Herbicide drift causes new problems

CARBONDALE, Ill. – It doesn’t matter if someone calls it “herbicide drift” or “chemical trespass” – if weed killer kills something besides the weeds, there may be a problem.

“Last fall, the U.S. Environmental Protection Agency announced it was considering a ‘zero tolerance’ policy for drift,” said Bryan G. Young, agronomist at Southern Illinois University.

“The original comment period ended in November, but they got so many responses it was extended through March,” he said. “I’m not sure what will come out of this, but the possibility has raised some concerns in the industry.

“One of the recommendations to reduce drift is to spray only when wind speed is 10 miles per hour or less. The Illinois Fertilizer and Chemical Association looked at recorded wind speeds for last year and found there were only three days that met this requirement.”

Accidental injury. Drift is getting more attention these days because more farmers are using weed killers that tend to cause noticeable injury to off-target plants.

Farmers used to apply weed killer to the soil before weeds emerged. But the development of new crops that could tolerate weed killers have allowed them to wait until the weeds come up, select the appropriate herbicide and then spray it directly on crops and weeds alike.

In many ways, that’s a more environmentally friendly process than laying down a broad spectrum weed killer over a field. When farmers can tailor their herbicide use to target particular weeds, it’s less risky to the environment and cheaper for them as well.

The downside. By raising the spray boom and using weed killers that act on plant leaves, farmers increase the odds that a passing breeze will catch the spray and deposit it on leaves that they don’t want to kill.

With the newer weed killers drift can occur even if the spray stays put.

Dicamba, the active ingredient in a popular “post-emergence herbicide,” as these weed killers are called, can turn into gas with the right combination of heat and humidity, and gas can go just about anywhere – with unfortunate results.

Farmers cannot do much to control vapor drift other than to avoid spraying on really hot days.

Reducing drift. But they can take steps to reduce the physical drift of spray particles:

* Keep the spray boom as low as possible.

“Low boom height minimizes the amount of time the herbicide is open to wind currents – it should be no higher than what the nozzle manufacturer recommends, which is generally less than two feet from your target,” Young said.

* Use nozzles with big openings and reduce the spray pressure.

* Add a drift-control agent to your tank.

* Watch the wind. “It’s been shown that drift dramatically increases as wind speeds increase over 10 miles per hour,” Young said.

“The right combination of these techniques can be used to manage herbicide drift while maintaining effective weed control,” he said.