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

Abstract

BACKGROUND 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 ae ha⁻¹) plus glyphosate (8.7 g ae 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. RESULTS Spray coverage decreased with carrier volume and ranged from 21 to 3%. Conversely, soybean injury increased as carrier volume decreased. Soybean height 14 d 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. CONCLUSION 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.