Patchy Response of Cheatgrass and Nontarget Vegetation to Indaziflam and Imazapic Applied After Wildfire in Sagebrush Steppe

Abstract

Control of nonnative grasses is needed where they are altering fire regimes and degrading rangelands, such as cheatgrass (Bromus tectorum) invasion of perennial sagebrush-steppe communities. Aerial broadcast of the pre-emergent and postemergent herbicide imazapic has been used for decades over vast areas to control cheatgrass after fire. Recent small-scale studies indicate that the pre-emergent herbicide indaziflam may provide more enduring cheatgrass control. We evaluated landscape-level vegetation responses to indaziflam sprayed in replicated areas at 66.7 g · ai · ha⁻¹, with and without imazapic (66.1 g · ai · ha⁻¹) over almost 500 ha of sagebrush steppe. Herbicides were strip-sprayed by helicopter in the fall of 2019 in subregions that either 1) had burned in the summer of 2019 and had moderate background cheatgrass invasion, 2) had burned in 2011 and became heavily invaded, or 3) were burned in both 2011 and 2019 and had intermediate invasion. Tarps were temporarily deployed to intercept herbicides and create untreated controls. Overall, indaziflam + imazapic had greater initial control of cheatgrass, but by 2023, both treatments led to similar ∼17 percentage-point reductions in cheatgrass cover. Cheatgrass individuals that “escaped” the herbicide treatment grew exceptionally large and fecund. There were no reductions in cover in any native vegetation type, including biocrusts, and nontarget increases in cover were observed for 1) deep-rooted perennial grasses treated with indaziflam + imazapic in the 2011 burn subregion and 2) the shallow-rooted Sandberg bluegrass (Poa secunda) treated with either herbicide in the 2011 or 2011 + 2019 burn subregions. Consideration of burn legacies, pretreatment landscape condition, and evenness of treatment application may improve restoration outcomes and help prioritize management allocation, timing, and treatment expectations.