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Dicamba has been the subject of lawsuits across the country, with landowners claiming the herbicide, when used by neighboring growers, was run over by their properties, killing valuable non-hardy crops.
Dicamba is sprayed in a formulation that contains an amine, a chemical agent that is supposed to hold the herbicide in place, preventing it from going into the air. Ongoing reports of crop damage despite these measures have previously shown, however, that it may not work as it should, particularly when the dicamba / amine formulation is sprayed with the world’s most commonly used herbicide, glyphosate, the main component. by Roundup.
Researchers from Washington University in St. Louis in the laboratory of Kimberly Parker, assistant professor in the McKelvey School of Engineering Department of Energy, Environmental and Chemical Engineering, have proposed a mechanism that describes how dicamba’s volatility is controlled by amines.
The discovery was published in October in Environmental sciences and technologies.
The factors that cause dicamba to volatilize, i.e. dispersion into the air, have been previously investigated in scientific studies conducted on fields and in greenhouses where researchers have measured the amount of dicamba transformed into a measurable gas in the air or by evaluating damage to plants.
But large gaps remained when it came to understanding the molecular processes at work, so Parker’s lab set out to fill them.
“We decided to approach it from a single direction,” said lead author Stephen Sharkey, a PhD student at Parker Lab. “We wanted to try and get into the chemistry behind the volatility process.”
He started by considering the interactions of molecules in the solid phase of the dicamba / amine formulation.
There are three amines typically used in commercial dicamba formulations. Sharkey considered these three commonly used amines and six others to gain a better, more generally applicable understanding of their properties and impacts on dicamba volatilization. How do amines interact with dicamba and this information can be used to find out why dicamba is still volatilizing?
Parker said there are a couple of common assumptions about what’s going on between amines and dicamba: the heavier amine acts as an anchor, thus weighing down the herbicide, or the volatilization is determined by pH levels.
Sharkey’s research showed something different. As for the three most widely used amines, he said, “the ones that work best have the most hydrogen bonded groups.” He went on to find the same results in the six additional amines.
The researchers also looked at how other molecules can affect these interactions. “We found an increase in the volatility of glyphosate in two of the three main amines,” Sharkey said. “One way dicamba products can be used is together with glyphosate as a way to kill many different weeds,” including those resistant to glyphosate and / or those resistant to dicamba.
The research team thinks it may be the case that glyphosate, which has many places where it can form hydrogen bonds, may interfere with dicamba’s ability to form bonds with amines. Essentially, glyphosate can drive a chemical wedge between the two by forming its own bonds with dicamba or amine molecules.
None of the other potential factors tested had as reliable or consistent an effect on volatility as the number of hydrogen bonding sites on the amine.
The team tested several variables, including temperature, reduction of the amine concentration relative to dicamba, acidity of the amine, vapor pressure of the amine, molecular weight of the amine, pH values of the solution. and the presence of glyphosate.
“We showed those weren’t primary determinants,” Parker said. “The hydrogen bond appeared to be the main factor. If the amine has more hydrogen bond functional groups, the volatility of dicamba is decreased compared to other amine formulations.”
Moving forward, this improved understanding of how dicamba and amines interact identifies a specific characteristic that can be modified to improve a formula’s ability to stay on a crop and away from surrounding fields. It also points to the benefits of studying herbicides in the laboratory in addition to the work done by other researchers in the field. This is something Parker and his team have done and will continue to do.
As for the next steps, Sharkey’s latest work is a look at how the introduction of more tolerant crops affects the use of herbicides. Parker said he would like a greater understanding of the effects of more complex chemicals on dicamba’s volatility.
“What about other chemicals on the surface of a leaf, for example?” she asked. “How could they further influence volatilization?”
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