Soybean Tolerance to ultra-low doses of dicamba: Hormesis or not

The widespread use of dicamba across the soybean growing regions of the United States resulted in increased off-target movement of this herbicide to susceptible vegetations. There are speculations that a drift of sublethal (ultra-low) doses of dicamba can enhance soybean growth and yield through a phenomenon called hormesis. Field studies were conducted during 2018 and 2019 to determine whether soybean growth and yield can be enhanced with ultra-low doses (0.0112–56 g ae ha-1) of dicamba, applied at three growth stages (V2, R1 and R2). There was no evidence that the ultra-low doses of dicamba (0.0112–56 g ae ha-1) increased any of the observed parameters, irrespective of the application timing. Doses of 0.56 g ae ha-1 and lower did not have a negative impact on plant dry matter, height and grain yield, even if causing visible symptomology. The estimated effective doses for 50% reduction in various growth parameters and yield (ED50) indicate that soybean is more sensitive at the R1 compared to the V2 or R2 stages. The estimated visual injury threshold that caused 5% yield loss were 53, 36 and 21% for V2, R1 and R2 stage of dicamba exposure, respectively. The proposed “No Observed Adverse Effect Level” (NOAEL), which is a safe exposure threshold, ranged from 0.0003 to 0.033 g ae ha-1. However, efforts should be made to avoid dicamba drift to susceptible soybean as there is no evidence of enhanced growth and yield with the ultra-low doses