Imazapic Activity in a Semiarid Climate in Downy Brome (Bromus tectorum)–Infested Rangeland and CRP Sites

Abstract

Chemical control of downy brome has focused on imazapic; however, imazapic efficacy in semiarid climates is unpredictable, possibly because of variable residual soil activity. Our objective was to characterize imazapic activity over 9 mo in rangeland and a Conservation Reserve Program (CRP) site following its application in the fall as affected by rate (0, 80, 160, 240 g ai ha−1) and quantity of plant residue (reduced, ambient). Greenhouse bioassays were conducted over two seasons (2010 to 2011 and 2011 to 2012) using soil collected at multiple dates after imazapic application. Quantity of plant residue did not affect downy brome biomass or response to imazapic. Imazapic reduced downy brome biomass (P < 0.05) across all sampling dates in both seasons, and the response to rates was consistent up to 200 d post application. Imazapic activity over time conformed to a biphasic model with activity being consistent, or slightly improving, up to about 160 and 150 d post application, and then dropping rapidly to the final sampling event 287 and 272 d post application in rangeland and at CRP sites, respectively. These results indicate that fall imazapic applications in semiarid climates persist into the spring, thus providing control of both fall-emerging downy brome seedlings and seeds that overwinter and emerge the following spring.

Integrated Management of Cheatgrass (Bromus tectorum) with Sheep Grazing and Herbicide

“…These results, showing that herbicides had minimal effect on B. tectorum, highlight the inconsistency in the ability of herbicides to reduce B. tectorum abundance, especially with only limited herbicide application (two years for spring glyphosate but only one year for fall applications due to low fall germination in 2016). Previous studies have measured the effective control of B. tectorum via imazapic [28,29,43,44] and rimsulfuron [30,45], although a review of imazapic efficacy studies (n = 24) in Montana showed effective but variable results, with one year of application [29].…”

“…Morris et al [11], however, determined that spring-applied glyphosate alone reduced B. tectorum cover for two years, and combining with imazapic provided even greater control. Imazapic has been widely used [28][29][30][31] because it provides residual soil activity, which is important for controlling annual plants such as B. tectorum, with early post-emergent fall application recommended [29].…”