July 1, 2010

Managing spray drift  improves pesticide efficacy by ensuring that the correct amount reaches the target, helping you get all the benefit you're paying for. Consider: If 7% of the pesticide does not reach the intended target, the cost of that amount of pesticide is wasted.

It is estimated that two-thirds of pesticide drift problems involve mistakes that could have been avoided. Drift takes the pesticide from the intended target, making it less effective, and deposits it where it is neither needed nor wanted. The pesticide then becomes an environmental pollutant in the off-target areas where it can injure susceptible vegetation, contaminate water, or damage wildlife. Drift cannot be eliminated, but using the proper equipment and application procedures can limit it to acceptable levels.

There are two kinds of drift:

• Particle drift is off-target movement of the spray particles.
• Vapor drift is the volatilization of the pesticide modules and their movement off target.

A Mississippi State University study analyzed data from more than 100 studies involving drift from ground sprayers. Of the 16 variables considered, three were most important.

1. Spray when the wind speeed is 10 mph wind or less.

   Wind Speed. When the wind speed was doubled, there was almost a 700% increase in drift when the readings were taken 90 feet downwind from the sprayer.
2. Boom Height. When the boom height was increased from 18 to 36 inches, the amount of drift increased 350% at 90 feet downwind.
3. Distance Downwind. If the distance downwind is doubled, the amount of drift decreases five-fold. Therefore, if the distance downwind goes from 100 to 200 feet, you have only 20% as much drift at 200 feet as at 100 feet and if the distance goes to 400 feet, you only have 4% of the drift you had at 100 feet.
   Check wind direction and speed when starting to spray a field.
   • Start spraying one side of the field when the wind is lower.
   • Spray only part of the field because of wind speed, wind direction, and distance to susceptible vegetation. The rest of the field can be sprayed when conditions change.

Spray Pressure and Sprayer Speed

Pesticide drift also can be reduced by using one of the new types of tips and by adjusting spray pressure. Higher spray pressures produce smaller droplets which are more susceptible to drift. If using a rate controller, be careful of increased speed. Since most rate controllers increase the pressure to maintain the same gpa when the speed increases, try to maintain the speed within 10%. For example, if you're applying 20 gpa at 8 mph at 40 psi and you increase the speed to 11 mph, the pressure will be 75.5 psi which will produce small particles prone to drift. Also, this pressure will be above the operating range of most tips. Drift reduction agents may be helpful.

New spraying technology such as the "blended pulse" can decrease the risk of drift by allowing flow rate to be controlled independently of spray pressure.

Nozzle Tip Selection

It is important to select the nozzle tip and pressure that delivers the spray particle size needed to give good efficacy with the pesticide. Always consult the pesticide label for information on application. This may include carrier rates, nozzle types, spray particle size, pressure, drift, additives, etc.

Table 1. Distances spray droplets travel with various wind speeds from 3-foot boom height.
Droplet Size	Wind Speed
(microns)	5 mph	10 mph	15 mph	20 mph
	Distance traveled in feet
100	24	48	72	96
200	9	18	26	35
400	5	9	14	18
500	4	7	10	14
600	3	6	9	12

Droplet Size. All nozzle tips produce a range of droplet sizes with low-drift nozzles reducing the number of small droplets (fines). Droplets less than 150 to 200 microns are most susceptible to spray particle drift. Nozzle tips come in several sizes within a given type of nozzle. Smaller nozzles designed to give a lower output will also produce a higher percentage of small droplets compared to larger tips.

If the air-induction spray nozzle tips produce the spray particle size needed for the pesticide and application method being used, they may be the best choice. Most of the standard air-induction spray tips give the best performance when used at about 70 psi. Check with the manufacturer to get their recommendations. Extended range air-induction tips are best used at around 40 psi. Again, check with the manufacturer for their recommendation.

You can manage spray drift damage.

1. Recognize and adjust for the factors that affect it:
   • wind speed (spray when it's less than 10 mph)
   • nozzle height
   • spray nozzle tip
   • pesticide selection
   • spray additive used
2. Keep spray a safe distance from susceptible vegetation.

Robert Klein
Extension Western Nebraska Crops Specialist