EFFECTS OF CHEATGRASS MANAGEMENT ON WILDFIRE RISK AND REFINING CHEATGRASS TREATMENT PRACTICES IN COLORADO FRONT RANGE GRASSLANDS

Abstract

Cheatgrass invasion is a critical threat to rangelands in the western United States. Cheatgrass invasion alters ecosystems by diminishing biodiversity and increasing fire risk by promoting fine fuel abundance and continuity. Live fuel moisture content (LFMC) is the water content in living vegetation and when reduced, increases the likelihood of wildfire ignition and potential rate of wildfire spread, making it a crucial predictor of wildfire risk. LFMC is often reduced in rangelands dominated by invasive annual grasses like cheatgrass (Bromus tectorum L.) because they senesce earlier in the season compared to native plants. Reductions in LFMC after annual grass invasion increase the likelihood of uncharacteristically frequent, large, and fast-growing wildfires that pose risks to human health, property, and native plant communities. While the impacts of cheatgrass invasion on fire regimes in sagebrush shrublands are well- understood, it is necessary to evaluate the impacts of cheatgrass management in different rangeland plant communities, including grasslands in semi-arid regions like Colorado’s Front Range. The herbicide indaziflam (Rejuvra®, Envu) is widely used to manage cheatgrass in rangelands and several studies have verified its effectiveness for reducing cheatgrass abundance. We evaluated the effects of indaziflam treatment on LFMC at six grassland study sites in 2024 and four grassland study sites in 2025 located along the Colorado Front Range. Indaziflamvtreatment increased LFMC in treated areas for portions of both summer growing seasons, indicating that treatments reduced wildfire risk for at least a portion of the year. We also investigated how to effectively control cheatgrass without impacting co- occurring non-target species. Indaziflam is strictly pre-emergent and will not control seedlings established prior to treatment. To promote consistent treatment effectiveness and increase short- term cheatgrass control, land managers often add a post-emergent herbicide as a tank mix with indaziflam to control seedlings that have already germinated and established prior to application. While this is an effective strategy to control cheatgrass, post-emergent herbicides may impact co- occurring non-target plants. We evaluated the effects of two different post-emergent herbicides, rimsulfuron (Matrix®, Corteva) and imazapic (Plateau®, BASF), applied with indaziflam at different rates to determine which post-emergent tank mix provides the most satisfactory short- term cheatgrass control with the least impact to co-occurring non-target plants over the course of two years. The results indicated that all treatments completely controlled cheatgrass by year two of the study, and there were no differences in perennial grass or bare ground cover. We anecdotally observed impacts (stunting) on native perennial grasses resulting from treatments that included rimsulfuron, but these were not observed after the first year of the study and reduction in perennial grass cover were never statistically significant, indicating that non-target impacts were minor and transient when they occurred.