt 80% ai wettable powder 1.1 20% ai pellets (3.2-mm 1.1 diameter) 20% ai pellets (1.6-mm 1.1 diameter) llndicates acid equivalent; ai indicates active ingredient. of treatment for acid-based herbicides are expressed on ah acid equivalent (ae) basis; other herbicides are on an active ingredient (ai) basis. percent or less. Threadleaf groundsel control was signifi- cantly better 0n plots treated with pelleted tebuthiuron than 0n plots treated with 2,4-D. Tebuthiuron pellets and sprays reduced densities of live threadleaf groundsel by 99 t0 100 percent at 583 days after the summer treatments were applied (Table 4). Threadleaf groundsel control was significantly better on plots treated with tebuthiuron, regardless of formula- tion, than on plots treated with 2,4-D. Other herbicides reduced threadleaf groundsel 48 percent or less (Table 4). Fall Treatments Soil water content was almost 13 percent, cloud cover was 100 percent, relative humidity was 94 percent, air temperature was 4°C, and soil temperature was 11°C at time of herbicide application on November 15, 1978. Precipitation during the 60-day period prior to treat- ment was 5. 7 cm and only 1. 7 cm of rainfall was received within 6O days after treatment (Table 3). Both mature threadleaf groundsel plants and seedlings were present and appeared to be vigorously growing. Sprays of 2,4-D + picloram (4:1) had completely controlled threadleaf groundsel at 120 days after treat- ment whereas 2,4-D sprays had controlled less than half of the weeds (Table 5). Sprays of 2,4,5-T + picloram (1:1) or picloram had reduced the density of threadleaf groundsel by 73 and 70 percent, respectively. All other herbicides had reduced weed densities 58 percent or less (Table 5). Sprays of 2,4-D + picloram (4:1) completely con- trolled threadleaf groundsel at 1 year following applica- tion in November 1978. Sprays of 2,4,5-T + picloram (1:1) and 5 percent (ae) picloram pellets reduced den- sities of threadleaf groundsel 95 percent (Table 5), and sprays of picloram, 2,4-D, and 2,4-D + dicamba (3:1) reduced densities 94, 83, and 81 percent respectively. Control was 68 percent or less on plots treated with the other herbicides. Control with 2,4-D + picloram (4:1) was not significantly better than that achieved with 2,4- D (Table 5). The complete control obtained with sprays of 2,4-D + picloram (4:1) continued through 485 days after treat- ment (Table 5). Sprays of 2,4,5-T + picloram (1') , * picloram and 5 percent picloram pellets reduced wee densities by 86 to 92 percent compared to untreated rangeland. All other treatments reduced threadleaf groundsel densities by 78 percent or less. Control with 2,4-D + picloram (4:1) (100 percent) was not signifi- cantly better than that achieved with 2,4-D (Table 5). Poor coverage of the soil surface with “the 10 percent picloram pellets compared to the 5 percent pellets prob- ably accounted for the reduced effectiveness. Winter Treatments Precipitation during the 60-day period prior to her- bicide applications on March 15, 1979 was 1.8 cm, and only 0.5 cm of rainfall was received within 60 days after treatment (Table 3). There was 100 percent cloud cover, air temperature was 12°C, humidity was 64 percent, soil water content was 10 percent, and soil temperature at 2.5 cm deep was 13°C on the morning herbicides were applied. Threadleaf groundsel foliage was severely re- duced, apparently because of cold temperatures, but some new foliar growth was evident. Sprays of 2,4-D + picloram (4:1) had reduced thread- leaf