Enlist^® cotton is a transgenic crop enabling over-the-top use of 2,4-D choline, glufosinate, and glyphosate for weed control, whereas non-Enlist^® cotton can be highly sensitive to low rates of such herbicides. When 2,4-D choline is applied to Enlist^® cotton fields, injury to the sensitive crops in the neighboring area may be caused by off-target movement of the herbicide. Physical drift and volatilization of 2,4-D choline can be major factors causing off-target movement of the herbicide applied. A field experiment was initiated at Marianna, AR, in 2018, and repeated at Keiser, AR, in 2019, 2020, and 2021, to evaluate injury to nearby sensitive cotton following an application of 2,4-D choline (1,065 g ai ha⁻¹) in mixture with glufosinate-ammonium. Occurrence and movement of volatile residues of 2,4-D from the treated area were also evaluated using high-volume air samplers established in the center and cardinal directions of the treated area 30 min after herbicide application. The herbicide mixture was applied to 4,000 m² in the center of a 40,000-m² XtendFlex cotton field when the crop reached the four- to six-leaf stage. Sensitive cotton plants in three downwind transects that were covered for 30 min after application did not show injury by 2,4-D, whereas non-covered plants along each of the downwind transects were injured. Non-covered plants in the upwind direction during application also showed no injury. The in-center air concentration of 2,4-D sampled for 72-h ranged from 3.3 to 4.3 ng m⁻³, and most volatile residues (5.0−25.5 ng m⁻³) were detected in air samplers established in the downwind direction. The overall results demonstrate that physical drift of 2,4-D choline along the downwind direction within 30 min after application is a major factor causing injury to sensitive cotton in the neighboring area following application of the herbicide. Moreover, understanding wind direction and possible shift throughout the entire application process is crucial to mitigate the herbicide damage by physical drift.

Michael Houston

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