ABSTRACT
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%).