groundsel density by 81 percent 60 days after winter application compared to untreated rangeland (Table 6). Other treatments reduced threadleaf groundsel den- sities by 67 percent or less. All treatments significantly reduced threadleaf groundsel densities, compared to 2,4-D, at 213 and 365 days after treatment (Table 6). Sprays of picloram, 2,4,5- T + picloram (1:1) and dicamba reduced live threadleaf groundsel densities by 100, 91, and 81 percent, respec- tively, at 213 days after application, and by 100, 92, and 87 percent, respectively, after 1 year. Tebuthiuron sprays and the 1.6-mm diameter pellets reduced thread- leaf groundsel densities by 91 percent at 1 year after application. The 3.2'-mm diameter pellets reduced den- sities only by 57 percent, probably because of poorer coverage. TABLE 4. PERCENTAGE REDUCTION IN THREADLEAF CROUNDSEL DENSITIES AT 98, 218, 271, 365, AND 583 DAYS FOLLOWING APPLICATION OF VARIOUS HERBICIDE TREATMENTS AT 1.1 KC/HA ON AUGUST 9, 1978 NEAR FORT DAVIS, TEXAS Days After Treatment Herbicides Formulation 98 218 271 365 583 None - 0 a1 0 a O bc 0 bcd 0 bc 2,4-D Liquid 43 ab 60 a —3 bc —22 abc —36 abc 2,4-D + dicamba (3:1) Liquid 56 b 15 a —63 ab —117 a —42 abc Dicamba Liquid 72 b 22 a —83 a —127 a —87 ab 2,4-D + picloram (4:1) Liquid 68 b 42 a -8 bc —111 a —127 a Picloram Liquid 96 b 68 a 12 c —25 ab —28 bc Picloram 5% pellets 87 b 99 a 46 cd 36 bcd 48 cd Picloram 10% pellets 88 b 97 a 50 cd 33 bcd 29 cd 2,4,5-T + picloram (1:1) Liquid 75 b 28 a 51 cd 1 bcd 27 cd Tebuthiuron Wettable powder 39 ab 93 a 92 d 87 cd 100 d Tebuthiuron 20% pelletsz 90 b 100 a 98 d 100 d 100 d Tebuthiuron 20% pellets3 85 b 70 a 100 d 93 d 99d ) ‘Means within a column followed by similar lower case letters are not significantly different at Ps0.05. 23.2-mm diameter. 31.6-mm diameter. 4 Spring Treatments ¢§\ There was no cloud cover, relative humidity was 67 percent, air temperature was 24°C, soil temperature was 26°C, and soil water content was less than 4 percent when herbicides were applied 0n May 15, 1979. Precipi- tation received during the 60-days prior to treatment was only 0.5 cm but 3.8 cm occurred during the 60-day period after treatment (Table 3). The threadleaf ground- sel population consisted of juvenile and mature plants in the vegetative growth stage. The foliage was fairly well- developed, but the plants did not appear to be growing. Sprays of 2,4,5-T + picloram (1:1) had reduced threadleaf groundsel density by 67 percent at 101 days after application compared to untreated rangeland (Table 7). Other treatments had reduced weed densities by 45 percent or less. Pelleted picloram reduced threadleaf groundsel den- sities by 84 to 100 percent at 222 days after application (Table 7). Sprays of 2,4-D controlled only a third of the weeds. Other herbicides controlled 7O percent or less of the threadleaf groundsel. Threadleaf groundsel was completely controlled at 312 days following spring application of 1O percent picloram pellets and densities were reduced by 75 to 82 percent on plots treated with 5 percent picloram pellets, sprays of picloram, or 2,4,5-T + picloram (1:1) (Table 7). The 3.2-mm diameter tebuthiuron pellets had reduced weed densities by 81 percent compared to untreated range- land. All other treatments reduced weed densities by 61 percent or less. Herbicide-Season Interaction Significant herbicide-season interactions were iden- tified in this experiment, based on percent reduction of threadleaf groundsel at approximately 3, 7, and 12 months subsequent to