Co-application of thifensulfuron with glyphosate accentuates soybean injury

Three trials (two in 2019 and one in 2020) were completed at the University of Guelph, Huron Research Station near Exeter, ON, to determine if the co-application of thifensulfuron with glyphosate accentuates soybean injury in glyphosate-resistant (GR) soybean. At 1, 2, 4, and 8 weeks after treatment (WAT), thifensulfuron (6 and 12 g a.i.·ha−1 representing the 1× and 2× rate, respectively) applied POST with no adjuvants caused up to 5% soybean injury. The addition of a non-ionic surfactant + UAN to thifensulfuron increased soybean injury to up to 24%. There was no decrease in soybean density, dry biomass, height, and yield, except soybean dry biomass was reduced up to 22% with the addition of adjuvants to thifensulfuron at the 2× rate. Glyphosate (1800 and 3600 g·ha−1 representing the 1× and 2× rate, respectively) applied POST caused no adverse effect on soybean injury parameters evaluated. The co-application of glyphosate + thifensulfuron at the 1× and 2× rates, without additional adjuvants, caused a synergistic increase in soybean injury at 1, 2, 4, and 8 WAT, and a synergistic decrease in dry biomass and height. All other interactions were additive. The co-application of glyphosate + thifensulfuron at the 1× and 2× rates, with additional adjuvants, produced a synergistic increase in injury at 1 (1× and 2× rate), 4 (1× rate), and 8 (1× rate) WAT in soybean. All other interactions were additive.