May 16, 2002

Engineer Offers Recommendations for Decreasing Spray Volumes and Drift

As sprayers begin making their way across the region’s crops, they bring with them the potential for spray drift – the fine spray drops that move away from their intended target. Eliminating all of those drift-susceptible droplets is impossible, says a North Dakota State University agricultural engineer, but applicators do have considerable control over the spray drops they are applying.

"Some applicators are reducing the spray volume for foliar application of herbicides based on spray equipment manufacturers’ recommendations," notes Vern Hofman of the NDSU Extension Service. "In some cases, applicators that had been applying 8 to 12 gallons per acre (gpa) are reducing this to 5 gallons per acre or even less." Sometimes the applicators are also increasing spray pressure to improve coverage with the reduced volume.

"These actions are usually not recommended," Hofman says. "Cutting application rates reduces drop size, which can reduce deposition on the target and increase drift potential." Hofman notes that droplets under 100 to150 microns are susceptible to drift. By comparison, a human hair is about 100 microns thick.

"Producing small drops may help improve crop coverage, but getting the fine drops to land on the intended plants may be difficult," he explains. "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."

Small drops produced from conventional sprayers lose their velocity very soon after they are produced. For example, a 50-micron drop will lose its velocity 3 inches from a nozzle and a 100-micron drop will lose its velocity 9 inches from the nozzle. Then, the drops depend on gravity to carry them to the target.

"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 the active ingredients of many sprays do not evaporate while their carrier, water, does so readily. The carrier evaporates, causing a smaller drop of concentrated spray that may be susceptible to drift.

Water in a spray formulation begins to evaporate immediately after the drop is formed. At 90F and 36 percent humidity, a 50-micron drop will evaporate to pure chemical in less than two seconds and will then be vulnerable to move in any wind.

Hofman notes that some equipment manufacturers are designing sprayers with a high-speed air stream (air-assist sprayers) that carries the spray drops to the target. In concert with that technology, some manufacturers are recommending the use of low amounts of carrier, a practice not listed on the label and therefore illegal with certain pesticides.

"The high-speed air stream may increase the problem of carrying spray past the leaf because a larger drop may be needed to break free from the air stream to deposit on the target," Hofman says. 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. "The problem occurs when the plant canopy is small early in the growing season. 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," he 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."

In addition, small drops have low momentum and very little energy to be driven into the plant canopy. Increasing pressure causes smaller drops to be produced with an increased potential to drift. Reducing spray volumes reduces drop size and increasing pressures reduces drop size even more with a resulting increase in drift potential.

New air-induction spray nozzles are available that do 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). 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. If an applicator uses extended range nozzles, operating pressures can be reduced to 15 to 20 pounds per square inch and the proper spray pattern will be maintained. "With those recommendations, larger drops are produced with fewer fines. That will help reduce the drift potential," he says.

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Source: Vern Hofman, (701) 231 7240, vhofman@ndsuext.nodak.edu
Editor: Tom Jirik, (701) 231-9629, tjirik@ndsuext.nodak.edu

Click here for a printable PDF version of this graphic. (32KB b&w graph)

Graphic:

A Droplet Approach to Controlling Spray Drift:
Size Does Matter

100 Microns -- human hair

Very fine droplets measuring less than 120 microns are collected efficiently by insects and needles on coniferous plants, but tend to remain in the airstream and are carried around the stems and leaves of weeds.

150 Microns -- sewing thread

300 Microns -- toothbrush bristle

Fine and medium droplets measuring between 120 and 350 microns deposit more efficiently on stems and narrow vertical leaves such as grasses when there is some air movement.

420 Microns -- staple

850 Microns -- paper clip

Coarse and very coarse droplets measuring more than 350 microns deposit most efficiently on large flat surfaces such as broad leaf weeds.