Reducing Euphorbia esula with a combination of sheep grazing and imazapic

Field studies were conducted in Idaho from 2002 to 2004 to determine whether summer grazing of sheep for 1 or 2 years before an autumn application of imazapic would enhance control of Euphorbia esula. E. esula, a perennial plant native to parts of Europe and Asia, has invaded the Great Plains and Rocky Mountains after its introduction into North America in the early 1880s and caused significant reductions in native plant biomass. Experiments were conducted to determine the impacts of 1 or 2 years of sheep grazing with or without a fall application of imazapic on E. esula and native plant populations. Sheep grazing was designed to remove reproductive parts from E. esula within a 10 d grazing period. Imazapic was applied at 210 g ae ha−1 with 1.25% (v/v) methylated seed oil. One year of sheep grazing did not alter measured vegetation components, but it did result in an increase of grass seed in the soil. Two years of sheep grazing increased the forb and grass cover component, increased the grass seed in the soil, and kept the E. esula seed bank from increasing. Application of imazapic reduced E. esula stem densities and cover and increased native forb cover. The combination of 1 or 2 years of sheep grazing and imazapic did not enhance the control of E. esula. However, 2 years of carefully timed sheep grazing followed with an imazapic application resulted in sustained productivity of plant biomass in the pasture. Because 2 years of sheep grazing prevented an increase in the E. esula seed bank, managers may have a better opportunity to establish desired vegetation in sagebrush steppe ecosystems after removing E. esula with imazapic.