NEWS for North Dakotans
Agriculture Communication, North Dakota State University
7 Morrill Hall, Fargo, ND 58105-5665

May 27, 1999

Ag Spray Droplet Size Relates to Coverage and Drift

Selecting the droplet size for spraying agricultural chemicals is a balancing act. Too small and they can drift away to other crops and plants. Too large and coverage is reduced on the target crop.

"What we're looking for is a good balance between the potential for drift and coverage," says Vern Hofman, an agricultural engineer with the North Dakota State University Extension Service. "The effectiveness of what is sprayed is largely determined by the amount of surface area of the spray that comes in contact with the pest. Small droplets offer significantly more surface area than large drops, and thus small droplets provide more effective coverage than large drops."

If the average droplet size in a spray pattern is doubled, the number of droplets is decreased by eight times and the amount of surface area is reduced usually by about half, Hofman says.

"It's an unfortunate fact that the most efficient, effective pesticide coverage can also be the most damaging to surrounding crops and the environment. The small droplets that maximize spray coverage are usually the ones that cause the most drift."

Aerodynamic drag determines how quickly droplets will fall to earth. Small droplets have higher drag and fall slowly; larger drops have lower drag and fall more quickly. For the same reason, wind influences the path of small droplets more than large drops, so small droplets will drift farther (or evaporate sooner) than large drops.

Selecting a spray nozzle involves a trade-off between effective spray coverage and drift reduction, Hofman says.

Because it's important to get the best performance from your pesticide and also important to reduce spray drift, the best spray nozzle would be the one that offers a combination of the most effective coverage and the most drift reduction. An average droplet diameter (VMD) of about 250-300 microns offers the best combination of effective coverage and drift reduction for post applicaton of many systemic and contact herbicides. Other pesticides and application methods may work best with other droplet sizes, Hofman cautions.

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Source: Vern Hofman (701) 231-7240

Editor: Tom Jirik (701) 231-9629

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