Field experiments were conducted at Kalispell, MT, and Corvallis, OR, to determine the optimum rate and application timing of imazamox for downy brome control in winter wheat. Crop injury occurred as a reduction in plant height and was minimal at Kalispell, never exceeding 10%. Crop injury at Corvallis was more severe and was dependant on application timing. No injury was observed with spring applications, but fall applications resulted in as much as 33% injury at the highest rate of imazamox. Fall applications generally provided more consistent control of downy brome, as evidenced by the lower dosage required to reduce downy brome dry weight by 50% (lower I₅₀ values). Nonetheless, spring applications generally provided control comparable with that of fall applications when imazamox was applied at the highest rate. The one exception was at Corvallis during 1997 to 1998, where spring applications failed to provide adequate control of downy brome even at the highest rate applied. Although imazamox generally provided excellent control of downy brome, wheat yield response to downy brome interference was negligible, declining by less than 10% in the absence of imazamox. The absence of a yield response to downy brome interference was attributed to the lack of competition for soil moisture from downy brome under the high-rainfall conditions of the experiment.

Nomenclature: Imazamox; downy brome, Bromus tectorum L. #³ BROTE; winter wheat, Triticum aestivum L.

Additional index words: Dose–response, reduced rates.