readthelabelPlease read the pesticide label prior to use. The information contained at this web site is not a substitute for a pesticide label. Trade names used herein are for convenience only. No endorsement of products is intended, nor is criticism of unnamed products implied.

Pesticide drift is the movement of a spray solution from the intended target to a place where it is not wanted, or the movement of spray droplets or pesticide vapors out of the sprayed area. In particular, herbicide spray drift can damage shelterbelts, garden and ornamental plants, cause water pollution, and damage non-susceptible crops in a vulnerable growth stage (2,4-D drift on wheat in the flowering or seedling stage, for example). Herbicide spray drift can cause non-uniform application in a field, with possible crop damage and/or poor weed control. In addition, insecticide spray drift can damage beneficial insect populations especially bees and natural predators of Montana agricultural pests. Drift is also costly from a financial standpoint. If only 50 percent of an applied solution makes it to the target, then you have wasted 50 percent of what you have applied. In all the above cases, the pesticide becomes an environmental pollutant, injuring susceptible plants, contaminating water, wildlife and even humans. Sadly, the majority of pesticide spray drift problems involve mistakes that could have been avoided by the applicator.

The effects of a spray solution once it leaves the nozzle are the responsibility of the applicator.

Given the threat of lawsuits, fines and human health issues, it is inconceivable that pesticide drift continues to be a problem. Yet every year there are reports of crop damage, damage to landscape plants, livestock contamination and human health problems. All contributable to drift, usually as a result of applicator error.

Remember when you spray a site you represent all pesticide applicators and a good portion of Montana agriculture. Irresponsibility in the field will only anger those whose only opinion of agriculture is the one you have given them.  

If drift problems become too frequent or too serious, some of our most useful pesticides could be withdrawn or their use severely restricted. 

In particular, some herbicides are much more of a problem than others. Some herbicides contain the active ingredient (a.i.) glyphosate (RoundupTM) which is non-selective -- it kills any plant it touches. On the other hand, the herbicide 2,4-D is selective and kills or damages only broadleaf plants. Because ester formulations of 2,4-D and occasionally Banvel can vaporize under high temperature and move to off target plant species, it is best to minimize applications during windy days or extremely hot weather to avoid problems.

Types of Drift

The best way to reduce drift is to understand the factors which cause it, most of which can be controlled by the applicator. It begins with knowing what drift is and how drift is best managed. There are two kinds of drift:

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

Factors of Drift

Dave Smith, a Mississippi State University ag engineer, analyzed data from over 100 studies involving particle drift from ground sprayers. Of the 16 variables he considered, three were most important.

  • Wind Speed. When the wind speed was doubled, there was almost a 70% increase in drift when the readings were taken 90 feet downwind from the sprayer. Spray when the wind speed is 10 mph wind or less.
  • Boom height. When the boom height was increased from 18 to 36 inches the amount of drift increased 350% at 90 feet downwind.
  • Distance downwind. If the distance downwind is doubled, the amount of drift decreases five-fold. 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. You may want to start spraying one side of the field when the wind is lower. Also it may be necessary to only spray part of a field because of wind speed, wind direction and distance to susceptible vegetation. The rest of the field can be sprayed when conditions change.

Other important factors that must be considered in drift management are spray pressure, nozzle size, nozzle orientation, operating speed, air temperature, relative humidity, shields on sprayers and nozzles, application rate and instructions from the manufacturer of the spray product.

Drift Reduction Nozzles

Many new nozzles for reducing drift are now available. Many of these use a pre-orifice which controls the flow rate. The exit orifice controls the pattern formation. The result is larger
drfittablespray droplets which are less susceptible to drift. Also, some of these nozzles can be used over a wider pressure range, which produces large droplets at low pressure and small droplets at high pressures. The ability of these nozzles to produce good spray patterns over a wide pressure range makes them an excellent choice to use with rate controllers which control the application rate by pressure changes.These drift reduction nozzles can still have a problem with drift especially when the sprayer speed is increased and pressure therefore is increased, resulting in smaller spray droplets. At slow speeds the spray droplets may be too large for good coverage. There are advantages and disadvantages with nozzles which produce large spray droplets. For most postemergence contact, herbicide coverage is important (see table)Nozzles which induce air are also available which use a pre-orifice. Early research work with these nozzles on drift reduction show most of the benefit as to particle size is from the pre-orifice.


