Does herbicide weed control increase livestock production on non-equilibrium rangeland?

Abstract

Herbicides have been applied extensively on rangelands to reduce forbs that were considered undesirable, which has been assumed to lead to an increase in grass production and ultimately to an improvement in livestock performance. While scores of research studies support the premise that forb suppression can increase desirable forage on plot-level studies, only a few studies have attempted to quantify the effect on livestock production within experimental units equivalent in area to production-scale pastures. We therefore tested the effect of herbicide on mixed prairie vegetation and on livestock gain on pastures typical in area, stocking rate, and composition to working ranches in the region. Picloram and 2,4-d at 0.15 and 0.56kg/ha, respectively, were applied aerially in spring of 2001 and again in spring of 2004 to two pastures. Two untreated control pastures were managed identically, including grazing management, but were not treated with herbicide. Canopy cover by species and cover of bare ground and litter were estimated in permanently located plots, and stocker cattle performance (average daily gain per head; ADG) and total cattle gain (kgha−1) were measured from 2000 to 2005. Forb cover was less (P<0.05) in herbicide treated pastures than in control pastures in the year of herbicide application (2001 and 2004) and for second growing season following the treatment (2 YAT) in the 2004 application. Grass cover varied more with annual precipitation than with treatment. Livestock ADG and gain ha−1 did not differ between treatments in any year or across years. Livestock production varied among years, reflecting variation in growing-season precipitation that is characteristic of non-equilibrium rangelands. Herbicide altered the plant community by reducing forb cover and increasing grass cover, but livestock production was not altered either on an individual basis (gain/head) or on an area basis (gain/ha). Livestock production per area (gain/ha) might be increased if grass production after herbicide treatment could be predicted reliably. However, predicting grass production is notoriously difficult in rangeland ecosystems that have high inter-annual variation in precipitation. Reliable adjustments in stocking rate to harvest grass released from forb competition is unlikely and therefore fraught with risk.