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Dicamba drift debacle decoded

Tom Wolfe blames inversions for much of the 2017 U.S. soybean damage

Saskatchewan Agriculture’s annual Crop Diagnostic School was held near Indian Head, Sask., this July. Tom Wolfe, owner of Agrimetrix Research & Training and sprayer specialist, was on hand to talk about sprayer technology and the dicamba problems that took place in the U.S. this summer.

“This year was a defining year for spray technology,” Wolfe told Crop Diagnostic School participants. “I’ve been in the business for nearly 30 years and this year something happened that has never happened before and hopefully will never happen again on a wide scale.”

The problem began when dicamba-resistant soybeans were brought to the market. These beans can survive being sprayed with dicamba, but non-resistant soybeans are very sensitive to the Group 4 herbicide that farmers may remember as Banvel.

Soybean growers in many parts of the U.S. have severe problems with glyphosate-resistant weeds like palmer amaranth and waterhemp. These two problematic weeds are also known to be resistant to Group 14 herbicides. So farmers were eager to plant Monsanto-DeKalb’s dicamba-tolerant soybeans, Xtend.

Along with the new beans, three new, low-volatility formulations of dicamba were released: XtendiMax (Monsanto), FeXapan (DuPont) and Engenia (BASF).

“All of those are three times less volatile than the previous Banvil II known as ‘Clarity’ in the U.S.,” Wolfe said.

U.S. farmers took to the new technology enthusiastically. About 25 million acres were seeded to Xtend soybeans and cotton.

“Most of those acres will have seen dicamba,” Wolfe said. “And we have a record number of dicamba drift complaints, more than we’ve ever seen or thought we would see.”

The state of Arkansas has been hit the hardest. Wolfe said that of the 3.3 million acres of soybeans seeded in Arkansas in 2017, 1.5 million were seeded with Xtend beans. Of the rest, there are “over one million acres documented with leaf cupping, which are classic dicamba symptoms.”

“What’s actually a disaster is the response of the industry to this,” Wolfe said. No company has taken ownership of the problem or offered to help affected growers. Instead, many parties are taking legal action.

On July 11, the state of Arkansas banned dicamba for agricultural use for 120 days and increased the penalty for misuse of up to $25,000, starting August 1.

The Arkansas Agriculture Department appointed a task force to look into dicamba use. On August 28, that task force recommended a ban on the use of dicamba after April 15, 2018.

Temperature inversions

Wolfe believes most of the dicamba damage was caused by growers spraying during temperature inversions.

“The air near the ground on a sunny day will be warmer than the air above it. There’s a gradient,” Wolfe said. When air moves up, it cools as it rises, but it will still be warmer than the air above it. In a normal temperature gradient, lower air is warmer.

But then the sun sets. “The soil now actually becomes a refrigerator for the air. So the air near the soil also cools, and it cools to a greater degree than the air above it.” This changes the temperature gradient. “Now you have air temperature rising with elevation. It’s the opposite. So the temperature profile is ‘inverted’ from a daytime condition.”

“Usually, air gets cooler with height, under an inversion, air gets warmer with height. And that is it. That defines an inversion. There’s nothing else to it.”

On a calm sunny day, you won’t likely see an inversion. For an inversion to form, Wolfe said, 90 per cent of the time the sun has to be absent. But on most summer nights, inversions will occur.

When there’s a clear sky and winds diminish in the afternoon, about three hours after the sun sets an inversion will begin. “It will begin from the ground up, and will form a stable layer of air. In this layer, air particles do not mix.” Because the air’s not mixing, chemical lingering in the air doesn’t have a chance to dilute.

“Drift that moves off target will move in a concentrated manner in the direction that, maybe, the land is sloped, or maybe the slightest of breezes will carry it. And when it gets there, it will be potent enough to kill things. It’s not diluted at all. That’s what we think happened in Arkansas,” Wolfe said.

“In Arkansas they’re seeing square miles of soybeans with identical symptoms over the whole field that can only be caused by an inversion. A regular drift event will have diminishing symptoms with distance, because of the dilution.”

Inversions are usually associated with calm conditions, Wolfe says. “If it’s windy, the inversion will be suppressed because of mechanical turbulence mixing that air back up.”

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