Long-term grass yields following chemical control of honey mesquite

Long-term herbaceous response data following herbicidal treatment of honey mesquite (Prosopis glandulosa Torr.) are needed to develop more accurate projections regarding economic feasibility of these treatments and to model ecological interactions between woody and herbaceous plants in rangeland systems. Our objective was to measure herbaceous yield and mesquite regrowth 10 or 20 years after mesquite was aerially sprayed with either mesquite top-killing or root-killing herbicides. Treatments evaluated included mesquite top-killing herbicides at 10-12 years (T10) and 19-21 years (T20) post-treatment, mesquite root-killing herbicides at 10-12 years (R10) and 19-21 years (1120) post-treatment, and an untreated control where mesquite were >30 years old (C30). Treatments were applied in the late 1970's or late 1980's. Grass yields, measured annually from 1998 through 2000, were quantified within patches of 3 perennial grass functional groups: cool-season mid-grasses, warm-season mid-grasses, or warm-season short-grasses. Cool-season annual grass yields were also quantified within these perennial grass patches. By 1998, mesquite canopy cover was 55, 47, 36, 24, and 12% in C30, T20, T10, R20, and R10 treatments, respectively. Warm-season mid-grass yields were most sensitive to differences in mesquite cover in all 3 years and declined sharply when mesquite cover exceeded 30%. Cool-season mid-grass yields declined slightly with increasing mesquite cover. Warm-season short-grass and cool-season annual grass yields were not related to mesquite cover, except in 2000 when warm-season short-grass yield beneath mesquite canopies increased with increasing mesquite cover. Results suggest that herbicide treatment life (defined by increased perennial grass yield in response to mesquite treatments) was at least 20 years for the root-killing herbicide, but no longer than 10 years for the top-killing herbicide.