Exposure of soybean to dicamba can result in leaf malformation and sometimes yield loss, but it is unclear how yield components are affected by exposure to low quantities of this herbicide. The objectives were to characterize soybean injury and quantify changes in seed yield and yield components of soybean plants exposed to dicamba, and determine if seed yield loss can be estimated from visual injury ratings. Nine dicamba rates (0, 0.06, 0.23, 0.57, 1.1, 2.3, 4.5, 9.1, and 22.7 g ae ha⁻¹) were applied at three growth stages (V2 – two trifoliates, V5-five trifoliates, or R2-full flowering soybean) to Beck's brand ‘342NRR’ soybean planted near Lafayette, IN, in 2009 and 2010 and near Fowler, IN, in 2009. Visually estimated soybean injury of 20% at the V2, V5, or R2 timing was 0.676 to 0.937 g ha⁻¹ dicamba at 14 d after treatment (DAT) and 0.359 to 1.37 g ha⁻¹ dicamba at 28 DAT. Seed yield was reduced by 5% from 0.042 to 0.528 g ha⁻¹ dicamba and a 10% reduction was caused by 0.169 to 1.1 g ha⁻¹ dicamba. The number of seeds m⁻², pods m⁻², reproductive nodes m⁻², and nodes m⁻² were the most sensitive yield components. Path analysis indicated that dicamba reduced seeds m⁻², pods m⁻², reproductive nodes m⁻², and nodes m⁻² which were the main causes of seed yield loss from dicamba exposure. The correlation of seed yield loss and visual soybean injury was significant (P < 0.0001) for both the V2 treatment timing (R²  =  0.92) and the V5 and R2 treatment timings (R²  =  0.91). Early-season injury rating of 8% at the V2 treatment and 2% at the V5 or R2 treatments caused 10% or more yield loss.

Nomenclature: Dicamba, soybean, Glycine max (L.) Merr.