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Uncovering dicamba’s wayward ways

Dicamba injury
Figure 1: Dicamba injury to soybean in a southwestern Minnesota field in 2017. Photo: Stephan Melson.

Historically, dicamba has been a challenging herbicide to keep in place. Its high vapor pressure makes it one of the most volatile herbicides.

At the same time Xtend soybean was released, a dicamba-tolerant variety, new formulations and labels were developed to contain dicamba's wayward ways.  

This new technology includes formulations with VaporGrip Technology (Xtendimax and FeXapan) and a new salt formulation (Engenia) designed to reduce volatility.

However, reduced volatility doesn’t mean no volatility. Recently published research found that non-Xtend beans showed dicamba exposure symptoms at 1/20,000 of the 1x rate, which is 0.5 pounds of acid equivalent (ae) formulation per acre. When you factor in even a small amount of volatility into this finding, you can understand the challenge of containing dicamba’s wayward ways.

Scale of off-target movement

Off-target dicamba movement wasn’t confined to just Minnesota during the 2017 growing season. According to the University of Missouri, there were 2,708 cases of dicamba-injured soybeans reported throughout the United States.

The area included 25 states and spanned from the Dakotas to Oklahoma to the East Coast. It’s estimated there were around 3.6 million acres of injured soybean. In Minnesota, 250 cases were reported to the Minnesota Department of Agriculture during 2017. This represents 265,000 acres of dicamba-injured soybean.

Routes of exposure

Off-target dicamba movement is possible via particle drift, tank contamination and volatilization (vapor drift). However, identifying how exposure occurred isn’t always obvious or easy.

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Temperature inversions

Temperature inversions occur when warm air rises into the atmosphere and cool air settles near the ground. With the light, warm air above the cool, denser air, no air-mixing occurs.

When herbicides are applied during a temperature inversion, small droplets can remain suspended in the air for long periods of time. These concentrated particles can then drift out of the target area when winds begin to move the following morning. Where and how far they move depends on wind speed and direction.

Typically, temperature inversions start at dusk and break up when air begins mixing again at sunrise.

The dicamba label doesn’t allow applications during temperature inversions. In addition, dicamba can only be applied between sunrise and sunset.

Jeffrey Gunsolus, Extension weed scientist

Reviewed in 2018

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