the four treatment dates. Essen- tially all herbicide sprays, except the wettable powder of tebuthiuron, more effectively controlled threadleaf groundsel 12 months after application in fall, winter, or spring than after summer application (Table 8). TABLE 5. PERCENTAGE REDUCTION IN THREADLEAF GROUNDSEL DENSITIES AT 120, 172, 212, 365, AND 485 DAYS FOLLOWING APPLICATION OF VARIOUS HERBICIDE TREATMENTS AT 1.1 KG/HA ON NOVEMBER 15, 1978 NEAR FORT DAVIS, TEXAS Days After Treatment Herbicides Formulation 120 172 212 365 485 None — 0 bc‘ 0 a 0 ab 0 a 0 a 2,4-D Liquid 43 bcd 68 bc 75 c 83 bc 78 b 2,4-D + dicamba (3:1) Liquid 58 bcd 69 c 63 bc 81 bc 75 b Dicamba Liquid 27 bcd 48 abc 76 c 57 abc 72 b 2,4-D + picloram (4:1) Liquid 100 d 100 c 100 c 100 C 100 b Picloram Liquid 70 cd 80 c 76 c 94 c 91 b Picloram 5% pellets -1 bc 64 bc 77 c 95 c 86 b Picloram 10% pellets 23 bcd 6 a 10 ab 26 ab 40 ab 2,4,5-T + picloram (1:1) Liquid 73 cd 82 c 78 c 95 c 92 b Tebuthiuron Wettable powder 29 bcd 47 abc 58 bc 56 abc 38 ab Tebuthiuron 20% pelletsz —81 a -6 a —48 a 52 abc 60 b Tebuthiuron 20% pellets3 ~13 ab 12 ab 40 bc 68 bc 75 b ‘Means within a column followed by similar lower case letters are not significantly different at Ps0.05. 23.2-mm diameter. 31.6-mm diameter. TABLE 6. PERCENTAGE REDUCTION IN THREADLEAF GROUNDSEL DENSITIES AT 60, 91, 213, AND 365 DAYS FOLLOWING APPLICA- TION OF VARIOUS HERBICIDE TREATMENTS AT 1.1 KG/HA ON MARCH 15, 1979 NEAR FORT DAVIS, TEXAS Days After Treatment Herbicides Formulation 60 91 213 365 None -- 0 a‘ 0 a 0 b 0 b 2,4-D Liquid -9 a —33 a —202 a —146 a 2,4-D + dicamba (3:1) Liquid 49 a 45 a 36 b 52 bc Dicamba Liquid 50 a 70 a 81 b 87 bc 2,4-D + picloram (4:1) Liquid 81 a 89 a 63 b 96 c Picloram Liquid 54 a 82 a 100 b 100 c Picloram 5% pellets 44 a 48 a 64 b 80 bc Picloram i 10% pellets —88 a —58 a 45 b 85- bc 2,4,5-T + picloram (1:1)I’ Liquid 67 a 85 a 91 b 92 bc Tebuthiuron Wettable powder 55 a 63 a 68 b 91 bc Tebuthiuron 20%‘ pelletsz —10 a —29 a 12 b 57 bc T-‘buthiuron 20% pellets3 —1 a 1 a 78 b 91 bc Means within a column followed by similar lower case letters are not significantly different at P<0.05. 23.2-mm diameter. 31.6-mm diameter. Tebuthiuron sprays applied in summer or winter tended t0 result in better control than fall or spring applications. However, sprays of 2,4-D applied in fall 0r spring were significantly more effective than those applied in sum- mer or winter, and there was a strong. trend toward increased effectiveness of fall treatments compared to spring treatments. There were no significant herbicide- season interactions with pelleted herbicides, and trends varied among formulations with both picloram and tebuthiuron (Table 8). Forage Response Blue grama and associated grasses, which were rapidly growing, were damaged by most herbicides applied in early August 1978 (Table 9). Sprays and pellets of tebuthiuron and picloram reduced standing crop of gras- ses by 3O to 42 percent at 3 months after application. Damage to grasses was intermediate on plots sprayed with 2,4,5-T + picloram (1:1) and least on plots sprayed with dicamba or 2,4-D. Associated grasses were not affected on plots sprayed with 2,4-D + dicamba (3:1) (Table 9). TABLE 7. Herbicides applied during fall, winter, or spring did not affect standing biomass of grasses compared to un treated rangeland, as determined by clipping within ' . ' fenced study area at the end of the 1979 growing season (Table 10). Also, grasses on the plots treated in August 1978 had recovered by the end