Avoiding Inversions

Know the warning signs before applying herbicides

It’s a beautiful summer evening. The sun is setting, winds have subsided and there’s not a cloud in the sky. Sounds like perfect conditions to work late into the evening and finish up your herbicide application, right? WRONG.

Ironically, these otherwise pleasant conditions are all warning signs that a temperature inversion is occurring.

Mandy Bish, senior research specialist at the University of Missouri (Mizzou), says farmers and applicators should avoid spraying during inversions at all cost this season.

What is a temperature inversion?

“Essentially, a temperature inversion is a very stable air mass,” says Bish. “It’s easier to understand what an inversion is if you first understand what it is not.”

Here’s what happens in the atmosphere on a typical summer day:

  1. The sun shines, and its energy hits the earth.
  2. The earth absorbs some of this energy and emits the rest.
  3. That released energy heats the air near the earth’s surface.
  4. Hot air near the earth’s surface expands and rises up and over cooler air.
  5. The cooler air settles in near the earth where it’s heated, expands and then rises.
  6. And so the natural daily cycle continues.

But Bish warns farmers and applicators of what can happen in the evenings when the daily air cycle is halted.

“When the sun sets in the evening, it no longer warms the earth,” says Bish. “And if there are no clouds in the sky, any energy that the earth emits will escape into space and the normal air shuffling will not occur.”

She further explains that when there is no mixing or shuffling of warm and cool air masses, the temperature near the earth’s surface is cooler than the air temperature above it. This causes a very stable air mass known as a temperature inversion.

Why should farmers be aware of this phenomenon?

Applying herbicides during an inversion period can contribute to off-target herbicide movement. This can cause damaging effects to neighboring crops.

“With a stable air mass, there’s a lack of wind,” says Bish. “It’s a place where herbicide particles can be suspended and eventually move to an unintended target once a horizontal wind gust blows through.”

Bish’s major concern this growing season is with dicamba herbicides, because most broadleaf plants are so sensitive to the chemical.

“Most, if not all, herbicide labels warn applicators not to spray during inversion periods, so inversions themselves are not new,” says Bish. “However, because broadleaf plants tend to be very sensitive to dicamba, it only takes a small amount of the herbicide moving off-target to injure unintended plants.”

For this reason, farmers must practice great stewardship with this herbicide, which includes not spraying into inversions.

“Labels for the newly approved dicamba products, FeXapan™, Engenia™ and XtendiMax®, are no different,” she explains. “They restrict spraying the products during inversions and even include a bit more detail about identifying inversions compared to some older labels.”

How can you tell when an inversion is occurring?

When an inversion sets in, it is usually around sunset on a clear night.

While Bish says the only way to confirm that an inversion is occurring is by monitoring air temperatures at different heights, there are two obvious indicators that farmers and applicators can watch for:

  1. No clouds in the sky
  2. Wind has died down (speeds under 3 mph)

During the months of June and July, Mizzou’s preliminary data at three different locations across the state show inversions setting in anywhere between 6-8 p.m. and lasting until 5-6 a.m. Inversions occurred approximately 50 percent of the nights in those months.

Based on this data, Bish encourages applicators to stop spraying in the evenings when the winds start dying and the skies clear. These conditions mean it’s likely an inversion is taking place.

“We know farmers like to be efficient with daylight hours,” says Bish. “And since there is still plenty of daylight during this timeframe in June and July, we’re concerned about applicators spraying into an inversion and herbicides drifting off-target.”

How can I learn more about temperature inversions?

Bish and her colleagues have conducted research and communicated about inversions for the past several years to increase awareness and improve herbicide stewardship in Missouri.

There’s not a whole lot of knowledge out there about inversions Bish explains. Last year, Mizzou conducted a survey among private and commercial pesticide applicators to gauge applicator feedback.

“While a lot of commercial applicators were able to correctly identify some conditions associated with a temperature inversion, many of our private applicators could not.”

When asked “What are some environmental ‘cues’ that often serve as signs of temperature inversions?” Bish says 25 percent of non-commercial Missouri pesticide applicators who participated in the survey responded with “I don’t know.”

Bish and Mizzou researchers are working to change this statistic. By using real-world observations surrounding last year’s dicamba injury in the Mid-South combined with preliminary research results on inversions, Mizzou spreads awareness about temperature inversions and how they can contribute to herbicide drift injury. Through farmer/applicator trainings held throughout the state, educational YouTube videos, social media outreach and more, Mizzou emphasizes the importance of herbicide stewardship moving forward.

For more information on Mizzou’s ongoing research and trainings, visit www.weedscience.missouri.edu