The Impact of the Addition of Urea Ammonium Nitrate to 2,4-D on the Control of Multiple-Herbicide-Resistant Canada Fleabane

Limited information exists on the impact of the addition of 28% urea ammonium nitrate (UAN) to 2,4-D, applied preplant (PP) to soybean, on the control of multiple-herbicide-resistant (MHR) Canada fleabane. A total of five field experiments were conducted over a two-year period (2020, 2021) in southwestern Ontario to determine if MHR Canada fleabane control with 2,4-D ester applied PP can be improved by adding UAN to the spray solution. Glyphosate + 2,4-D ester applied PP with various rates of UAN caused no soybean injury. The control of MHR Canada fleabane decreased when UAN was added to the spray solution, especially at higher rates evaluated. At 4 weeks after treatment (WAT), the predicted percent UAN concentration in the spray solution that caused a 5, 10, 20, or 50% decrease in MHR Canada fleabane control was 12, 17, 25, and 52%, respectively. At 8 WAT, the predicted percent UAN concentration that caused a 5, 10, 20, or 50% decrease in MHR Canada fleabane control was 10, 13, 18, and 33%, respectively. At 8 WAT, the predicted percent UAN concentration that caused a 5 and 10% increase in MHR Canada fleabane density was 36 and 73%, respectively. At 8 WAT, the predicted percent UAN concentration that caused a 5, 10, 20, or 50% increase in MHR Canada fleabane biomass was 2, 4, 8, and 19%, respectively. The predicted percent UAN concentration that caused a 5, 10, and 20% decrease in soybean yield was 1, 3, and 12%, respectively. This study concludes that the addition of 28% UAN, especially at the higher concentration, to 2,4-D ester reduces control and increases the density and biomass of MHR Canada fleabane.