Influence of Droplet Size on Efficacy of the Formulated Products Engenia™, Roundup PowerMax®, and Liberty®

As auxin-type herbicide-resistant crops become commercially available, nozzle selection will become a highly important variable for maintaining efficacy of herbicide solutions while minimizing off-target movement. Field experiments were conducted in 2013 and 2014 in Keiser, AR, to evaluate interactions among the N,N-bis-(aminopropyl)methylamine form of dicamba formulated as Engenia™, the potassium salt of glyphosate formulated as Roundup PowerMax®, and glufosinate formulated as Liberty® applied with three different nozzle types. Three TeeJet nozzles with an 11004 orifice (Turbo TeeJet [TT], Air Induction Extended Range [AIXR], and Turbo TeeJet Induction [TTI]) were used. To supplement the field data, droplet spectra for each nozzle and tank mixture combination were determined at the West Central Research and Extension Center in North Platte, NE. For most herbicide treatments and nozzle combinations, Palmer amaranth control 4 wk after treatment was > 95% both years. In 2013, TT nozzles provided 96% control of barnyardgrass and TTI nozzles provided 89% control, averaged across herbicides, except for Engenia alone. A similar effect of nozzle selection was observed in 2014. When treatments were applied to 20-cm-tall barnyardgrass in 2014, compared with 8-cm-tall plants in 2013, an antagonistic effect was observed when Engenia was tank-mixed with Roundup PowerMax. The weed control data correlated with the droplet spectrum analysis such that as volume median diameter (Dᵥ₅₀) increased from TT nozzles to the TTI nozzles, efficacy decreased for most tank mixtures. Results from the droplet analysis showed that Dᵥ₅₀ relative to water decreased for Liberty alone and not when tank-mixed with Engenia or Roundup PowerMax. These results suggest that nozzle selection will play a key role in maximizing efficacy of POST applications in dicamba-resistant crops. Additionally, evaluating droplet spectra of potential dicamba-containing tank mixtures is critical for producing desired droplet size to minimize off-target movement.Nomenclature: Dicamba; glufosinate; glyphosate; barnyardgrass, Echinochloa crus-galli (L.) Beauv.; Palmer amaranth, Amaranthus palmeri S. Wats.