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Research Highlights

Research Highlights
Exploring Off-Target Movement and Management of Dicamba and 2,4-D Herbicides

The atmospheric condition of inversion plays a key role in off-target herbicide drift. Visible signs of inversion include low-lying fog. Photo: United Soybean Board

By Carol Brown

For several years, the use of the herbicides dicamba and 2,4-D has been a sensitive issue. Regulations and usage limitations are outcomes of unintended consequences of herbicide application, including off-target drift and crop damage in neighboring fields. 

Soybean researchers have been exploring these issues to find solutions that are acceptable to all stakeholders including the producer, the neighboring farmer, and the herbicide manufacturer.

Professor Kevin Bradley is the state Extension weed scientist in the Division of Plant Sciences and Technology at the University of Missouri. He has been researching weed management and the dicamba dilemma for several years. Through projects supported by the Missouri Soybean Merchandising Council and United Soybean Board, Bradley is finding answers to questions that continue to arise.

“For several years we’ve been looking at dicamba, its potential to move in the air, and other implications from its use,” says Bradley. “When Enlist soybeans came onto the market, we added 2,4-D herbicides to the study as well. Formulations of these herbicides have changed since weed scientists began looking at them years ago, but they still need to be evaluated for our conditions in Missouri and the Midwest.”

Bradley included weather and other environmental factors in the equation in order to determine how these herbicides were affecting the crop fields in which they were applied and their effects on neighboring fields.

“Environmental factors, including surface temperature inversions, play a huge role in this whole issue,” he says. “Dr. Mandy Bish and I started working with our Missouri state climatologist, Dr. Pat Guinan, on these issues in 2015 in anticipation of what we thought may be a problem with dicamba.”

Surface temperature inversions have been found to be a key player in off-target dicamba drifting. According to the National Weather Service, the atmospheric condition of inversion happens in stable air masses where cooler air is near the earth’s surface and warmer air is on top. Smoke and pollution, including herbicide droplets, are trapped in the cooler air and not allowed to rise up and away from the surface. Inversions also lead to air moving horizontally near the surface and create the potential to carry small droplets or vapor much farther than normal. Conditions for inversion include clear skies and calm winds near sunrise or sunset. 

Studying inversion was an extensive exercise in weather data analysis, Bradley says. He and his research team receive thousands of lines of weather data that come in every few minutes from weather stations across the state.

“We were one of the first to have data on the frequency of inversions and when they are most likely to happen, and we’re still studying this today. None of this would have been possible without the Missouri Soybean Merchandising Council’s funding that helped us add more weather stations around the state,” Bradley remarks. “We’ve learned more about inversions than we’ve ever known through the weather data we’ve collected.”

Farmer Recommendations and Cautions

“We encourage farmers not to do their herbicide applications during mid- to late-afternoon, because of the strong possibility of inversion setting in,” Bradley advises. “It seems to be a good time because it is calm with low winds, but these are indicators of inversions. With dicamba, it can escape as a gas. If the gas gets moved into the atmosphere, that’s when we can have wide-scale, off-target movement, which can affect large areas and acres of crop fields.”

Bradley and Bish’s early study results in Missouri regarding herbicide movement due to inversions resulted in an expanded research project funded by the United Soybean Board and led by Bryan Young at Purdue University. The multi-state research team explored 2,4-D secondary drift movement and whether 2,4-D volatilizes, meaning turn to vapor, like dicamba does. Bradley defines secondary drift as the movement of herbicide well after the sprayer has left the field and not due to wind.  

“We found that the likelihood of secondary drift from 2,4-D choline (the active ingredient in the Enlist products) to be much less likely of a problem than dicamba,” he says. “We detected 2,4-D in the air approximately 12 hours after application, but with dicamba, we were able to consistently detect this active ingredient in the air for 72 hours after application.” 

Over his years of research, Bradley says he has learned many things about dicamba behavior. One finding is that soil pH can play a role in dicamba volatility. The lower the pH, the more likely that dicamba can volatilize. Also, if the crop field is close to large waterbodies such as a river or lake, dicamba injury is more likely to occur due to cool air drainage.

As long as weeds continue to be a nuisance in crop fields, scientists like Bradley, Bish, Young, and others will continue to study how they can be managed best to benefit farmers, their crops and their profitability. 

This project was funded by the soybean checkoff. To find research related to this research highlight or to see other checkoff research projects, please visit the National Soybean Checkoff Research Database.