Off-target Movement of DGA and BAPMA Dicamba to Sensitive Soybean

Abstract

It is well established that dicamba can cause severe injury to soybean that is not resistant to dicamba. Dicamba-resistant (DR) cotton became available in 2015, followed by DR soybean in 2016; in late 2016 came the release of new dicamba formulations approved for topical use in cotton and soybeans. Until this approval, use of dicamba was limited to primarily corn, small grains, range and pasture, and eco-fallow acres. Hence, studies were conducted in 2015 and 2016 to examine off-target movement of two dicamba formulations using non-DR soybean as a bio-indicator. Diglycolamine (DGA) andN,N-Bis(3-aminopropyl)methylamine (BAPMA) dicamba were applied simultaneously at 560 g ae ha–1in the center of two side-by-side 8-ha fields to vegetative glufosinate-resistant soybean. On the same day, a rate response experiment was established encompassing nine different dicamba rates of each formulation. Results from the rate response experiment indicate that soybean is equally sensitive to DGA and BAPMA dicamba. In 2015, a rain event occurring 6 to 8 h after application of the large drift trial probably limited off-target movement by incorporating some of the herbicide into the soil. As a result, secondary drift was less in 2015 than in 2016. However, minimal secondary injury (<5%) occurred 12 m farther into DGA dicamba plots in 2015. In 2016, secondary movement was decreased by 72 m when BAPMA dicamba was used compared to DGA dicamba. Appreciable secondary movement of both DGA and BAPMA dicamba is possible following in-crop applications of either formulated product to soybean in early to mid-summer. Additionally, the risk for secondary movement of BAPMA dicamba is slightly less than for DGA dicamba.

Dicamba Losses to Air after Applications to Soybean under Stable and Nonstable Atmospheric Conditions

“…Volatility of these new formulations, which have been shown to be less volatile than older dicamba formulations in extensive humidome research, has remained a point of concern (Long et al, 2016: Mueller, 2015). Recent studies of sensitive soybean plants placed in the field 30 min after dicamba applications has indicated that both formulations can volatilize in the field setting (Jones et al, 2019). Air sampling data from this study provide additional support for the ability of each formulation to volatilize in the field.…”

Off-target movement assessment of dicamba in North America

“…The tarp at this site was rested on the plants to ensure no risk for physical drift. In Arkansas, 19-L buckets covering three non-DR soybean plants were used similar to that used in other dicamba research (Jones et al 2019). Plant injury ratings were collected at 28 d after application (DAA) in Wisconsin and 21 DAA at all other sites.…”

“…To further reduce injury to sensitive plants due to OTM of dicamba, numerous restrictions have been added to the dicamba labels, including nozzle type, approved mixtures, exclusion of ammonium sulfate, carrier volume, boom height, application speed, wind speed and direction, and buffer zones. Research in Arkansas, Missouri, Tennessee, Nebraska, and Indiana has reported that under certain environmental conditions, these new formulations of dicamba can still volatilize and move to nontarget areas, even when applied according to the manufacturers' recommendations (Jones et al 2019;Norsworthy et al 2018). A national survey conducted by the University of Missouri in 2017 reported soybean injury on 1.3 million ha in the United States (Bradley 2017).…”

Herbicide drift exposure leads to reduced herbicide sensitivity in Amaranthus spp.

“…Spray drift is the part of the pesticide application deflected away from the target area during or following applications 5 . Glyphosate, 2,4-D, and dicamba spray drift have been reported to cause severe injury on sensitive vegetation and crops, especially when best practices are not adopted during applications [6][7][8][9][10][11][12][13] .…”