Honey Mesquite Control and Forage Response in Crane County, Texas

Replicated field plots of honey mesquite (Prosopis giizndulosa var. glandulosa) were aerially treated with herbicides in 1977 near Crane, Texas. Plots were evaluated for 3 years to determine efficacy of nine herbicide formulations. Of the herbicides studied 3,6-dichloropicolinic acid was the most effective. Sprayed plots produced twice as much forage as unsprayed areas with several species of grass showing significant increases in production. Forb response was not significantly different between treated and untreated plots. Most of the forage response occurred 1 m from the tree base rather than at 3 and 5 m from the tree. Herbicides remain an economical and efficient method for controlling large areas of mesquite. However, the controversies surrounding the use of 2,4,5-T [(2,4,5 trichlorophenoxy) acetic acid] have threatened its continued use. Two new herbicides, triclopyr {[(3,5,6-trichloro-2-pyridinyl)oxy]acetic acid ) and 3,6-DPA (3,6dichloropicolinic acid), have recently been evaluated for mesquite control (Jacoby et al. 1980, 1981). Aerial spraying of mesquite with 2,4,5-T and 1: 1 mixture of 2,4,5-T ipicloram (4-amino-3,5,6-trichloropicolinic acid) or dicamba (3,6-dichloro-o-anisic acid) has provided temporary suppression of mesquite as well as increased forage production (Scifres 1980). Degree of forage response following mesquite control has been related to: (1) mesquite size and density (Williams 1976); (2) rainfall following treatment (Cable 1976, Martin 1975); (3) degree of control (Dahl et al. 1978); and (4) the condition and composition of the understory herbaceous community (McDaniel et al. 1978, Scifres and Polk 1974). Generally, most dramatic forage responses following brush suppression have occurred in arid to semiarid areas where competition between brush and grass is critical. Studies conducted in Arizona by Cable (1976), Martin and Tschirley ( 196 I), and Parker and Martin ( 1952) have shown increased forage production from mesquite control over long periods of time following treatment. Specific competitive factors between mesquite and associated grasses have been delineated. Tiedemann and Klemmedson (1973) found that soils were more fertile under mesquite trees than in the open interspaces. Certain plants including Arizona cottontop (Digitaria californica), bristlegrass (Setaria texana), and bush muhly (Muhlenbergia porterii) were found to adapt to shaded conditions beneath mesquite canopies, while plants such as black grama (Bouteloua eriopoda) were shade intolerant (Tiedemann et al. 1971). Brock et al. (1978) determined that cool-season forage species found mainly in the canopy zone decrease following mesquite control in northcentral Texas. Cable (1977) defined zones of Authors are associate professor, research associate, and former research associate, Texas Agr. Exp. Sta. , Vernon 76384, and assistant to manager, Surface Lands, University of Texas Lands, Midland 79701. Manuscript received June 18, 1981. Technical article TA-16917 from the Texas Agric. Exp.Sta. Study was a cooperative investigation of Texas Agr. Exp. Sta. and University of Texas Lands. The authors express their appreciation to J.R. Bell, Garth Grizzle, J.B. Averitt,and Tomas Dominguez with the Soil Conservation Service, U.S. Dep. Agric. for their assistance in collecting the forage production data. We also wish to thank Billy Carr, Manager, Surface Leasing, University Lands for suggestingand supporting this study. Funds for this research were provided by the University of Texas Lands-Surface Leasing, Midland Texas. Herbicides were provided by Dow Chemical Co., U.S.A. , and Velsicol Chemical Co. Application of herbicides was by H & W Flying Service, Midland, Texas. 424 moisture use around mesquite plants with moisture depletion occurring rapidly near tree bases, decreasing with depth and distance from the tree. Thomas and Sosebee (1978) found that mesquite in western Texas relies on a system of shallow lateral roots when moisture is available in the upper soil profile and utilizes a deep tap root system during drought. The objectives of this study were to determine relative efficacies of registered and experimental herbicides for mesquite control and to estimate plant responses of associated forage species in sprayed and nonsprayed plots. Study Area and Methods Studies were conducted on rangeland 6 km northwest of Crane in western Texas. Climate is semiarid with an average of 33 cm annual rainfall occurring mainly in late spring and summer. Soils in the study area are sands and sandy loams in the Penwell (Ustic Torripsamments), Jalmar and Pyote (Ustalfic Haplargids) series (Watson 1978). Soils are mainly sandy loams in the southern end of the area becoming more sandy to the north. Native vegetation on the area is dominated by shrubs including honey mesquite, catclaw acacia (Acacia greggii), and fourwing saltbush (Atriplex canestens). Grasses dominate the understory and include Wright threeawn (Aristida wrightii), sand dropseed (Sporobolus cryptandrus), mesa d ropseed (Sporobolus jlexuosus), bristlegrass, Arizona cottontop, black grama, bush muhly, and hooded windmillgrass (Chloris cucullata). Major forbs are leatherweed croton) (Croton pottsii) and broom snakeweed (Xanthocephalum sarothrae). The study area was subdivided into 30 plots (102 X 402 m each) to accommodate a randomized complete block design comprised of nine treated plots and an untreated check plot in each of three blocks. The area was blocked to account for soil variation. Blocks were delineated by four parallel access paths bulldozed 402 m apart. The experimental design was modified by randomizing order of treatment application but using the same order of treatment installation in each block. Treatments were offset in each block to insure that similar treatments were not together in adjacent blocks. Herbicides were aerially applied in late May 1977. Each treatment consisted of six parallel spray swaths each measuring 12 by 402 m. Untreated buffer zones 30 m wide were maintained between plots to minimize cross-treatment herbicide movement and to facilitate field evaluations. All herbicide treatments were applied at a 0.56 kg/ha (a.e.) rate in 9.4 L/ha volume of I:7 (v:v) diesel oil and water emulsion. Herbicides were mixed for each treatment in an open vat with constant recirculating agitation. Measured amounts of herbicide and diesel oil were poured simultaneously from separate containers into a premeasured amount of recirculating water to form the emulsion. The aircraft was calibrated prior to treatment, loaded through the hopper, and following treatment, drained of residual spray solution which was measured to substantiate proper treatment volume. Aircraft systems were washed and purged between each treatment to prevent herbicide contamination among treatments. Herbicides included in the experiment were: 2,4,5-T’ alone and in combination (1: 1) ‘2,4,5-T as the propylene glycol butyl ether ester JOURNAL OF RANGE MANAGEMENT 35(4), July 1982 Table 1. Mean plant defoliation (%) and mortality (%) of honey mesquite treated in May 1977 and evaluated 18 and 30 months following treatment near