Abstract
Traditional two-pass weed management strategies need to be compared with new strategies in glyphosate/dicamba-resistant soybean. Weed control, soybean yield, partial profitability and environmental impact (EI) were evaluated in glyphosate/dicamba-resistant soybean using dicamba applied alone or in a tank-mix with dimethenamid-P applied preemergence (PRE). Trials were conducted at three locations during 2014 and 2015. Several PRE herbicides provided excellent control of broadleaf and grass weeds. Dicamba provided > 91% control of broadleaf weeds, and the addition of dimethenamid-P improved grass control. All weed species at the trial locations were controlled > 94% following a postemergence (POST) application of glyphosate. Weed interference reduced soybean seed yield 33% where no herbicide was applied. A single POST application of glyphosate had the lowest EI. Several treatments improved early-season weed control and reduced early-season weed density and biomass compared to glyphosate and had similar EI values. In this study, there was no benefit to yield or partial profit by including a PRE herbicide for weed management; however, the inclusion of multiple modes-of-action in a herbicide program may reduce the selection for herbicide-resistant weeds.
Highlights
Soybean is most susceptible to yield loss during the critical weed-free period (CWFP) that begins at V1 and ends at V4 [5] [6] [7], which is the reason why herbicide applications are typically made prior to soybean emergence and soybeans are maintained weed-free through the end of the vegetative growth period prior to flowering
The glyphosate-only treatment received a POST application of glyphosate (900 g∙ae.∙ha−1); the level of weed control was equal to the weedy check prior to the POST application
There were differences in early-season weed control among the herbicide treatments, weeds were controlled by the follow-up POST application of glyphosate
Summary
Weed control was greatly simplified when glyphosate-resistant (GR) soybean was commercialized in Ontario in 1997. Glyphosate is a Group 9, non-selective herbicide that controls over 300 weed species [8]. It has a wide margin of crop safety, low toxicity, and a flexible application window [6] [9] [10]; these factors contributed to the high adoption rate of GR soybean. The sequential application of a single herbicide with one mode-of-action may select for herbicide-resistant (HR) weed biotypes; for this reason, the repeated use of a single mode-of-action is strongly discouraged [13]. The growing concern of selecting HR weed biotypes has prompted research for weed management solutions that can fill the agronomic gaps left by using only glyphosate
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