A field spray drift study to determine the downwind effects of isoxaflutole herbicide to nontarget plants

Abstract Spray drift buffers are often required on herbicide labels to prevent potential drift effects to nontarget plants. Buffers are typically derived by determining the distance at which predicted exposure from spray drift equals the ecotoxicology threshold for sensitive plant species determined in greenhouse tests. Field studies performed under realistic conditions have demonstrated, however, that this approach is far more conservative than necessary. In 2016, the US Environmental Protection Agency estimated that isoxaflutole (IFT), a herbicide used to control grass and broadleaf weeds, could adversely affect downwind nontarget dicot plants at distances of ≥304 m from the edge of the treated field due to spray drift. This prediction implies that a buffer of at least 304 m is required to protect nontarget plants. To refine the predicted buffer distance for IFT, we conducted a field study in which sensitive nontarget plants (lettuce and navy bean, two to four leaf stage) were placed at various distances downwind from previously harvested soybean fields sprayed with Balance® Flexx Herbicide. The test plants were then transported to a greenhouse for grow out following the standard vegetative vigor test protocol. There were three trials. One had vegetation in the downwind deposition area (i.e., test plants placed in mowed grass; typical exposure scenario) and two had bare ground deposition areas (worst-case exposure scenario). For both plant species in bare ground deposition areas, effects on shoot height and weight were observed at 1.52 m but not at downwind distances of ≥9.14 m from the edge of the treated area. No effects were observed at any distance for plants placed in the vegetated deposition area. The field study demonstrated that a buffer of 9.14 m protects nontarget terrestrial plants exposed to IFT via spray drift even under worst-case conditions. Integr Environ Assess Manag 2022;18:757–769. © 2021 Bayer. Integrated Environmental Assessment and Management published by Wiley Periodicals LLC on behalf of Society of Environmental Toxicology & Chemistry (SETAC). KEY POINTS A field study was conducted in which sensitive non-target plants (lettuce and navy bean, 2–4 leaf stage) were placed at various distances downwind from previously harvested soybean fields sprayed with isoxaflutole (IFT), an herbicide used to control grass and broadleaf weeds. Significant effects of IFT spray drift were observed for only a short distance downwind of the treated area (i.e., 1.52 m in the worst-case trials where test plants were placed on bare ground). No effects were observed downwind of the treated area in the more realistic trial where test plants were placed in a vegetated field margin. The no observed effect distance for IFT was 9.14 m, at least 33-fold less than the no effect distance predicted by the US EPA using their standard approach of comparing a sensitive effect endpoint from a greenhouse vegetative vigor study to a Tier 1 AgDrift spray drift curve.

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