Herbicide Environmental Impact Mitigation

The interactions of different herbicides and what they mean for herbicide drift are being studied. In recent years, herbicide drift has caused more damage to soybean fields and other crops and trees across the Midwest particularly plant grown from seeds that have not been genetically modified to be herbicide-tolerant. Drift onto unintended plants causes leaves to curl and shrivel, potentially causing permanent crop damage.

To learn more about how different chemical agents interact in herbicide formulations, a team of researchers at Washington University in St. Louis’ McKelvey School of Engineering is developing a framework to understand how the pieces fit together, according to Kimberly M. Parker, assistant professor of energy, environmental, and chemical engineering. Stephen M. Sharkey, a fourth-year doctoral student in Parker’s lab, and Brent J. Williams, associate professor of energy, environmental, and chemical engineering, are her collaborators in this work. Their findings appeared in the journal Environmental Science & Technology.

By 2020, approximately 90% of all corn, cotton, and soybeans planted in the United States would have been genetically modified to tolerate one or more herbicides such as glyphosate, dicamba, or 2,4-dichlorophenoxyacetic acid (2,4-D). As a result, Sharkey discovered, the associated herbicides for the tolerant crops have typically seen increased use. Before the release of dicamba-tolerant crops, dicamba was used on 2% of all U.S. soybeans in 2014-15, but it was used on 21% of all U.S. soybeans in 2017-18, after the release of dicamba-tolerant crops.

We want to figure out why these formulations have different volatility levels. Our focus has been on the amines that are included in dicamba formulations and how the chemical properties of the amines influence the volatility of dicamba.

Kimberly M. Parker

Dicamba and 2,4-D herbicides unintentionally affect other nontargeted plants through herbicide drift, either as primary or secondary drift. Primary drift occurs shortly after application when sprayed droplets containing herbicide molecules are carried by the wind to off-target crops.

Secondary drift occurs over longer time periods when a herbicide converts from a liquid or solid state to a vapor and then drifts away from the targeted crops as a result of air temperature, wind, moisture, and herbicide formulation. Because of higher temperatures and larger plants that cover the soil, this change into a vapor state, known as volatilization, may be exacerbated when dicamba and 2,4-D are applied to herbicide-tolerant crops later in the season.

Mitigating environmental impact of herbicides

While there are regulations in place regarding when and how these herbicides should be sprayed, drift remains an issue that has resulted in a slew of lawsuits in recent years. As a result of these lawsuits, several dicamba products were withdrawn from use and required re-approval by the United States Environmental Protection Agency (EPA).

Dicamba contains an amine, a chemical agent intended to keep the herbicide in place instead of volatilizing into the atmosphere.

“We want to figure out why these formulations have different volatility levels,” Sharkey, who previously published a paper on amines in dicamba formulations, said. “Our focus has been on the amines that are included in dicamba formulations and how the chemical properties of the amines influence the volatility of dicamba.”

Furthermore, they investigated the extent to which herbicide use is linked to the introduction of genetically modified crops. They discovered that the introduction of herbicide-tolerant crops influences herbicide use rates and practices. While newer products such as dicamba and 2,4-D have reduced glyphosate’s reliance, dicamba and 2,4-D have been used more frequently since crops designed to tolerate them were introduced to the market. Following the introduction of dicamba-resistant crops in 2015, the use of dicamba increased by a factor of 2.3 in just one year, from 2016 to 2017.

Sharkey also combed through data on the various herbicide products, as well as the regulations and deregulations governing how they are to be used. To reduce drift, the EPA restricts the use of dicamba and 2,4-D, but those regulations for soybeans and corn have remained relatively unchanged since 2014. Other requirements aimed at reducing the drive include limiting the types of nozzles used when spraying herbicides and requiring a buffer area.

“Continued progress is needed to improve practices that prevent drift, including the design of chemical formations, and to understand the impact of herbicides after they enter the atmosphere,” Parker said. “By defining the factors that contribute to herbicide drift and characterizing the atmospheric processes that influence its impact, we may be able to develop new prevention strategies.”