Soybean Response to Dicamba Applied at Vegetative and Reproductive Growth Stages

Abstract

Availability of soybean with dicamba resistance will provide an alternative weed management option, but risk of dicamba injury to sensitive crops from off-target movement and spray tank contamination is of concern. Research conducted at multiple locations and years evaluated soybean injury and yield response to POST applications of the diglycolamine salt of dicamba. Dicamba was applied at the two to three trifoliate stage (V3/V4) at 4.4, 8.8, 17.5, 35, 70, 140, and 280 g ae ha−1(1/128 to 1/2 of the recommended use rate of 560 g ae ha−1). Soybean injury 7 d after application was 20% following dicamba at 4.4 g ha−1and increased to 89% at 280 g ha−1. At 14 d after application, injury for the same rates increased from 39 to 97%. In a separate study, dicamba was applied at first flower (R1) at 1.1, 2.2, 4.4, 8.8, 17.5, 35, and 70 g ha−1(1/512 to 1/8 of use rate). Soybean injury 7 d following dicamba application was 19% at 1.1 g ha−1and increased to 64% at 70 g ha−1. For the same rates of dicamba, injury from 7 to 14 d after application increased no more than 4 percentage points. For dicamba rates in common for the timing studies, soybean injury 14 d after treatment was greatest for application at V3/V4, but the negative effect on mature soybean height and yield was greatest for application at R1. For dicamba at 4.4 g ha−1(1/128th of use rate), soybean yield was reduced 4% when applied at V3/V4 and 10% when applied at R1. For 17.5 g ha−1dicamba (1/32 of use rate), yield was reduced 15% at V3/V4 and 36% at R1. Based on yield reductions for 4.4 and 17.5 g ha−1dicamba, soybean at flowering was around 2.5 times more sensitive compared with vegetative exposure.