Yield of Dicamba-Tolerant Soybean as Influenced by Micro-Rates of 2,4-D

Abstract

Like other auxin herbicides, 2,4-D is typically associated with increased risk for drift-related damage to non-target crops. A study was conducted in 2019 near Concord, NE to investigate the impact of 2,4-D micro-rates on dicamba-tolerant (DT) soybean yield. The experiment used a randomized complete block design (RCBD) with four replications and a split-plot arrangement. Main plots consisted of three 2,4-D application times [second trifoliate (V2); beginning of flowering (V7/R1); and full flowering (R2)] and subplots consisted of six micro rates of 2,4-D (1/5; 1/10; 1/50; 1/100; 1/500; and 1/1000 of the label recommended dose of 1,120 g ae ha-1) and a check with no herbicide applied. Crop injury was visually assessed at 7, 14 and 21 days after treatment (DAT). Grain yield was also collected. Increase in 2,4-D dose increased soybean injury and reduced yield. Less than 1/10 of the label recommended dose of 2,4-D caused 5-20% injury regardless of application time. V2 and R2 were 1.9- and 2.6-fold, respectively, more sensitive to 2,4-D injury than the R1 stage at 21 DAT. The effective dose of 2,4-D required to cause 5% soybean injury at 21 DAT at V2, R1 and R2 was 49.27, 94.26 and 36.59 g ae ha-1, respectively. In regards to yield reduction, the V2 and R1 stages were 4.2- and 3.5-fold, respectively, more sensitive to 2,4-D than the R2 stage. Preliminary data analysis showed that dose of 2,4-D of 7.17-8.72 g ae ha-1 reduced soybean yield by 5% (0.2 Mg ha-1) at the V2 and R1 stages. These results show that off-target movement of 2,4-D can significantly reduce soybean yield and therefore should be avoided.