Imazapic Effects on Competition Dynamics Between Native Perennial Grasses and Downy Brome (Bromus tectorum)

Downy brome inhibits revegetation efforts following ecosystem disturbance. Imazapic is a commonly used herbicide for downy brome management, but more information is needed regarding effective application timing for restoration efforts. We wished to determine (1) if native species establishment exhibited a tradeoff between downy brome competition and injury from herbicide and (2) if this differed between pre- and postemergent applications of imazapic. We used a standard replacement series design and overlaid herbicide treatments. Nine weeks after planting, aboveground biomass was harvested and relative yield (RY) indices calculated. Both imazapic applications reduced downy brome biomass by 91% or more (P < 0.05). Imazapic caused drastic reductions in native biomass but less than what was caused by downy brome competition (P < 0.05). Natives were less injured by a pre-than postemergent application (P < 0.05). In situations where downy brome may impact restoration efforts, pre-emergent applications of imazapic at 70 g ai ha(-1) (0.06 lb ai ac(-1)) may reduce downy brome with less negative impacts on newly-seeded native grasses than post-emergent applications. Ensuring sufficient proportions of native species seeds on restoration sites may reduce downy brome.