Glyphosate-Tolerant Soybean Yield Loss and Yield Response to Micro-Rates of 2,4-D as Influenced by Growth Stage

Abstract

With the introduction of 2,4-D-tolerant crops, the use of 2,4-D and the risk of drift in non-2,4-D tolerant crops including soybean are likely to increase. To understand the impact of 2,4-D drift on glyphosate-tolerant (GT) soybean, a study using a randomized complete block design with four replications and a split-plot arrangement of treatments was conducted in 2019 near Concord, NE. 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. Soybean injury was visually assessed at 7, 14 and 21 days after treatment (DAT). Grain yield was also collected using a small-plot combine. In general, there was an increase in soybean injury and reduction in grain yield with increase in 2,4-D dose. GT soybean was more sensitive to 2,4-D injury at R2 than V2 and R1 stages. Less than 1/10 of the label recommended dose of 2,4-D caused 5-20% injury to GT soybean. Based on estimates of the effective dose of 2,4-D required to cause 5% injury, GT soybean was 1.4- and 1.2-fold more sensitive to 2,4-D at R2 (44.88 g ae ha-1) than V2 (61.78 g ae ha-1) and R1 (53.12 g ae ha-1) stage, respectively. This increase in GT soybean sensitivity to 2,4-D injury has ultimately resulted in a significant reduction in grain yield especially at the R2 stage. Preliminary data analysis showed that 2,4-D dose of 0.33 g ae ha-1 reduced GT soybean yield at R2 by 5% (0.22 Mg ha-1) compared with 54.58 and 1.77 g ae ha-1 at the V2 and R1 stage, respectively. These results show that 2,4-D drift poses a risk to GT soybean and can result in significant yield losses; therefore, it is crucial that 2,4-D drift is prevented especially at the R2 stage.