LAUSR

Field experiments were conducted across multiple sites in 2012 and 2013 to describe sensitivity of soybean to 2,4-D (six doses) and dicamba (seven doses) at V3 and R1 growth stages. Further experiments were conducted under greenhouse conditions in 2017 and 2018 to compare soybean response to several dicamba herbicides across a broader range of doses than those tested in the field. Soybean yield loss was 6.1-times greater from 2,4-D exposure at V3 compared to R1 and 1.4-times greater from dicamba exposure at R1 than at V3. In V3 exposures, soybean was 15.4-times more sensitive to dicamba than 2,4-D and 134.4-fold more sensitive to dicamba when exposed at R1. Plant injury and height correlations to grain yield resulted in coefficients ranging from 0.65 to 0.91. In greenhouse experiments, five dicamba products were tested at up to 19 doses and as low as 0.002 g ae ha-1 (3.6 x 10-6 % of maximum single use-rate); however, no differences were observed among formulations used in dicamba-resistant crops versus traditional formulations. A no observable effects dose was not identified due to responses observed even at the lowest doses tested although hormesis effects were observed in plant height. These data suggest that the sensitivity of soybean to dicamba is much greater than what has previously been reported. However, as has been indicated by previous work that injury does not always result in yield loss.