Soybean Yields as Influenced by Ultra-Micro Rates of Dicamba: Hormesis or Not?

Abstract

There are speculations that a drift of sub-lethal or ultra-low doses of dicamba herbicides to soybean can increase the yield through the phenomenon called hormesis. Thus, there is a need to evaluate the impact of ultra micro-rates of dicamba on yields of sensitive soybean. Field study was conducted in 2018 and 2019 at Concord, NE. The study was arranged as a split-plot design with ten dicamba micro-rates, 3 application times and 4 replications. Dicamba rates included 0; 1/10; 1/100; 1/1000; 1/5000; 1/10000; 1/20000; 1/30000; 1/40000 and 1/50000 of the 560 g ae ha-1 (label rate) of XtendiMax. The 3 application times were V2 (2nd trifoliate), R1 (beginning of flowering) and R2 (full flowering) stages of soybean development. Application of 1/5000 to 1/10 of dicamba label rate caused 20 to 80% visual injury with the greatest injury at R1. A 1/10 of the dicamba label rate could cause 23 to 78% soybean yield loss depending on the growth stage of exposure; with the greatest yield loss (78%) at the R1 stage. In general, our preliminary study suggested that there was no evidence that sub-lethal doses of dicamba could increase the yield of soybean irrespective of the growth stage of dicamba exposure, suggesting that there is no hormesis occurring.