Weed Control in Dicamba-Resistant Soybean with Glyphosate/Dicamba Applied at Various Doses and Timings

Seven field trials were completed over a three-year period (2016 to 2018) in southwestern Ontario, Canada, to assess weed control in conventional-till dicamba-resistant (DR) soybean with glyphosate/dicamba (2:1 ratio) applied postemergence (POST) at 3 doses (900, 1350, and 1800 g.ae.ha(-1)) and 3 application timings (up to 5, 15, and 25 cm weeds). There was minimal soybean injury (<= 2%) from treatments evaluated. Glyphosate/dicamba applied at application timing of up to 5, 15, and 25 cm weeds, controlled Amaranthus spp. (pigweed spp.) 87-96, 94-99, and 99%; Ambrosia artemisiifolia (common ragweed) 93-99, 97-99, and 99-100%; Chenopodium album (lambsquarters) 89-99, 95-100, and 99-100%; Echinochloa crus-galli (barnyardgrass) 81-84, 94-96, and 96-97%; Setaria faberi (giant foxtail) 37-90, 77-98, and 99-100%; and Setaria viridis (green foxtail) 94-96, 99, and 99-100%, respectively. Additionally, glyphosate/dicamba applied POSTat 900, 1350, and 1800 g.ae.ha(-1) controlled Amaranthus spp. 90-97, 95-98, and 97-99%; A. artemisiifolia 95-98, 97-99, and 99-100%; C. album 92-99, 95-100, and 98-100%; E. crus-galli 84-88, 93-94, and 95-96%; S. faberi 74-95, 75-97, and 79-98%; and S. viridis 98, 98-99, and 98-100%, respectively. Weed interference reduced DR soybean yield as much as 51% compared to the highest yielding treatments. Results indicate that glyphosate/dicamba applied POSTat the label doses can provide an adequate control of troublesome weeds in DR soybean. Weed control was generally most consistent when glyphosate/dicamba was applied at the highest registered dose in Ontario (1800 g.ae.ha(-1)) and when weeds were up to 25 cm tall.