The introduction of 2,4-D–resistant soybean will provide an additional POST herbicide site of action for control of herbicide-resistant broadleaf weeds. The introduction of this technology also brings concern of off-site movement of 2,4-D onto susceptible crops such as sensitive soybean and tomato. The 2,4-D formulation approved for use in 2,4-D–resistant soybean restricts application of the herbicide to nozzles that produce very coarse to ultra-coarse droplet spectrums. The use of larger droplet spectrums for broadcast applications can reduce herbicide deposition onto target weeds and thus influence herbicide efficacy. Field experiments were conducted to evaluate the influence of nozzle design on herbicide deposition onto target plants and the resulting efficacy of a POST application of 280 g ha−1 glyphosate plus 280 g ha−1 2,4-D. The TTI11004 nozzle produced an ultra-coarse droplet spectrum and reduced coverage and deposition density on spray cards as compared with the XR11004 and TT11004 nozzles that produced medium droplet spectrums. The AIXR11004 nozzle also reduced deposition density on spray cards but did not reduce coverage. Herbicide solution deposition onto glyphosate-resistant Palmer amaranth, tall waterhemp, giant ragweed, and horseweed ranged from 0.28 to 0.72 µl cm−2 and was not influenced by nozzle design. Herbicide efficacy was reduced by the TTI11004 nozzle on Palmer amaranth and horseweed compared with the AIXR11004, TT11004, and XR11004 nozzles when applications were made to either high densities of plants or plants exceeding the labeled height. The use of the AIXR11004 and TTI11004 nozzles that are listed as approved nozzles for glyphosate plus 2,4-D applications on 2,4-D–resistant soybean did not reduce herbicide deposition onto four of the most troublesome broadleaves and did not reduce herbicide efficacy when applied in conjunction with lower weed densities and smaller weeds.

Dicamba-resistant soybean technology provides an additional site of action for POST control of herbicide-resistant broadleaf weeds in soybean but also raises concern of off-site movement and damage to sensitive crops in adjacent fields. Dicamba formulations approved for use on dicamba-resistant soybean require applicators to use nozzles producing large droplets to reduce the risk of spray-particle drift. The use of nozzles with relatively larger droplet spectra can reduce herbicide deposition on target weeds, especially if a filtering effect from the crop canopy occurs. Experiments were conducted to evaluate the influence of broadcast nozzle design on the deposition and efficacy of 280 g ha−1 glyphosate plus 140 g ha−1 dicamba applied POST to four herbicide-resistant weed species. The TTI11004 nozzle, the original nozzle labeled for dicamba applications on dicamba-resistant soybean, reduced deposition coverage and density on spray cards compared with the TT11004 and XR11004 nozzle. The AIXR11004 nozzle produces a very coarse droplet spectrum and did not reduce coverage on spray cards, though it did reduce deposition density. Herbicide solution deposition onto Palmer amaranth, tall waterhemp, giant ragweed, and horseweed ranged from 0.41 to 0.52, 0.55 to 0.87, 0.49 to 0.58, and 0.38 to 0.41 µl cm−2, respectively. Nozzle design and droplet spectrum did not influence the deposition of herbicide solution onto the target weed, as all nozzles were equivalent for all species and site-years. Herbicide efficacy was not influenced by nozzle design, as weed control and plant height reduction were similar for all species. The results of this experiment show that the use of the TTI11004 nozzle for dicamba applications to dicamba-resistant soybean will provide acceptable herbicide deposition and efficacy when applied under the label requirements of weed height and carrier volume.