Enhancing efficacy of herbicides to control cheatgrass on Montana range, pasture, and Conservation Reserve Program (CRP)

Abstract

Chemical control of cheatgrass has recently focused on imazapic; factors such as application rate and timing and the presence of plant litter can influence imazapic's efficacy. Herbicides minimally impact the seedbank so integrating a seed-killing pathogen like Pyrenophora semeniperda may result in more effective and sustainable control. My research objectives were to 1) test the effect of imazapic application rate and timing and plant litter on cheatgrass and desired plant species in range and Conservation Reserve Program (CRP) lands, 2) conduct a soil bioassay to determine imazapic persistence as affected by imazapic rate, presence of plant litter, and time after herbicide application, and 3) determine whether the fungal pathogen P. semeniperda combined with a single imazapic application would provide greater control of cheatgrass than either strategy used alone. Objective 1 was carried out in range and CRP lands over two years with a factorial combination of four imazapic rates, two litter manipulation treatments and/or two application timings. In general, all three imazapic rates were equally effective in controlling cheatgrass compared to the non-sprayed control. Litter manipulation treatments had little effect on imazapic efficacy, but early application of imazapic resulted in more consistent cheatgrass control. Objective 2 was conducted in the greenhouse using soil samples collected over a six month period from the field study for Objective 1. Cucumber and cheatgrass were used as indicator species. All three herbicide rates reduced both species' biomass below that of the control. Again, litter manipulation had a minimal effect, and imazapic was found to persist through the following spring after spraying. Objective 3 was explored in a greenhouse experiment using a factorial combination of two imazapic treatments, two P. semeniperda treatments, and three seeding depths. Pyrenophora semeniperda reduced cheatgrass emergence, while cheatgrass biomass and density were affected by imazapic and the integration of imazapic and P. semeniperda. Imazapic and P. semeniperda did not favorably interact to reduce biomass and density; however, integrating these two tools holds promise as P. semeniperda can reduce the seedbank, and imazapic can control seedlings that escape pathogen-caused mortality.