Strawberries, an important Florida crop, are grown on raised beds covered with plastic mulch. The plastic mulch provides good control of many weeds, but some problem species can emerge from the transplant hole during crop establishment. POST herbicide options for broadleaf weed control within the strawberry bed is limited to clopyralid, which only provides suppression. Strawberry canopy shielding may be responsible for the observed incomplete control with clopyralid application for problematic broadleaf weed species such as black medic and Carolina geranium. Two field experiments were established on mature strawberries to evaluate spray penetration through the canopy. The first examined spray penetration through the canopy of multiple strawberry cultivars at various distances from the crown. The second examined the effects of application volumes and nozzle selection on spray penetration. Cultivar selection had no effect on spray penetration through the canopy. In the first study, when applying at 281 L ha−1, the area around the planting hole (0 to 5 cm from the crown) had 8% coverage below the canopy while the area below the canopy edge (10 to 15 cm from the crown) had 27% coverage. In the second study, increasing the application volume from 187 to 375 L ha−1 increased coverage by 81%. Increasing the application volume from 375 to 740 L ha−1 increased coverage 33% with maximal coverage of 53% at 740 L ha−1. Nozzle type (standard even flat spray tip, Drift Guard, or TwinJet nozzles) did not affect coverage or deposition volume below the canopy. Overall, mature strawberry canopies demonstrated similar spray droplet penetration across cultivars with increased penetration with increased distance from the crown. Penetration increased with increasing application volume, but the nozzle types used in this experiment did not affect penetration. Additional research is needed to better define the effect of application volume on herbicide efficacy.

Sprayer applicator-controlled variables, such as nozzle selection and spray volume, will become increasingly important for making labeled POST applications of dicamba in next-generation cropping systems. A field experiment was conducted in 2013 and 2014 at the Northeast Research and Extension Center in Keiser, AR. Tank mixtures of Engenia (a new form of dicamba), glyphosate, glufosinate, and S-metolachlor were applied with TeeJet AIXR, AITTJ60, and TTI nozzles. Two nozzle sizes, 11003 and 11006, were used to vary spray volume from 94 L ha−1 to 187 L ha−1, respectively. For barnyardgrass, a significant decrease in control was observed when spray volume was reduced for glyphosate + dicamba in 2013. In 2014, an overall decrease in control was observed for the TTI nozzle when spray volume was reduced to 94 L ha−1, averaged across all herbicide treatments. The addition of the product S-metolachlor to glyphosate + glufosinate + dicamba significantly reduced the droplet spectra for all nozzle types. For example, adding S-metolachlor into the tank-mix decreased the volume median diameter (Dv50) for the TTI nozzle at 187 L ha−1 spray volume from 789 μm to 570 μm. The results from this research demonstrate that using low spray volume and coarser nozzles could reduce efficacy of the herbicides on the weed species evaluated. Nozzle selection and spray volume have key roles in maximizing efficacy of POST applications in dicamba-resistant crops. Additionally, evaluating droplet spectra of potential dicamba-containing tank-mixtures is critical for producing the desired droplet size to minimize off-target movement.

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.

Sprayer applicator–controlled variables, such as nozzle selection and ground speed, will become increasingly important for making labeled POST applications of dicamba in next-generation cropping systems. Typically, nozzle orifice sizes and ground speeds differ greatly between small-plot research applications, from which efficacy recommendations are made, and commercial field applications. However, little research has been conducted to compare applications made with backpack sprayers and tractor sprayers. Thus, a field experiment was conducted in 2013 and 2014 at the Northeast Research and Extension Center in Keiser, AR. Tank mixtures of Engenia™ (N, N-Bis-(aminopropyl) methylamine form of dicamba), Liberty (glufosinate-ammonium), and Liberty + Engenia were applied with TeeJet XR, TT, AIXR, AI, and TTI nozzles at 5 km h−1 and 20 km h−1. Two nozzle sizes (110015 and 11006 rated at 0.58 L min−1 and 2.27 L min−1 at 276 kPa, respectively) were used to keep spray volume constant at 141 L ha−1, whereas ground speed was varied. Weed control ratings were typically lower at 5 km h−1 than at 20 km h−1. For example, Palmer amaranth control 4 WAT in 2013 with glufosinate and the TTI nozzle was 89% at 5 km h−1 and 96% at 20 km h−1. More differences between speeds were observed for the coarser nozzles, such as the TTI and AI, as opposed to finer nozzles, such as the XR and TT. Results from this research suggest increasing orifice size increases droplet size and that other factors related to applications at faster speeds (e.g., higher droplet velocity, disruption of the crop canopy) may influence the efficacy of herbicide applications. However, increasing ground speed is not a recommended means for increasing efficacy of herbicide applications.