Urea ammonium nitrate as the carrier for preplant burndown herbicides

Abstract

Preplant weed control is a common practice for many small grain farmers. The timing of these applications often coincides with starter nitrogen (N) fertilizer application. Co-application of the herbicides and N fertilizers, such as urea-ammonium nitrate (UAN), can reduce the number of trips across the field, labor costs, and the costs of N and herbicide applications. However, there is a dearth of information on the effect of herbicide-N fertilizer mixtures on herbicide efficacy. Field studies were conducted in the summer of 2021 and 2022 to evaluate the effect of UAN (32-0-0) rate (0%, 25%, 50%, 75%, and 100% of carrier volume) on the efficacy of three non-selective herbicides (glyphosate [1260 g ae ha−1], paraquat [560 g ai ha−1], and tiafenacil [74 g ai ha−1]). There was no effect of UAN volume on herbicide efficacy. The addition of UAN did not reduce the efficacy of glyphosate, paraquat, or tiafenacil. At 3 weeks after herbicide application, glyphosate efficacy ranged from 92% to 94% (broadleaved weeds) and 97% (grassy weeds). Paraquat efficacy ranged from 63% to 87% (broadleaved weeds) and 87% (grassy weeds). Tiafenacil efficacy ranged from 52% to 74% (broadleaved weeds) and 70% (grassy weeds). Higher application volume may be needed to increase the efficacy of contact herbicides such as paraquat and tiafenacil.