Management of annual brome invasion within northern mixed grassland using indaziflam

Abstract

Annual brome grasses are some of the most widespread and problematic weeds in North America and present a challenge in managing rangeland, with control options needed. The herbicide indaziflam has shown promise in long-term annual brome reduction on western U.S. rangeland, though how these results apply to northern temperate grasslands of western Canada remains uncertain. In chapter 2 we studied the effect of indaziflam rate (0, 37.5, 75, and 150 g ai ha-1) and timing (fall vs spring application) on long-term biomass and density of annual brome at two sites in Canada’s northern Mixedgrass Prairie. Reductions in brome were not evident until the second growing season following treatment. During the third growing season, the current recommended rate (75 g ai ha-1) of indaziflam reduced brome biomass and density by at least 90% at both sites. By the fourth season indaziflam continued to reduce brome biomass by 11 and 66%, and brome density by 76 and 95%, at the two sites, respectively. Reductions in brome biomass and density occurred at rates as low as 37.5 g ai ha-1 but were not as reliable as higher rates of 75 and 150 g ai ha-1. The timing of indaziflam application had less impact on long-term brome reduction. A single application of indaziflam can reduce annual brome, including corn brome (Bromus squarrosus L.), in northern Mixedgrass Prairie grasslands for up to four years. In Chapter 3, to understand how non-brome northern temperate grassland vegetation responds to indaziflam herbicide treatment, we studied the effect of indaziflam rate (0, 37.5, 75, and 150 g ai ha-1) and timing (fall vs spring application) on plant species cover, richness, diversity, total plant biomass, perennial grass biomass, and forb biomass. Indaziflam application did not affect the cover, richness, or diversity of non-brome plant species until the fourth year after treatment, at which time species richness and diversity were reduced by most indaziflam treatments at both sites. In the fourth year, indaziflam treatment also altered the cover of the three most abundant grasses but did not affect the three most abundant forbs. Total biomass first decreased in the second year, then increased in the third, and stabilized in the fourth year following indaziflam treatment. Perennial grass biomass either increased or remained the same and did not decline under any indaziflam treatment in any year. Forb biomass was not affected by indaziflam treatment. Next, we studied the effect of indaziflam rate (0, 37.5, 75, and 150 g ai ha-1) on seedling emergence from the soil seedbank treated with indaziflam two years prior. Indaziflam reduced forb emergence but did not reduce perennial grass emergence. Finally, we studied the effect of indaziflam rate (0, 75, and 150 g ai ha-1) on root and shoot biomass of four species of actively growing perennial grasses. Indaziflam reduced root and shoot biomass of four-month-old perennial grasses grown in greenhouse conditions. Our study provides an improved understanding of how indaziflam affects plant community composition and biomass within the northern Mixedgrass Prairie where native grasslands are relatively less invaded by annual brome grasses.