Glyphosate-resistant downy brome (Bromus tectorum) control using alternative herbicides applied postemergence

Downy brome is a troublesome facultative winter-annual grass weed that invades agricultural and nonagricultural lands in western North America and can cause substantial crop yield losses particularly in no-till winter wheat. Glyphosate-resistant (GR) downy brome was identified in southern Alberta in 2021, representing the first confirmation of a GR grass weed in Canada. This study was designed to evaluate alternative herbicides and herbicide mixtures applied postemergence (POST) for control of GR and glyphosate-susceptible (GS) downy brome populations at the seedling stage under a controlled environment. The GR downy brome did not exhibit cross-resistance to other herbicides applied POST. Quizalofop alone or in combination with imazamox, imazamox + bentazon, or imazamox/imazethapyr, and glufosinate mixed with either clethodim or tiafenacil resulted in =80% visible control, plant mortality, and reduction in biomass of both GR and GS downy brome populations 21 d after treatment. Diligent stewardship of these remaining herbicide options is warranted since downy brome populations with resistance to herbicides that inhibit acetyl-CoA carboxylase or acetolactate synthase have been reported in neighboring states.