Tank mixture of glyphosate with 2,4-D accentuates 2,4-D injury in glyphosate-resistant corn

Six field trials were conducted at Ridgetown, ON, over a 2-yr period (2015 and 2016) to determine the tolerance of two corn hybrids to 2,4-D (560 and 1120 g a.i. ha(-1)) and glyphosate (1800 g a.e. ha(-1)) applied alone or in combination at stage VI, V3, or V5. In DeKalb DKC52-61 corn, 2,4-D caused as much as 24%, 16%, 11%, and 11% visible injury at 1 wk after each postemergence herbicide application (WAT), 2 WAT, 4 wk after the last postemergence herbicide application (WA-C), and 8 WA-C, respectively. Plant stand was not affected, but plant height decreased by 5 cm at 560 g a.i. ha(-1) and 7% at 1120 g a.i. ha(-1). As the application timing was delayed from stage VI to V5, there was a trend to increased injury at both 2,4-D rates. Corn yield decreased 8% with 2,4-D applied at 560 g a.i. ha(-1) and 12% at 1120 g a.i. ha(-1). In Pioneer P0094AM corn, 2,4-D caused as much as 16%, 9%, 7%, and 7% visible injury at 1 WAT, 2 WAT, 4 WA-C, and 8 WA-C, respectively. Plant height was not affected, but goosenecking and brace root malformation were increased as the rate of 2,4-D was increased. There was generally no difference between glyphosate rates (1800 vs 0 g a.e. ha(-1)) at the VI com stage but visible injury, goosenecking, and brace root malformation at other application timings was as much as 15%, 3%, and 19% greater when 2,4-D was applied in a tank mixture with glyphosate, respectively. Yield was reduced by 12% when 2,4-D (1120 g a.i. ha(-1)) was applied with glyphosate in the tank mixture.