Managing Spray Drift
June 18, 2008

Drift of pesticides away from the target is an important and costly problem. The proximity of individuals and sensitive sites to the pesticide application, the amounts of pesticide drift, and toxicity of the pesticide are important factors in determining the potential impacts from drift. A number of factors influence drift, including weather conditions, topography, the crop or area being sprayed, application equipment and methods, and decisions by the applicator.

Drift can occur by two different means:

Vapor Drift occurs when a chemical vaporizes after being applied to the target area.  The vapors are then carried to another area where damage may occur.  The amount of vaporization that occurs depends largely on the air temperature and formulation of the pesticide being used.

Physical Droplet Drift is the actual movement of spray particles away from the target area.  Many factors affect physical drift, but one of the most important is droplet size.  Small droplets fall through the air slowly, so they are carried farther by air movement.

Results of many studies indicate that spray density required for effective weed control varies considerably with plant species, plant size and condition as well as herbicide type, additives and carrier used.  Although excellent coverage can be achieved with extremely small drops, decreased deposition and increased drift potential limit the minimum size drop that will provide effective pest control.

The objective in applying pesticides is to achieve uniform spray distribution while retaining all the spray droplets within the intended spraying area. Spray liquid may have a velocity of 60 feet per second or more when leaving a nozzle.  The speed is reduced due to air resistance and the spray material breaking into small drops.

Drift is not always harmful.  It depends on the pesticide being used, the targeted pest and the non-target organisms or objects that are downwind or adjacent to your target area.  Keep in mind that if you have considerable drift downwind, you will be losing pesticide.

Drift Control

Because all nozzles produce a range of droplet sizes, the small, drift-prone particles cannot be completely eliminated, but drift can be reduced and kept within reasonable limits.

1.  Because all nozzles produce a range of droplet sizes, the small, drift-prone particles cannot be completely eliminated, but drift can be reduced and kept within reasonable limits. Use adequate amounts of carrier. This means larger nozzles, which in turn usually produces larger droplets. Although this will increase the number of refills, the added carrier improves coverage and usually increases the effectiveness of the chemicals. Smaller droplets will be produced with lower spray volumes, resulting in a greater drift hazard.

2.  Avoid using high pressure. Higher pressures create fine droplets; 40 PSI should be considered the maximum for conventional broadcast spraying.

3.  Use a drift-reducing nozzle where practical. They produce larger droplets and operate at lower pressure than the equivalent flat-fan nozzle.

4.  Many drift-reducing spray additives which can be used with regular spray equipment are available today.

5.  Use wide angle nozzles and keep the boom stable and as close to the crop as possible.

6.  Spray when wind speeds are less than 10 mph and when wind direction is away from sensitive crops.

7.  Do not spray when the air is completely calm or when an inversion exists.

8.  Use a shielded spray boom when wind conditions exceed prime pesticide application conditions.

Excerpt from NDSU Extension publications.




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