Glyphosate-Resistant Horseweed (Conyza canadensis) Control with Dicamba in Alabama

The development and spread of glyphosate-resistant (GR) horseweed has increased the use of dicamba as an alternative herbicide treatment. Research evaluated suspected glyphosate-resistant horseweed populations from DeKalb (GR-1) and Cherokee (GR-2) counties, Alabama, for response to glyphosate, dicamba, and glyphosate + dicamba. Populations used for resistance determination were tested at rosette and bolt growth stages. Glyphosate resistance evaluation treatments ranged from 0 to 36.0 kg ae ha(-1). Data confirmed that GR-1 and GR-2 horseweed populations were 3.0 to 38 times more resistant to glyphosate than the susceptible population, according to population, data type, and growth stage at treatment. GR-1 and GR-2 populations were further evaluated for response to dicamba. Dicamba was applied at 0 to 1.12 kg ai ha(-1), both with and without the addition of glyphosate at 1.12 kg ae ha(-1). All populations had similar tolerance to dicamba, with the exception of GR-2 treated at the rosette growth stage, which had similar to 2-fold greater tolerance. When glyphosate was tank-mixed with dicamba, the response of GR populations was similar to that of dicamba alone. Therefore, any potential resistance-management benefit of tank-mixing dicamba with glyphosate may be negated when one is attempting to control GR horseweed. Conversely, adding glyphosate to dicamba drastically enhanced control of the susceptible population at both growth stages.