Be Wary of Inversions That Can Cause Herbicide Drift

As herbicide applications are made during the 2017 growing season, a main concern will be the possibility of herbicide drift injuring sensitive crops.

The normal culprit of off-target movement during herbicide applications is thought to be excessive winds. However, the absence of wind can result in off-target chemical applications, as well. To make matters worse, it's more difficult to detect.

What is the weather anomaly that can cause this type of off-target herbicide movement? It's called a temperature inversion.

An inversion is a really stable air mass, says Mandy Bish, University of Missouri research specialist.

In normal weather conditions, the earth absorbs solar radiation and emits that energy to its surroundings. The air around the surface of the earth warms up, expands, and rises, says Bish. Then cooler air settles down toward the ground until it's warmed.

This cycle of shuffling air continues throughout the day, and it allows for herbicide particles to disperse. Once the sun sets on a clear night, the earth is no longer being warmed by the sun. As the soil surface cools, the warm air rises, and the cooler air settles near the ground. This is the onset of an inversion. The cool, more dense air remains under the less dense warm air, and the two air masses do not mix.

Right conditions

There are a couple of conditions that create inversions.

• Clear sky at night. If there are no clouds in the sky, any heat left in the atmosphere will escape into space, Bish says.
• Lack of wind. The warm and cool air aren't being mixed together. "These conditions are good indicators that an inversion is forming or occurring," says Bish.

If herbicide applications are made when the air near the ground is cooler than the air above it and the winds are less than 3 mph, it is likely during a temperature inversion, explains Bish.

The particles may get stuck in this stable air mass and become suspended in the air instead of reaching the target plants, she says.

These particles will remain suspended, and then they are susceptible to horizontal wind gusts capable of blowing them off target.

If you're unsure if an inversion is forming, look for these indicators associated with inversions:

• Visible dew
• Low-lying fog
• Clear skies
• Horizontal smoke patterns

As always, read and follow the herbicide label when making applications.

Most herbicide labels instruct you not to spray during an inversion. Both the XtendiMax and Engenia labels have detailed information about avoiding application in inversions and checking for wind speeds less than 3 mph.

"In our preliminary research, about 90% of the time that an inversion occurs in June or July in Missouri, the wind speed is less than 3 mph," says Bish.

This isn't a new phenomenon. Inversions have been occurring for a long time, but the consequences before were not always this significant.

Broadleaf plants are more sensitive to dicamba than to many other herbicides. It would only take a little bit of dicamba caught in an inversion to result in visible damage, she explains.

"We've probably been able to get away with spraying during inversions in the past because neighboring crops were either resistant to the same herbicide or weren't as sensitive to the herbicide as they are to dicamba," says Bish. "This allowed us to be a little more lax or complacent. Additionally, in the past, we sprayed dicamba earlier in the growing season when there may have been less sensitive crops growing nearby."

Bish predicts this will change. "Now, we have to be cognizant of it because other plants will be sensitive to dicamba," she says. "Not every field will have the same trait."

What we know

"We've been monitoring inversions at application heights for two years," says Bish.

Preliminary results show that inversions will normally last overnight.

"A typical inversion starts to set up around 6 p.m. or 7 p.m. and will last for 10 or 11 hours," she says.

Bish worries that applicators won't think twice about spraying near 7 p.m., because during the summer, there is still daylight, even though inversions are forming.

"We need to be on top of stewardship and really follow the label with dicamba," she says.