Effect of carrier volume and spray quality on glyphosate-resistant soybean response to sublethal dicamba exposure

Field experiments were conducted across three sites in Mississippi in 2018 to evaluate carrier volume and spray quality effects on glyphosate-resistant soybean response to dicamba. Treatments consisted of dicamba (5.6 g a.e. ha^-1 ) plus glyphosate (8.7 g a.e. ha^-1 ) applied to soybean at R1 using 140, 105, 70, 35, 14, or 7 L ha^-1 . Each carrier volume was applied with TT11002 and XR110015 nozzles which resulted in Fine and Coarse spray qualities, respectively. A colorimetric dye was included in spray solutions to quantify spray coverage of each treatment.

Spray coverage decreased with carrier volume and ranged from 21% to 3%. Conversely, soybean injury increased as carrier volume decreased. Soybean height 14 days after treatment (DAT) was reduced 34% to 37% from carrier volumes of 70 to 140 L ha^-1 ; however, carrier volumes of 14 and 7 L ha^-1 resulted in 45% height reductions. By 28 DAT soybean height was similar among volumes of 35 to 140 L ha^-1 (39% to 42% reduction); however, volumes of 14 and 7 L ha^-1 resulted in 46% and 51% reductions, respectively. Grain yield was reduced 14% from treatment at 140 L ha^-1 and reductions increased with decreased carrier volume to 41% loss at 7 L ha^-1 . Averaged across carrier volumes, Fine and Coarse sprays caused 30% and 26% yield loss, respectively.

These data suggest that carrier volume profoundly affects soybean response to dicamba. Therefore, soybean response to sublethal dicamba doses applied at a constant carrier volume may not reflect physical drift exposure. © 2021 Society of Chemical Industry.