Assessment of Dicamba and 2,4-D Residues in Palmer amaranth and Soybean

Off-target movement of 2,4-D and dicamba are sometimes to blame as the cause of symptoms observed in weeds growing in production fields. Pesticide regulatory authorities routinely sample tissue of weeds or crops from fields under investigation for potential illegal use of auxin herbicides. This research aimed to determine if analytical tests of herbicide residue on soybean or Palmer amaranth vegetation treated with dicamba or 2,4-D could be used to differentiate between rates applied and how the residue levels decay over a one-month interval. Four rates of each herbicide (1X, 0.1X, 0.01X, and 0.001X) were applied, with a 1X rate of dicamba and 2,4-D assumed to be 560 and 1065 g ae ha-1, respectively. Experiments included dicamba- and 2,4-D-resistant soybean (Xtend and Enlist traits, respectively) and Palmer amaranth categorized by size (8-15 cm, 20-30 cm, and 35-50 cm in height). Analytical results showed that herbicide residues were detected above detection limits of 0.04 µg g-1 for dicamba and 0.004 µg g-1 for 2,4-D, respectively, particularly for samples treated with a 1X and 0.1X rate of dicamba or 2,4-D. Non-detections were frequent, even as early as 2 to 3 days after treatment (DAT), with 0.01X and 0.001X rates of 2,4-D or dicamba. Dicamba residues declined rapidly on Xtend soybean treated with dicamba, and 2,4-D residue in Enlist soybean. The severity of auxin symptomology generally agreed with the ability to detect dicamba or 2,4-D residue in plant tissue for Palmer amaranth, while, for soybean, this was not always the case. Hence, detecting dicamba or 2,4-D residues in both Palmer amaranth and soybean vegetation, along with visible symptoms on both plants during investigations, would generally indicate an earlier direct application of the auxin herbicide rather than off-target movement being the cause of detection.