Persistence of dicamba residue in harvested soybeans

Adoption of dicamba-tolerant soybeans contributed to widespread reports of chemical trespassing on adjacent, sensitive soybeans. Reports of the impact of dicamba on sensitive soybeans (Glycine max L.) have been well documented; however, the potential for dicamba carryover into harvested beans from sensitive plants has largely been overlooked. Field trials in central Missouri focused on assessing the injury and yield response of sensitive soybeans to concentrations of dicamba as low as 0.25% of the use rate (10 µL L⁻¹ dicamba). In both 2018 and 2019, dicamba-sensitive soybeans were planted in conventional row spacing and treated with 10–300 µL L⁻¹ dicamba at both V3 and R1 soybeans. Dicamba symptoms were visible in less than 7 days after application (DAA); significant injury was observed at 10 µL L⁻¹ and persisted through the duration of the study (28 DAA). Injury levels reached almost 50% with 300 µL L⁻¹ dicamba. Step-wise increases in soybean yield losses occurred with increasing dicamba concentrations and reached 50% with 300 µL L⁻¹ dicamba. Yield losses were up to 10% greater for R1 versus V3 soybeans treated with the same dicamba concentration. Dicamba residues in bean tissue ranged from 0.72 to 0.81 mg kg⁻¹ for 150 to 300 µL L⁻¹ dicamba, and residues were similar for beans at both V3 and R1. Dicamba persisted in beans harvested up to 122 days after plant exposure to dicamba. Although dicamba residues were within limits established by the EPA (10 mg kg⁻¹), residues exceeded that allowed in marketed, organic soybeans (0.5 mg kg⁻¹).