Evaluating Impacts of Imazapic Treatments on Mule Deer Forage Quality, Quantity, and Composition

The invasion of exotic annual grasses including cheatgrass (Bromus tectorum), medusahead (Taeniatherum caput-medusae), and North Africa grass (Ventenata dubia) have drastically altered grassland ecosystems across the western United States. Continuous efforts have been made to reclaim these ecosystems through the application of annual grass selective herbicides. The effectiveness of these herbicides and the response of vegetation post application is well documented, but research is lacking in how these treatments directly affect mule deer (Odocoileus hemionus) forage quality, quantity, and composition throughout the year. Our objectives were to evaluate the efficacy of using imazapic (Open Range G, Wilbur-Ellis) to improve summer mule deer forage and assess the potential negative effects of the herbicide on winter forage availability. In fall of 2020, imazapic was applied to fifteen, 8-ha plots established within telemetered mule deer home ranges located in grassland habitat surrounding Heppner, Oregon, United States. Paired treatment and control plots were monitored for 2 yr in the summer and fall of 2021 and summer of 2022. Our results suggest that imazapic applications can be used to increase the nutritional quality of forage but may temporarily reduce the quantity of forage available in the first summer after treatment. In the second summer, forage biomass increased significantly but crude protein and digestible energy contents of forage were no longer superior within treatment plots. Imazapic applications had no effect on winter forage biomass or nutritional quality of forage available. Imazapic treatments appear to be an effective strategy to improve summer mule deer forage quality and quantity without adversely affecting winter food sources.