ABSTRACT
Three field experiments were
conducted during 2016 and 2017 in southwestern Ontario, Canada to evaluate the
effect of glyphosate/dicamba (2:1 ratio) applied at the V2/V3 or V4/V5 growth
stage on dicamba-resistant (DR) soybean injury, growth response and yield. At 1 DAA,
glyphosate/dicamba at 450, 900, 1350, 1800 and 3600 g·ae·ha-1 caused 0, 0.3, 1.8, 3.3 and 5.8% growth response (leaf droop) when applied at
the V2/V3 growth stage and 0.3%, 1.3%, 1.5%, 2.3% and 4.5% growth response when
applied at V4/V5 growth stage in DR soybean, respectively. The growth response
was similar at 3 DAA. This response was transient with no growth response
observed at 7 DAA, 2 WAB, 4 WAB, 8 WAB, and late Sept. (R8 growth stage).
Glyphosate/dicamba at 450, 900, 1350, 1800 and 3600 g·ae·ha-1 caused 0.0, 1.4%, 3.6%, 5.7% and 10.7% injury (speckled chlorosis and necrosis
and leaf distortion) when applied at the V2/V3 growth stage and 0.5%, 0.7%,
2.0%, 3.7% and 6.9% injury when applied at the V4/V5 growth stage in DR
soybean, respectively. The injury observed was transient with no injury
observed at 4 and 8 WAB and late Sept. (R8).
There was no impact of glyphosate/dicamba at various rates evaluated on
maturity and seed yield except at 1350 g·ae·ha-1 (V2/V3) and 3600 g·ae·ha-1 (V4/V5) which
caused a 6% reduction in yield of DR soybean yield compared to the weed-free control. Based on these results, glyphosate/dicamba
at the labelled rates can be safely applied at the V2/V3 and V4/V5 growth stage
in DR soybean. However, care is needed to avoid spray overlaps as the 2× rate can result in
significant crop injury and yield reduction in DR soybean.