Surface Temperature Inversions and Risk of Off-Target Herbicide Damage in the Soybean- and Cotton-Growing Regions of the US

Abstract

Core Ideas Temperature inversions form most nights between 15 May and 30 June. Temperature inversions commonly form before sundown under winds greater than 3 mph. Frequency of temperature inversions in 2017 were similar to those in 2012 through 2016. The approval of the dicamba-tolerant trait in soybean and cotton as well as new dicamba spray formulations to control weeds in cotton and soybean fields has resulted in broad usage of dicamba-based herbicides. As dicamba-based herbicide usage increased, reports of non-target crop damage dramatically increased. Temperature inversions have been implicated in this off-target damage. Short-term climatologies of surface inversions at six locations across the soybean- and cotton-growing regions of the US were developed for the period 15 May through 30 June between 2012 and 2017 using National Climate Reference Network measurements. Inversions occurred most nights, occasionally collapsing and then reforming in the same night. Inversions often formed before or around sunset as the surface temperature cooled more rapidly than the overlying air. Inversion development was similar for most of the soybean-growing region with the greatest change in temperature difference between the air and the ground occurring at sundown. Higher wind speeds corresponded to decreased inversion intensity and shortened the duration of the inversion. Differences in inversion conditions in 2017 versus those of 2013–2016 were not consistent; consequently, the increased occurrence of off-target damage by dicamba in 2017 cannot be explained by an increase in the number or intensity of inversions or differences in winds during inversions. The presence of inversions when they are not expected (before sunset, after sunrise and when winds exceed 3 mph) increases the risk of unexpected off-target crop damage. Since inversions are commonly a local phenomenon, predicting inversion occurrence requires understanding the conditions at specific locations.