Tolerance of Corn (<i>Zea mays</i> L.) to Early and Late Glyphosate Applications

Abstract

Fifteen field experiments were conducted from 2009 to 2012 in Ontario, Canada and Michigan, USA to determine the tolerance of corn ( Zea mays L.) to early (spike or 1- to 2-leaf stage) or late (8- or 10-leaf stage) applications of 900, 1800, 3600, or 7200 g &middot ae &middot ha -1 of glyphosate. Postemergence applications were evaluated for corn injury, cob length and deformity, crop moisture at harvest, and yield in the absence of weed competition. In the early application experiment, no visible injury was detected with applications of up to 3600 g &middot ae &middot ha -1 ; however, 1.4% injury was observed 4 weeks after treatment (WAT) when 7200 g &middot ae &middot ha -1 was applied to 1- to 2-leaf stage corn. Yet by harvest, the observed injury was transient as yields were similar to the untreated control regardless of glyphosate dose or timing. In the late application experiment, visible injury tended to increase with glyphosate dose. In addition, for corn treated with 7200 g &middot ae &middot ha -1 at the 10-leaf stage, injury increased over time as 6%, 11%, and 12% injury was observed 1, 2, and 4 WAT, respectively. Similar to the visible injury of vegetative tissue, cob deformity and reductions in yield tended to increase with glyphosate dose, but this response varied and the data were pooled into two environment groups. For example, in one environment group, corn treated with 7200 g &middot ae &middot ha -1 at the 8- and 10-leaf stage had a 9.5% and 14.6% reduction in yield, respectively. Whereas in another environment group, corn yields were similar to the untreated control regardless of glyphosate dose or timing. This research demonstrated that commercially available corn hybrids have tolerance to glyphosate at doses greater than what has been previously published or could reasonably be expected during spray overlaps in a field.