| Summary: | The adoption of auxin-tolerant crops has increased awareness regarding herbicide off-target movement. Deposition aids are promoted as a possible solution to off-target movement, although their effect on spray canopy deposition are not well understood. Studies were conducted to determine the impact of deposition aids tank-mixed with herbicides on spray droplet size and canopy deposition. Commonly used herbicides were applied on soybean and cotton in combination with deposition aids (oil, polymer, and guargum). Interactions between herbicide solution and deposition aid influenced droplet size parameters for both cotton and soybean herbicides tested herein (<i>p</i> ≤ 0.0001). Generally, the addition of polymer and guargum deposition aids increased spray droplet size, whereas the addition of oil deposition aid decreased droplet size for some treatments. When herbicides were combined, the inclusion of deposition aids did not influence overall spray deposition on cotton (<i>p</i> = 0.82) and soybean (<i>p</i> = 0.72). When herbicide solutions were evaluated individually, the advent of deposition aids had inconsistent results with cotton and soybean spray deposition being unaffected, increased, or even decreased depending on the herbicide solution tested. For example, the polymer-based deposition aid increased spray deposition on cotton for applications of glyphosate + dicamba + <i>S</i>-metolachlor resulting in 1640.6 RFU (relative fluorescence units). However, the same deposition aid decreased spray deposition on cotton for applications of glyphosate + dicamba + acetochlor (1179.3 RFU). Although deposition aids influenced spray deposition on cotton and soybean for some herbicide combinations, their use should be determined on a case-by-case scenario.
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