NEWS for North Dakotans
Agriculture Communication, North Dakota State University
7 Morrill Hall, Fargo, ND 58105-5665
May 18, 2000
Drifting sprays waste money, reduce the effectiveness of pesticides and can damage 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," says Vern Hofman, an agricultural engineer with the North Dakota State University Extension Service. Probably the most important threat from spray drift is the potential damage to other crops in the area. 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. Applicators need to consider all those factors and make appropriate adjustments to minimize the potential for drift, Hofman says.
Droplet Size
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 how well small drops are deposited on the targeted pest. "The smaller the drop, the greater the risk of drift," Hofman says.
Drops less than 100 microns (about the thickness of a 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 its velocity in 3 inches) from the nozzle and evaporate rapidly. Rather than reaching the target, the pesticides contained in water droplets become very small aerosols that 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:
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 herbicides to obtain adequate coverage. 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
"Reduce drift by mounting the spray boom closer to the ground while being careful not to disrupt the uniformity of the spray pattern," Hofman says. Wind speed and drift increase with height. 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, thereby offsetting some of the advantages of a lower boom height.
Hofman also advises applicators to use the lower end of the nozzle operating pressure range if the pesticide allows. Higher pressures generate more small droplets. "Try not to use pressures that exceed 40 to 45 pounds per square inch (psi). Extended-range nozzles are capable of operating at 15 to 20 psi while providing a uniform spray pattern," he says. Remember that flow rate will go down as pressures are reduced, so the sprayer will need to be recalibrated.
An increase in nozzle size will create larger droplets that 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 15 GPA,"Hofman recommends.
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. But as spray volume is reduced, the droplet size will decrease, increasing the potential for drift," Hofman notes.
Research has also shown that control of some broadleaf weeds with contact herbicides declines as spray volume is reduced. However, Hofman notes that reduced volumes 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 the reduced spray volume, some applicators increase spray pressure from a normal 30 to 40 psi to 60 to 80 psi. "They believe they can force small droplets into the crop canopy to increase coverage, but small drops will quickly lose their velocity and evaporate before they reach the plant," Hofman says. "In addition, the small droplets have less momentum and insufficient energy to be driven into a plant canopy."
Increased pressure should not be used as a substitute for spray volume. Hofman recommends keeping pressures below 40 psi. To increase coverage, increase spray volume.
Some applicators are starting to use newer drift-reducing nozzles, Hofman says. All of those nozzles contain a pressure-reducing chamber so the spray drop produced is larger with fewer fine drops. The latest addition to this group of nozzles induces air into the spray drop. "This type of nozzle is excellent for systemic type herbicides. It should not be used for contact type pesticides, which require a smaller drop for good coverage," he says.
Climatic Conditions
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.
"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," Hofman advises.
To minimize the damage done by drift, Hofman recommends that applicators 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.
Temperature also influences atmospheric stability, as well as the presence of air turbulence and inversions, Hofman says. An inversion can occur when the air is very calm, with very little mixing. This condition makes it easy for small spray drops to move slowly downwind. "That means extremely calm conditions can pose a significant risk for pesticide drift," Hofman says. "Wind doesn't always have to be a factor."
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," Hofman says.
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," he says.
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Source: Vern Hofman (701) 231-7240
Editor: Tom Jirik (701) 231-9629
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