Evaluation of Delayed Glyphosate Burndown in No-Till Soybean

Application of a preplant or preemergence glyphosate burndown is routine in most no-tillage production systems of annual crops, however, time of application may influence overall weed control and grain yield. Six field trials were conducted over a three-year period (2008, 2009 and 2010) near Ridgetown, Ontario, Canada to determine the effect of glyphosate burndown application timing in glyphosate-resistant (GR) no-tillage soybean [Glycine max (L.) Merr.]. Soybean growth was reduced 5%, 10% and 20% when the burndown was delayed to 1, 5 and 12 days after planting (DAP), when evaluated one week after application (WAA), respectively. By 8 WAA, predicted burndown timing increased to 9, 14 and 23 DAP, and was 10, 17 and 26 DAP at 12 WAA, to reduce soybean growth by 5%, 10% and 20%, respectively. Similarly, burndown application at 14, 21 and 30 DAP corresponded to a soybean yield reduction of 5%, 10% and 20%. Seed moisture content was generally insensitive to burndown timing; 80 and 140 DAP were required to increase moisture by 5% and 10%, respectively. This research indicates that delaying glyphosate burndown by up to 14 DAP, approximately VE to VC growth stage, has the potential to result in a modest reduction in soybean yield (5%).