of the 1979 growing season. Damage incurred on plots treated during sum- mer 1978 was apparently short-term since most of the plants had recovered by the second‘, autumn after her- bicide applications. CONCLUSIONS Sprays of 2,4-D, dicamba, or 2,4-D + dicamba (3:1) at 1.1 kg/ha did not effectively control threadleaf groundsel on Deep Upland and Igneous Hill and Mountain range sites in the Davis Mountains. Fall applications of 2,4-D tended to be more effective than applications at other seasons. However, almost 7 months lapsed before three- fourths of the threadleaf groundsel plants died following fall treatments. The relatively high toxicity of threadleaf PERCENTAGE REDUCTION IN THREADLEAF GROUNDSEL DENSITIES AT 101, 222, AND 312 DAYS FOLLOWING APPLICATION OF VARIOUS HERBICIDE TREATMENTS AT 1.1 KG/HA ON MAY 15, 1979 NEAR FORT DAVIS, TEXAS Days After Treatment Herbicides Formulation 101 222 312 None — 0 a‘ 0 ab 0 a 2,4-D Liquid 31 a 33 a-d 49 abc 2,4-D + dicamba (3:1) Liquid —6 a —31 a -2 a Dicamba Liquid 21 a 29 a-d 30 ab 2,4-D + picloram (4:1) Liquid 32 a 39 a-d 45 abc Picloram Liquid 25 a 65 bcd 82 bc Picloram 5% pellets 16 a 84 cd 79 bc Picloram 10% pellets 16 a 100 d 100 c 2,4,5-T + picloram (1:1) Liquid 67 a 57 bcd 75 bc Tebuthiuron Wettable powder 45 a 67 bcd 61 abc Tebuthiuron 20% pelletsz 19 a 70 bcd 81 bc Tebuthiuron 20% pellets?’ 27 a 22 abc 34 abc ‘Means within a column followed by similar lower case letters are not significantly different at P€0.05. 23.2-mm diameter. 316mm diameter. TABLE 8. MEAN PERCENT REDUCTION IN LIVE THREADLEAF GROUNDSEL NUMBERS AT APPROXIMATELY 12 MONTHS FOLLOWING SUMMER, FALL, WINTER, OR SPRING APPLICATIONS OF VARIOUS HERBICIDE TREATMENTS AT 1.1 KC/HA IN 1978-1979 NEAR FORT DAVIS, TEXAS 12 Months Herbicide Formulation Summer Fall Winter Spring 2,4-D Liquid ~22 a‘ a3 b -146 a 49 b 2,4-D + dicamba (3:1) Liquid —117 a 81 b 52 b -2 b Dicamba Liquid —127 a 57 b 87 b 30 b 2,4-D + picloram (4:1) Liquid —111 a 100 b 96 b 45 b Picloram Liquid —25 a 94 b 100 b 82 b Picloram 5% pellets 36 a 95 a 80 a 79 a Picloram 10% pellets 33 a 26 a 85 a 100 a 2,4,5-T + picloram (1:1) Liquid 1 a 95 b 92 b 75 b Tebuthiuron Wettable powder 87 a 56 a 91 a 61 a Tebuthiuron 20% pelletsz 100 a 52 a 57 a 81 a Tebuthiuron 20% pellets3 93 a 68 a 91 a 34 ‘Means within a row followed by similar lower case letters are not significantly different at P€0.05. 23.2-mm diameter. 31.6-mm diameter. 6 ‘t . 1:3 ‘it'- 4»! ..» I groundsel t0 livestock dictates that control be manifested flquickly and that a high proportion of the weeds be killed effectively reduce livestock poisoning. Sprays of 2,4-D + picloram (4:1) at 1.1 kg/ha during the fall completely controlled threadleaf groundsel for almost 16 months and winter applications effectively controlled the weeds for 1 year. However, applications in spring or summer were no more effective than 2,4-D alone at the same rate. Picloram at 1.1 kg/ha applied as sprays in the fall maintained a 91 percent reduction in threadleaf groundsel densities for almost 16 months and winter applications completely controlled the weeds for 1 year. Spring or summer applications of picloram sprays did not effectively control the weeds. Foliar sprays of 2,4,5-T + picloram (1:1) at 1.1 kg/ha applied in fall or TABLE 9. MEAN GRASS STANDING CROPS ON NOVEMBER 15, 1978 FOLLOWING APPLICATION OF VARIOUS HERBICIDE TREATMENTS AT 1.1 KG/HA ON AUGUST 9, 1978 NEAR FORT DAVIS, TEXAS Standing crop Herbicide Formulation (kg/ha) None —- 963 a1 2,4-D + dicamba (3:1) Liquid 887 ab Dicamba Liquid 806 b 2,4-D Liquid 776 b 2,4,5-T + picloram (1:1) Liquid 709 bcd Picloram 10% pellets 670 d-g Tebuthiuron 20% pelletsz 608 fg Tebuthiuron 20% pellets3 604 fg Picloram 5% pellets 587 g Picloram Liquid 573 g Tebuthiuron Wettable powder 563 g ‘Means within a column followed by the same letter are not significantly different at P<0.05. 