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Steps to stop spray drift
June 21, 2005

Cutting spray rates? Don't increase drift

Cutting herbicide rates may sound like a good idea, but if you're not careful your weed control can drift away in the wind.

"Producing small drops may help improve crop coverage, but getting the fine drops to land on the intended plants may be difficult," says
Vern Hofman, NDSU extension agricultural engineer.

"Some of those very fine drops will remain in the spray stream created by the spray pattern and move around leaves instead of landing on them. A drop needs to have enough mass to break loose from the stream to deposit on a leaf. Small drops may not be able to do so."

A 50-micron drop will lose its velocity 3 inches from a nozzle. A 100-micron drop will lose its velocity 9 inches from the nozzle. "When the droplet has lost its velocity, even gentle breezes may carry them out of the target field," Hofman says

The problem is compounded because water in a spray formulation evaporates faster than the chemical. At 90 degrees F and 36% humidity, a 50-micron drop will evaporate into pure chemical in less than two seconds and will then be vulnerable to drift in any wind.

Air assist cautions

A study in Canada has shown increased drift from air-assisted sprayers early in the growing season because the high-speed air stream hits the ground and rebounds, carrying spray with it. Fine drops then remain in the dissipating air stream.

"If herbicides are being applied with an air-assist sprayer, it may be the best to reduce airflows so the rebounding air does not increase drift," Hofman says.

To compensate for reduced spray volumes, some applicators may increase operating pressure from 30 to 40 pounds per square inch to 50 to 60 pounds per square inch or more, believing they can drive small drops into the crop canopy and increase coverage.

"In reality, the opposite occurs," Hofman says. "Smaller drops are being produced that are losing velocity very quickly after they leave the nozzle. At the same time, evaporation is reducing their size more, making them more susceptible to drift."


Air induction nozzles

New air-induction spray nozzles are available that reduce drift potential, Hofman notes. They produce large drops even at higher pressures (50 pounds per square inch and above is the optimum for several of the nozzles). But until more research is done, they should only be used with systemic herbicides. "These nozzles produce large drift-resistant drops, but may reduce coverage as well," he says.

Air-induction nozzles are available in small sizes, but if better coverage is necessary, Hofman advises applicators to use higher spray volumes and flat-fan nozzles. Higher volumes produce larger spray drops that will be more resistant to drift. He recommends keeping pressures under 40 psi with flat-fan nozzles. You can reduce the pressure 15-20 pounds if using extended range nozzles.


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