Response of dicamba-resistant soybean cultivars to postemergence dicamba dose exposure

Abstract Dicamba-resistant (DR) soybean cultivars are essential elements in managing broadleaf weeds in modern production systems. However, limited information is available regarding yield reductions associated with dicamba rates that were previously registered for postemergence weed control and off-label dicamba rates in these cultivars. This study aimed to characterize and quantify the effects of postemergence dicamba applications on two DR soybean cultivars. Field trials were conducted in 2022 and 2023, with dicamba applied at 0 to 1,440 g ae ha⁻¹ during the V5 to V6 stages. Visible injury increased with dicamba rate, reaching 18% (Cultivar A) to 20% (Cultivar B) at 1,440 g ae ha⁻¹ at 3 d after treatment, but symptoms declined to <10% by 4 wk after treatment (WAT). Chlorophyll fluorescence was not significantly affected at 2 and 4 WAT. Height reduction at 4 WAT occurred only at the highest dicamba rate (1,440 g ae ha⁻¹), but differences disappeared by maturity. Dry biomass reduction was also dose-dependent, reaching 16% for Cultivar A and 10% for Cultivar B at the highest rate. Pod reduction in DR soybean was minor (<3.5%) and not significant. Applications of dicamba from 288 to 864 g ae ha⁻¹ resulted in minimal yield reductions (<5%) and no significant biomass reduction. At a dicamba dose of 1,152 g ae ha⁻¹, yield reductions reached 7% and 9% for Cultivars A and B, respectively, while the highest rate (1,440 g ae ha⁻¹) resulted in yield reductions of 12% (Cultivar A) and 14% (Cultivar B). Despite over-the-top application restrictions, these results confirm that DR soybean cultivars tolerate rates (≤720 g ae ha⁻¹) of dicamba that were previously registered for postemergence weed control with minimal (<5%) yield reduction and recover rapidly from transient injury. However, applications above these rates can reduce yield by up to 14%, highlighting the importance of adhering to recommended dicamba use guidelines.