23.2-mm diameter. 31.6-mm diameter; winter reduced threadleaf groundsel densities 92 per- cent for more than a year, whereas spring or summer applications did not provide satisfactory control. Tebuthiuron applied as sprays or pellets at 1.1 kg/ha during late summer reduced threadleaf groundsel den- sities 99 to 100 percent for 19 months, whereas applica- tions in fall, winter, or spring resulted in erratic control. Five percent picloram pellets were generally more effec- tive for threadleaf groundsel control than were 1O per- cent picloram pellets. However, summer applications of picloram pellets effectively controlled the weeds for only about 7 months, whereas more than 7 months lapsed following fall and spring applications, and 1 year lapsed following winter applications, before three-fourths of the weeds were killed. Fall or winter applications of foliar active herbicides were generally more effective for threadleaf groundsel control than spring or summer applications, presumably because seedlings were emerging and actively growing in the fall and winter. Low soil water contents usually severely limit plant growth during late winter, spring, and early summer in the Davis Mountain area and threadleaf groundsel populations during late summer consisted mostly of mature, flowering plants. The sus- ceptibility of most plants to foliar-active herbicides de- creases as the plants mature and flower and as growing conditions become less favorable. Blue grama and other grasses associated with thread- leaf groundsel incurred moderate-to-severe short-term damage following application of most herbicides during late summer, presumably because the plants were suc- culent and rapidly growing following summer rains. However, no damage was caused by herbicide applica- tions during fall, winter, or spring when growing condi- tions were less favorable and the associated species were mature or dormant. TABLE 10. MEAN GRASS STANDING CROPS (KG/HA) ON NOVEMBER 3, 1979 FOLLOWING GROUND APPLICATION OF VARIOUS HERBICIDE TREATMENTS AT 1.1 KG/HA ON FOUR DIFFERENT DATES IN 1978 NEAR FORT DAVIS, TEXAS Standing crop (kg/ha) Date Treated August Nov. March May Herbicide Formulation 1978 1978 1979 1979 None —- 925 a‘ 807 a 685 a 710 a 2,4-D Liquid 553 a 929 a 815 a 578 a Dicamba Liquid 452 a 528 a 678 a 535 a Picloram Liquid 574 a 642 a 850 a 477 a Tebuthiuron Wettable powder 431 a 603 a 578 a 664 a 2,4-D + dicamba (3:1) Liquid 718 a 628 a 693 a 703 a 2,4,5-T + picloram (1:1) Liquid 782 a 746 a 1166 a 513 a 2,4-D + picloram‘. (4:1) Liquid 675 a 761 a 850 a 685 a Picloram " 10% pellets 553 a 531 a 667 a 736 a Picloram 5% pellets 675 a 649 a 743 a 1051 a Tebuthiuron 20% pelletsz 617 a 1073 a 348 a 1051 a ,r“huthiuron 20% pellets3 682 a 589 a 502 a 556 a Means within a column followed by similar lower case letters are not significantly different at P<0.05. 23.2-mm diameter. 31.6-mm diameter. LITERATURE CITED Allison, L. E. 1965. Organic carbon. In C. A. Black (Ed) Methods of SoilAnalysis. (Part II). Amer. Soc. Agron., Madison, Wis. p. 1367- 1378. Barkley, T. M. 1978. North American Flora. SeriesII, Part 10. The New York Botanical Garden, Bronx, New York. p. 50-118. Briske, D. D. and B. I. Camp. 1982. Water stress increases alkaloid concentrations in threadleaf groundsel (Senecio longilobus). Weed Sci. 30: 106-108. Clawson, A. B. 1933. The American groundsel, species of Senecio as stock poisoning plants. Vet. Med. 28: 105-110. Correll, S. and M. C. Iohnston. 