Dicamba Air Concentrations in Eastern Arkansas and Impact on Soybean

Abstract Damage to non–dicamba resistant (non-DR) soybean [Glycine max (L.) Merr.] has been frequent in geographies where dicamba-resistant (DR) soybean and cotton (Gossypium hirsutum L.) have been grown and sprayed with the herbicide in recent years. Off-target movement field trials were conducted in northwest Arkansas to determine the relationship between dicamba concentration in the air and the extent of symptomology on non-DR soybean. Additionally, the frequency and concentration of dicamba in air samples at two locations in eastern Arkansas and environmental conditions that impacted the detection of the herbicide in air samples were evaluated. Treatment applications included dicamba at 560 g ae ha–1 (1X rate), glyphosate at 860 g ae ha–1, and particle drift retardant at 1% v/v applied to 0.37-ha fields with varying degrees of vegetation. The relationship between dicamba concentration in air samples and non-DR soybean response to the herbicide was more predictive with visible injury (generalized R2 = 0.82) than height reduction (generalized R2 = 0.43). The predicted dicamba air concentration resulting in 10% injury to soybean was 1.60 ng m–3 d–1 for a single exposure. The predicted concentration from a single exposure to dicamba resulting in a 10% height reduction was 3.78 ng m–3 d–1. Dicamba was frequently detected in eastern Arkansas, and daily detections above 1.60 ng m–3 occurred 17 times in the period sampled. The maximum concentration of dicamba recorded was 7.96 ng m–3 d–1, while dicamba concentrations at Marianna and Keiser, AR, were ≥1 ng m–3 d–1 in six samples collected in 2020 and 22 samples in 2021. Dicamba was detected consistently in air samples collected, indicating high usage in the region and the potential for soybean damage over an extended period. More research is needed to quantify the plant absorption rate of volatile dicamba and to evaluate the impact of multiple exposures of gaseous dicamba on non-targeted plant species.