Establishment of native perennial grasses in the presence of downy brome and imazapic

Background/Question/Methods

Cheatgrass (Bromus tectorum), one of the most significant weeds in the western United States, easily colonizes disturbed areas and inhibits revegetation efforts following ecosystem disturbance. The herbicide imazapic is used in revegetation efforts with variable success to manage cheatgrass. This study investigated three native perennial grasses important for restoration projects in the region: western wheatgrass (Pascopyrum smithii), needle and thread (Hesperostipa comata), and blue grama (Bouteloua gracilis). The objectives of this study were to determine (1) if native species establishment and early growth exhibited a tradeoff between cheatgrass competition and injury from herbicide application, (2) if this differed between imazapic applied at 70 g ai ha^-1 prior to or following cheatgrass emergence. A standard replacement series design was used with 3 proportions of each of the three native species (25, 50, and 75%) relative to cheatgrass, along with a monoculture treatment for each species. Three different herbicide treatments (no herbicide, pre-emergent, and post-emergent applications) were compared. The 39 treatment combinations were replicated 5 times in a randomized complete block design. After 9 weeks, aboveground biomass was harvested, dried, and relative yield indices calculated to assess competitive ability.

Results/Conclusions

Aboveground biomass for native species and cheatgrass differed depending on native proportion and herbicide application (p < 0.05). Regardless of timing or native species proportion, herbicide application reduced cheatgrass biomass. In the absence of herbicide, cheatgrass biomass was much greater than native biomass although it decreased as the seeding proportion of native species increased. Herbicide reduced native species biomass. In general, western wheatgrass and needle and thread biomass was reduced less by a pre-emergent than a post-emergent application. Blue grama biomass was reduced by both herbicide application timings. Our results suggest that, in situations where cheatgrass may impact restoration efforts, a pre-emergent application of imazapic at 70 g ai/ac will reduce cheatgrass and have less negative impacts on newly-seeded native grasses than post-emergent applications. Additionally, ensuring sufficient proportions of native species on the site may reduce cheatgrass in such situations.