Glyphosate-resistant horseweed control in glyphosate/glufosinate/2,4-D-resistant soybean with one- and two-pass herbicide programs

Glyphosate-resistant (GR) biotypes of horseweed were first confirmed in southern Ontario in 2010 and have spread across southern Ontario. A total of four field experiments were conducted between 2021 and 2022 to determine GR horseweed control with one-and two-pass herbicide programs in glyphosate/glufosinate/2,4-D-resistant (GG2R) soybean. 2,4-D choline/glyphosate DMA, halauxifen-methyl, and saflufenacil applied preplant (PP) controlled GR horseweed by 59%, 72%, and 78% 8 wk after postemergence (POST) application (WAA-POST); there was no improvement of GR horseweed control when 2,4-D choline/glyphosate DMA was added to saflufenacil; in contrast, there was improved GR horseweed control when saflufenacil was added to 2,4-D choline/glyphosate DMA. Glufosinate and 2,4-D choline/glyphosate DMA applied POST controlled glyphosate-resistant horseweed by 71% and 86%, respectively, 8 WAA-POST. Two-pass herbicide programs of a PP followed by POST application provided greater GR horseweed control than a PP or POST herbicide applied alone. Glufosinate or 2,4-D choline/glyphosate DMA applied POST following 2,4-D choline/glyphosate DMA or halauxifen-methyl applied PP improved GR horseweed control by 29% to 38% and 24%, respectively at 8 WAA-POST. The application of 2,4-D choline/glyphosate DMA applied POST following saflufenacil applied PP improved control by 20% 8 WAA-POST; there was no improvement of GR horseweed control when glufosinate was applied POST following saflufenacil applied PP or when either POST herbicide was applied following saflufenacil + 2,4-D choline/glyphosate DMA applied PP. When used in a two-pass program, 2,4-D choline/glyphosate DMA POST provided 2% to 3% greater control of GR horseweed than glufosinate.