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soybean field at sunset Jill Loehr
AVOID THESE CONDITIONS: Here is an example of when you don’t want to spray in rolling topography. A temperature inversion is likely.

New dicamba rules aim to avoid temperature inversions

Weed scientists say U.S. EPA’s new rule to stop spraying earlier in the day should help avoid issues with temperature inversions that cause drift.

Along with the extension for using dicamba herbicides on Xtend soybeans, U.S. EPA issued several new guidelines with the goal of reducing off-target crop injury. If any of those new rules will help, it will most likely be the restrictions on hours when dicamba can be sprayed. That’s how Bill Johnson, Purdue University weed control specialist, views the label changes.

“We still have a lot to learn about temperature inversions and how they impact herbicide drift,” Johnson says. “The truth is, we don’t know exactly how particles act and how they could move during an inversion.

“What we do know is that temperature inversions which could affect movement are more likely in most of Indiana during early evening and early morning hours.”

The new EPA label changes for XtendiMax, Engenia and FeXapan don’t allow spraying these products until one hour after sunrise. They also require that spraying stops two hours before sunset.

Joe Ikley, the new weed control specialist in North Dakota, worked with Johnson in Indiana in 2018. After EPA announced this rule change, he went back and looked at weather data to see if it would have made a difference in 2018.

“We found that if spraying stopped two hours before sunset, there would have been fewer times when an inversion might have been present in many areas,” Ikley says. “The only place where that might not hold true is in low-lying bottomland positions.”

Based on Ikley’s findings, Johnson believes stopping applications earlier in the evening is important. “This change could prevent someone from spraying right into the beginning of a temperature inversion,” he says. “Curtailing spraying earlier could result in fewer drift complaints.” 

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