Control of Bitterweed with Herbicides

Abstract

Herbicide 24-D was less effective for bitterweed control when applied at air temperatures below 14” C compared to applications at 22” C even when soil water was adequate for active plant growth. There was no difference in bitterweed control whether the ester or amine formulations of 2,4-D were used. At 22” C air temperature, 1.12 kg/ha of 2,4-D •t dicamba (3:1), picloram, or 2,4,5-T ipicloram (1:l) did not improve short-term control (36 to 182 days) of bitterweed compared to 2,4-D alone at the same rate. However, at 14’ C temperature or when bitterweed were in advanced phenological states (75% with inflorescences), these herbicides provided excellent short-term bitterweed control whereas 2,4-D was inconsistent. Mixtures of dicamba with 24-D slightly improved residual bitterweed control, compared to the same rate of 2,4-D alone. Picloram at 0.56 to 1.12 kg/ha controlled 60 to 100% of the bitterweed populations for a year or more following applications in winter or spring. Tebuthiuron at 0.56 to 1.12 kg/ha was not as effective as 2,4-D at 1.12 kg/ha relative to initial bitterweed control, but provided excellent residual control after 1 year following winter application. Bitterweed (Hymenoxys odorata DC.) occurs throughout most of the southern Great Plains and Southwest (Clawson 1931), and is recognized as a major poisonous plant on the Edwards Plateau of Texas (Sperry 1949). Death losses of sheep attributed to bitterweed poisoning on the Edwards Plateau average 1 to 6% annually (Sultemeier 1961). Bitterweed is a major factor, along with predators and unavailability of labor, causing the 50% decline in sheep numbers over the past 30 years in West Texas. Boughton and Hardy (1937) reported that the acute LD50 of fresh seedling bitterweed in sheep averaged 1.3$%c of body weight, but may be as low as 0.5% of body weight during prolonged drought. Dollahite et al. (1973) reported acute LD50 values for air-dried bitterweed of 3.6 to 8.5 g/kg (0.36 to 0.85% of body weight). Hymenoxon, a sesquiterpene lactone, has been isolated as a toxic principle in bitterweed (Kim et al. 1975). Sperry (1949) stressed the importance of good grazing management for avoiding bitterweed poisoning. However, Authors are associate professor, professor, research associate, and assistant research scientist, Texas Agricultural Experiment Station at San Angelo, College Station, San Angelo, and Corpus Christi, respectively. Thts report is approved by the Director, Texas Agricultural Experiment Station as TA-14837. The authors wish to express their appreciation to J.R. Scifres for typing and manuscript preparation. Also, the authors wish to acknowledge George Sultemeier and Robert E. Steger, Texas Agricultural Extension Service, and numerous County Agricultural Extension agents for assistance in installation and evaluation of these experiments. Land for this research was provided by Reginald Atkinson, Bill Scott and Hal Noelke, Mertzon; Bill Pfluger, San Angelo; Tony Allen, Ozona; S.K. Horwood, Sterling City; and Jimmy Powell, Ft. McKavett, Texas. Manuscript received February 8, 1979. JOURNAL OF RANGE MANAGEMENT 33(6), November 1960 reduced cover of forage plants following periodic droughts and short-term overgrazing results in continual bitterweed poisoning on the Edwards Plateau. Also, many ranchers hesitate to utilize mechanical brush control practices since soil disturbance encourages bitterweed infestations. Broadcast application of 2,4-D ([2,4dichlorophenoxy] acetic acid) ester is effective for short-term control when soil water is favorable for bitterweed growth (Sperry and Sultemeier 1965). Bunting and Wright (1974) reported excellent control of bitterweed with an unidentified 2,4-D amine applied between November 1 and January 15, apparently regardless of plant phenology or environmental conditions. However, bitterweed control with 2,4-D has severe limitations, including: (1) limited control of seedlings emerging after treatment; and (2) unsatisfactory control of bitterweed when growing conditions are unfavorable or when bitterweed is in advanced phenological stages. Environmental conditions, as they regulate plant growth and physiology, strongly influence weed control with foliarapplied, translocated herbicides such as 2,4-D. Sperry and Sultemeier (1965) reported that 20 to 25% soil-water contents were critical for controlling 90% or more of the bitterweeds with 2,4-D on clay soils. However, the influence of air or soil temperatures on herbicidal control of bitterweed has