Considerations for Postemergence Dicamba Use in Wisconsin Soybean Cropping Systems

Abstract

Soybean [Glycine max (L.) Merr] is an important component of annual cropping systems in Wisconsin and throughout the Midwest United States. Novel herbicide-resistant soybean traits (i.e. Roundup Ready 2 Xtend®, glyphosate- and dicamba-resistant) are being adopted to diversify chemical control options postemergence (POST) to address widespread occurrence of glyphosate-resistant broadleaf weeds. Proper application timing, overreliance and concerns regarding off-target movement (OTM) of dicamba may limit the success of dicamba-resistant (DR) soybean technology. A field experiment was conducted in Wisconsin at three sites in 2018 and four sites in 2019 to determine optimum dicamba POST timing and consider the value of a layered residual POST approach. Treatments consisted of preemergence (PRE) fb POST diglycolamine salt of dicamba with VaporGrip® technology (DGA+VG) plus glyphosate in DR soybean at three timings: early POST (V1-V2), mid-POST (MPOST, V3-V4), and late POST (V5-V6/R1); with or without acetochlor as a layered residual POST. Laboratory experiments were conducted in 2019 to assess the role of formulation type, glyphosate and spray additives on solution pH. Two low tunnel field volatility experiments were conducted in 2019 to assess the influence of glyphosate and application timing of two dicamba formulations, DGA+VG and sodium salt of dicamba with diflufenzopyr (NA+DIF), and 2,4-D choline on soybean symptomology. In field experiments, POST application timing impacted weed control, weed seedbank deposits, and soybean grain yield at site-years with troublesome species (e.g. giant ragweed [Ambrosia trifida L.] and waterhemp [Amaranthus tuberculatus (Moq.) J.D. Sauer]). MPOST (V3-V4) application timing was observed to optimize season long weed control while reducing weed seedbank deposits and preserving crop yield potential. In laboratory experiments, the addition of glyphosate strongly influenced pH of spray mixtures with dicamba or 2,4-D choline, often resulting in a final solution pH <5.0. Random forest analyses indicated inclusion of glyphosate was the most influential variable on pH change. Low solution pH (<5.0) for dicamba mixtures may be of concern due to increased likelihood of more volatile free dicamba acid. In the low tunnel volatility experiments, an adaptation of Area Under the Disease Progress Stairs, Area Under the Injury over Distance Stairs (AUIDS), was utilized to make treatment comparisons in the field low tunnel field volatility experiments. The addition of glyphosate was not found to influence soybean symptomology (p = 0.366). Greater soybean symptomology was observed for dicamba treatments (AUIDS ranged from 1 to 36) than 2,4-D-choline treatments (AUIDS <2). Higher air temperatures and lower wind speeds helped explain the greater dicamba symptomology (AUIDs >10) observed in two of four application timings. Collectively, our results indicate a two-pass program, PRE fb MPOST, can be a successful weed control approach in Wisconsin. Growers should make applications when favorable weather conditions (low temperature, adequate wind) are present during and following application. Future work is needed to fully understand the interactions between environmental conditions and tank-mix components on dicamba OTM.