June 12, 2001

Ag Engineer Outlines Strategies for Reducing Spray Drift

Drifting spray wastes money, reduces the effectiveness of pesticides and can cause serious damage to surrounding crops, trees, wildlife and water supplies. Keeping pesticide applications on target is a key to having the maximum impact on weeds, insects and diseases while minimizing costs, notes a North Dakota State University agricultural engineer.

"Probably the most important threat from spray drift is the potential damage to other crops in the area," says Vern Hofman of the NDSU Extension Service. "Some crops, as well as trees and other native vegetation are extremely sensitive to herbicides. An unintended application from drift can have devastating results."

Major factors that contribute to pesticide drift are droplet size, equipment, application methods, and environmental conditions, Hofman notes. Applicators need to consider all those factors and make appropriate adjustments to minimize the potential for drift.

Droplet Size is Important

Atomizing the spray solution into very small droplets may increase coverage, but applicators need to consider the potential for evaporation, drift out of the target field, canopy penetration and where the small drops will be deposited. The smaller the drop the greater the risk of drift, Hofman notes.

Drops less than 100 microns (about the thickness of human hair) lose their velocity soon after leaving the spray nozzle. They are in free fall within a few inches (a 50 micron drop loses it’s velocity in 3 inches) from the nozzle and evaporate rapidly. Rather than reaching the target, the pesticide contained in water droplets become very small aerosols, which will remain in the air until picked up in falling rain.

Drops over 150 microns resist evaporation much more because of their larger surface area. The potential for drift rapidly decreases with these larger drops.

"In reality, a range of droplet sizes is needed to deposit pesticides on the wide variety of plant types, sizes and shapes that are in the field," Hofman says.. The following suggests how different size spray drops vary in effectiveness:

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

"To effectively control weed and insect pests, the actual range of droplet sizes depends on the specific pesticide being used, the kind and size of the target plant, and weather conditions," Hofman says. A few nozzles are specifically designed to reduce drift by reducing the amount of small, driftable droplets in the spray pattern.

Insecticides and fungicides generally require smaller droplets than herbicide applications to obtain adequate coverage, he says. For foliar herbicides, research suggests that droplet sizes in the range of 100 to 400 microns do not significantly differ in weed control effectiveness, unless application volumes are extremely high or very low. Exceptions to this guideline may exist for specific herbicides.

Equipment and Application Methods Also Play a Role

"Reduce drift by mounting the spray boom closer to the ground while being careful not to disrupt the uniformity of the spray pattern. Wind speed and drift increase with height," Hofman notes. The correct spray height for each nozzle is determined by the nozzle spacing and the spray angle. Wide-angle nozzles can be placed closer to the ground than narrow-angle nozzles. Nozzles spaced 20 inches apart should be 18 inches above the target for 80 degree tips and 15 inches for 110 degree tips. However, wide-angle nozzles also produce smaller droplets, offsetting some of the advantages of a lower boom height.

"Applicators are also advised to use the lower end of the nozzle pressure operating range if the pesticide label allows. Higher pressures generate more small droplets," Hofman notes. Try not to use pressures that exceed 40 pounds per square inch (psi). Extended-range nozzles are capable of operating at 15 to 20 psi while providing a uniform spray pattern. "Remember that flow rate will go down as pressures are reduced, so the sprayer will need to be recalibrated," he says.

An increase in nozzle size will create larger droplets, which are less likely to move off-target. If you use nozzles that put out 5 to 10 gallons per acre (GPA), increase to nozzles that put out 10 to 12 GPA, Hofman advises.

Some applicators are reducing the spray volume of foliar herbicides from the commonly used 7 to 10 GPA to 5 GPA or less. "When you reduce spray volume, the herbicide concentration must increase to maintain the same dose of active ingredient," he notes. "But as spray volume is reduced, the droplet size will decrease, increasing the potential for drift. Pesticide labels specify application rates which should be followed as the label is the law."

Research has also shown that control of some broadleaf weeds with contact herbicides declines as spray volume is reduced. However, reduced volumes usually have little effect on weed control with most herbicides, as long as the chemical is applied properly. It is best to follow chemical label recommendations on application rates.

"To compensate for reduced spray volume, some applicators increase spray pressure from 30 to 40 psi to 60 to 80 psi," Hofman says. "They believe they can drive small droplets into the crop canopy to increase coverage. The actual result is the opposite. Small drops will quickly lose their velocity and evaporate before they reach the plant. In addition, small droplets have less momentum and insufficient energy to be driven into a plant canopy. Larger spray drops will maintain their velocity and are able to be driven into the plant canopy."

Increased pressure should not be used as a substitute for spray volume, Hofman says. It is recommended to maintain pressures below 40 psi, and if you need increased coverage, increase spray volume (GPA).

Newer drift-reducing nozzles are becoming popular. All of them contain a pressure reducing chamber in the nozzle so the spray drop produced is larger with fewer fine drops. Research studies show excellent results in reducing drift, Hofman notes.

The latest addition to this group of nozzles is the "air induction" type. These nozzles pull air into the nozzle and introduce air bubbles into the spray drop increasing the drop size. When the drop hits the target it explodes spreading spray mix over a larger area of the plant leaf. "This type of nozzle is excellent for systemic type herbicides," Hofman says. "It should not be used for contact type herbicides, which require smaller drops for good coverage."

Climatic Conditions are Another Key

"Wind speed and direction, temperature, relative humidity, and atmospheric stability all affect spray drift. Wind speed is usually the most critical meteorological condition. The greater the wind speed, the farther small droplets will be carried," Hofman says.

"There is no maximum wind speed to serve as a guideline in all situations, but try to spray when the wind speed is less than 10 miles per hour," he says.

To minimize the damage done by drift, applicators should determine if sensitive crops are downwind. To greatly reduce damage to sensitive plants, leave a buffer zone at the downwind edge of the spray area. After the wind has died down or changed direction, spray the buffer zone. The size of the buffer zone is determined by the pesticide being sprayed and the sensitivity of the adjacent crop.

Temperature and humidity affect the amount of drift that occurs through evaporation of spray particles. Although some spray is lost through evaporation under all atmospheric conditions, losses are reduced significantly in cool, damp conditions, Hofman notes.

Temperature also influences atmospheric stability, as well as the presence of air turbulence and inversions. An inversion can occur when the air is very calm, with very little mixing. An inversion contains cool dense air near the earth’s surface with warmer air at higher elevations. This condition makes it easy for small spray drops to remain suspended in the air and move slowly downwind.

"That means extremely calm conditions can pose a significant risk for pesticide drift; wind doesn’t always have to be a factor," Hofman says.

Inversions often occur in early morning or late evening. "You can recognize an inversion by observing a column of smoke. If the smoke does not dissipate, or if it moves downwind without mixing vertically, conditions are not good for spraying," he notes.

"The best way to avoid the kind of drift associated with these atmospheric conditions is to eliminate the formation of very small droplets in the spray. Once you’ve eliminated those very small droplets, you’ve drastically reduced the effects of weather stability factors on drift potential," Hofman says. "Larger spray drops will resist atmospheric conditions much better than smaller drops."

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Source: Vernon Hofman, (701) 231-7240, vhofman@ndsuext.nodak.edu
Editor: Tom Jirik, (701) 231-96290, tjirik@ndsuext.nodak.edu