Due to non-Xtend soybeans’ sensitivity to dicamba, injury symptoms aren’t reliable indicators of yield loss.
Slight dicamba injury doesn’t always result in significant yield loss, according to published research. However, yield losses from dicamba exposure can be dramatic.
The level of yield loss depends on several factors. These include:
Soybean growth stage at the time of exposure.
Growing conditions for the remainder of the growing season.
Soybean growth stage
Research suggests that if minor exposure occurs during early vegetative development, yield loss is less likely compared to when soybean has entered reproductive development.
University of Nebraska researchers applied six micro-rates of dicamba to non-tolerant soybeans that ranged from 1/10 (equivalent to 3 tablespoons per acre) to 1/1000 of the labeled rate. These rates were applied at three different soybean stages:
- Early vegetative (V2)
- Late vegetative/early flowering (V7/R1)
- Full flowering (R2)
Researchers found non-tolerant soybeans were most sensitive to dicamba exposure in the V7/R1 stage. Soybeans exposed to 1/10 of the labeled Engenia rate at this stage yielded an average of only three bushels per acre (Table 1).
Early vegetative soybeans (V2) were least sensitive to dicamba exposure in this study. Yield loss at the 1/10 rate was still significant at 59 percent, but much less than soybeans exposed at V7/R1.
Soybeans exposed at full flowering had higher yields than those of V7/R1, but less than those exposed at V2.
Table 1 shows average yield and percent yield losses after University of Nebraska researchers applied 1/10 of the labeled Engenia rate at different soybean growth stages.
Table 1: Losses of non-tolerant soybean exposed to dicamba. Source: University of Nebraska.
|Growth stage at application||Average yield||Average yield loss|
|Check (no dicamba)||59 bushels per acre||0%|
|Early vegetative (V2)||24 bushels per acre||59%|
|Late vegetative/early flowering (V7/R1)||3 bushels per acre||95%|
|Full flowering (R2)||19 bushels per acre||68%|
Dicamba exposure dose
Similar to injury symptoms, the impact on yield increases with the level of dicamba exposure.
Findings from the Nebraska study include:
Even very small concentrations of dicamba (1/1000 of the labeled rate) at V2 reduced yields by an average of three bushels per acre.
As the exposure increased to 1/10 of the labeled rate, yield losses approached 60 percent (Figure 2).
Similar research at Purdue in 2009 and 2010 studied eight dicamba rates ranging from 1/10,000 to 1/25 of the labeled rate.
They found dicamba reduced yields of non-tolerant soybeans by 10 percent with exposures as low as 1/1064 to 1/510 of the labeled rate. Keep in mind, a 1/1000 rate is equivalent to 1/10 of one teaspoon per acre.
These studies looked at single exposures of dicamba at different rates and soybean growth stages. To date, no data describe yield effects on soybean exposed more than once.
Drought conditions during reproductive growth stages can influence yield.
In the Purdue study, one of the site-years received only 2.75 inches of rain from July through September. The drought stress amplified dicamba exposure’s impact on yield. Even at an extremely low rate of 1/3333 of the labeled rate, yields were reduced by 10 percent.
Hager, A. (2017). The dicamba dilemma in Illinois: Facts and speculations.
Ikley, J. & Johnson, B. (2017). Response of Roundup Ready soybean yield to dicamba exposure.
Knezevik, S. (2018). Research on the impact of dicamba micro-rates on non-tolerant soybean.
Reviewed in 2021