Off-Target Herbicide Movement: What You See Is Not What You Get

Off-target herbicides may be invisible, but that doesn’t mean they don’t exist.

Off-target herbicides damage crops, ornamentals and other vegetation in nearby fields when droplets move off of the soil or intended plants and onto nontarget plants, says University of Missouri Plant Sciences research specialist Mandy Bish.

Pesticide applicators have control over many factors that contribute to the off-target movement of herbicides, such as spray tank contamination and making applications when it is too windy. “However, volatility, or the ability of the herbicide to evaporate, is a property of the chemical itself. This makes it tricky to understand and hard to detect, yet impactful when it happens,” Bish says.

Bish and other researchers at MU showed their findings at the annual MU Pest Management Field Day on July 7.

To demonstrate volatility of an herbicide, researchers sprayed bare ground plots with the herbicide dicamba. Thirty minutes after application, they placed potted tomato and pepper plants on plastic film on the sprayed test plot and on an unsprayed plot 40 feet away and downwind of the treated plot.

After six hours, they removed the plants and grew them in the greenhouse for nearly two weeks, monitoring the plants for dicamba injury.

Within one week, the tomato plant in the untreated plot that was downwind of the dicamba application began showing dicamba injury. By two weeks after application, both the pepper and tomato plants had cupped leaves.

Plants in the treated plots had much less injury.

“Herbicide droplets may initially land on target plants and/or soil,” Bish says. “Conditions that favor evaporation instead of absorption can allow herbicide to move back into the air.”

Moisture, temperature, ground cover and wind speed influence movement.

Herbicide formulations matter as well. Low-volatile herbicide formulations are essential in minimizing injury of nontarget plants due to volatility.

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Temperature inversions can also affect off-target herbicide movement. Inversions occur when the air temperature near the ground is cooler than at higher altitudes. This causes a stable air mass with little wind. When this occurs, spray particles remain suspended instead of reaching the target surface.

Most inversions occur around sunset to sunrise. Bish says the following conditions may indicate temperature inversions:

  • Wind speeds less than 2-3 miles per hour.
  • Dew or frost. After reaching their target, spray particles may evaporate before being absorbed.
  • Horizontal smoke patterns, which indicate horizontal wind movement.
  • Ground fog in low-lying areas.
  • Clear evening skies, which indicate a lack of clouds to trap warm air in the atmosphere.