Of all the factors that led to the off-target movement of dicamba in 2017, volatility remains the most controversial.
Older formulations, such as Banvel and Clarity, have long been known to volatilize when sprayed under certain environmental conditions. Engenia, FeXapan, and XtendiMax were all marketed as dicamba formulations substantially lower in volatility than previous formulations.
Volatility is the movement of an herbicide as a gas or vapor after a spray application hits its target. It results from molecules of the chemical vaporizing from the surface of the plants or soil into the air.
While the industry has repeatedly acknowledged low volatility does not mean “no volatility,” it also attributed most of the off-target movement in 2017 to application issues, contaminated tank mixes and/or other types of human error.
Wading into this debate results in a quagmire of data with many of the studies still ongoing. While EPA acknowledged volatility as a potential problem when it revised labels last fall, the revisions did not specifically address volatility.
Several weed scientists and researchers were asked to suggest what applicators can do to limit the potential of this type of secondary movement.
TAKE YOUR TEMP
Volatility is largely driven by temperature, said Tom Mueller, a University of Tennessee weed scientist, who has been working with dicamba compounds and measuring their movement for more than a decade.
“My field data shows dicamba emissions from Engenia and XtendiMax are lower compared to Clarity,” Mueller said. “But some dicamba still comes off the treated area.”
Based on laboratory studies, Mueller said dicamba emissions following application appear to be directly related to temperature, with more dicamba emissions detected as the temperature increases.
“While the new formulations of Engenia and XtendiMax resulted in reduced emissions compared with Clarity, these field and laboratory studies suggest volatility from all the dicamba formulations tested could contribute to dicamba drift,” he concluded. He did not test FeXapan, which is identical to XtendiMax in composition.
The generic recommendation is to stop spraying before temperatures reach 85 degrees Fahrenheit. Ohio State, University of Illinois and Purdue University suggest 80 F as the upper limit.
“There also appears to be a minimum temperature of about 60 degrees (F) where dicamba emissions from all formulations essentially cease to occur,” noted Mueller.
Calendar date doesn’t dictate lower temperatures, but University of Illinois weed scientist Aaron Hager said the majority of the volatility issues last year can be traced to postemergence applications made when temperatures were higher.