1970. Manual of the Vascular Plants of Texas. Texas Res. Found., Renner, Texas. 1881 pp. Day, P. R. 1965. Particle fractionation and particle size analysis. In C. A. Black (Ed) Methods of Soil Analysis. (Part I). Amer. Soc. Agron., Madison, Wis. p. 545-567. Dollahite, I. W. 1972. The use of sheep and goats to control Senecio poisoning in cattle. Southwest. Vet. 25: 223-226. Ediger, R. I. 1970. Revision of section Suflruticosi of the genus Senecio (Compositae). SIDA. 3: 504-524. Gardner, W. H. 1965. Water content. In C. A. Black (Ed) Methods of Soil Analysis. (Part I). Amer. Soc. Agron., Madison, Wis. p. 82-127. Kingsbury, I. M. 1964. Poisonous Plants of the United States and Canada. Prentice-Hall, Inc. Englewood Cliflls, New Iersey. 626 pp. Manske, R. H. F. 1931. The alkaloids of Senecio species. Can. I. R1 5. 651-659. Mathews, F. P. 1933. Poisoning of cattle by species of groundsel. Texas Agr. Exp. Sta. Bull. 500. 13 pp. Norris, I. I. 1951. The distribution and chemical control of species of ‘ Senecio, Astragalus, and Baileya in the highlands range of West Texas. Ph.D. Diss. Texas AézM University. College Station, Texas. 95 PP- 7i Patraw, P. M. 1953. Flowers of the southwestern mesas. Southwestern Monuments Assoc. Popular Series, No. 5., Gila Pueblo, Globe, Arizona. 112 pp. Peech, M. 1965. Hydrogen ion activity. In C. A. Black (Ed) Methods of Soil Analysis. (Part II). Amer. Soc. Agron., Madison, Wis. p. 914- 926. Rickett, H. W. 1969. Wildflowers of the United States. Vol. III. McGraw-Hill Book Co., San Francisco. 634 pp. Sperry, O. E., I. W. Dollahite, G. O. Hoffman, and B. I. Camp. 1964. Texas plants poisonous to livestock. Texas Agr. Exp. Sta. Bull. 1028. 57 pp. Vardiman, P. H. 1952. Poisonous plant conditions in Big Bend and surrounding areas of Texas. Southwest. Vet. 5: 423. ACKNOWLEDGMENTS The authors express appreciation to the Houston Livestock Show and Rodeo for partial financial support of this research, to Billy and Tommy Weston for providing land for the research, to Dr. Charles E. Gates for assistance with statistical analyses, to Brad Lisenbee for assistance with computer pro- gramming, to R. L. Potter, I. L. Petersen, and Rex Cochran for soil analyses, and to Mrs. Sonnie Olin for manuscript preparation and typing. /' APPENDIX Scientific Names of Plants and Animals Mentioned in Text Common Name Animals Pronghorn antelope Plants Babywhite aster Black grama Blue grama Hairy grama Hook threeawn Spiney goldenweed Scientific Name Antilocapra americana Leucelene ericoides Bouteloua eriopoda Bouteloua gracilis Bouteloua hirsuta Aristida hamulosa M achaeranthera pinnatifida Metric Units — English Equivalents Metric Unit Centimeter (cm) Hectare (ha) Kilogram (kg) Kilograms per hectare (kg/ha) Kilometer (km) Liter (l) Meter (m) Millimeter (mm) Square meter (m2) (Degrees centigrade X 1.8 + 32) English Equivalent 0.394 inches 2.47 acres 2.205 pounds 0.983 pound per acre 0.62 statute mile 0.264 gallon 3.28 feet 0.0394 inches 10.758 square feet Degrees Fahrenheit 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. ‘i All programs and information of The Texas Agricultural Experiment Station are available to everyone without regard to race, ethnic origin, religion, sex, or age. 1.2M—11-82