Remember, the environment and human safety are the top priority of any activity. There are no excuses for mishandled herbicides when human safety is the issue. With the larger number of people coming into contact with agriculture, we need to be sensitive to their lack of knowledge of agricultural issues. Understanding drift and knowing how others have learned to manage it will help most producers avoid problems. Bottom line, we are responsible for the injury we cause and are accountable for it. Keeping pesticides confined to the target area is an on-going problem. We can't blame our neighbors if they get upset because our pesticides drift onto their property. This publication offers suggestions for reducing problems from pesticide drift.

  • Low-volatile (LV) esters are not really low volatile. Indeed, they are less volatile than the old high-volatile ones (butyl esters), but the LV esters are still considerably more volatile
    than amines. LV esters are more susceptible to movement because gases can move farther than spray droplets, and can come off of previously sprayed plants or soil. Choose the amine form if there are susceptible plants in the area.
  • Even nonvolatile chemicals can drift. Small spray droplets can move considerable distances in some weather conditions.
  • Keep spray droplets as large as practical. For most pesticide usage, especially with 2,4-D type herbicides, a minimum size of 0.2 gal/min (for example, Spraying Systems 8002) flat fan nozzle tips and a maximum of 30 psi pressure are sufficient for good coverage. Smaller nozzle tips or higher pressure can produce too many "fines," or small droplets, which can easily move laterally to non-target areas. Some herbicide labels call for application at higher pressure. Apply these products with extra caution. Flood-type nozzles can reduce spray drift by producing larger droplets at low pressure. They produce a less precise pattern than flat fan nozzles, but in many situations they are satisfactory. Consider using a new generation of flat fan nozzles designed for lower pressures when the precision of the flat fan is required.
  • A windscreen may reduce drift. A windscreen around the boom and reaching near the sprayed surface may reduce drift. To avoid a chimney effect, place windscreens above the boom. Because the spray pattern cannot be seen by the operator, sprayers can be equipped with tip monitors to detect plugged nozzle tips.
  • A drift-control adjuvant, such as Nalcotrol, may help reduce the production of small droplets, thereby reducing drift.
  • Proper timing of herbicide application can help avoid damage to nearby plants. For example, grapes are readily injured by 2,4-D-type herbicides (such as Crossbow). The greatest damage to fruit production seems to be when drift occurs after the fruiting cluster has emerged but before bloom, generally mid- to late-May. Always avoid drift, but in areas where grapes are grown, not spraying during sensitive stages may be the safest approach. Observe the same principle with other sensitive plants in your area.
  • Use wide-angle nozzle tips to keep the boom low. Research indicates doubling the boom height quadruples drift. Of course, the drift potential from aerial application is considerably higher than from ground application.
  • The biggest single weather factor involved in drift is wind. Even relatively light breezes can carry small droplets a long distance. Generally, spraying early in the morning is preferable to afternoon or evening. If you are spraying near sensitive crops, limit your applications to times when winds do not exceed 5 mph. Spraying when slight winds are away from sensitive crops may be safer than spraying when the air is calm.
  • Consider not spraying areas nearest to sensitive crops. Leave a buffer zone.
  • Do not apply pesticide to dusty soil which might later be carried on winds to sensitive crops or aquatic areas.
  • Do not apply pesticides to areas where treated soil is likely to be carried by water to where sensitive crops are grown.
Avoiding chemical trespass is the responsibility of each pesticide user. This requires intelligent management and great care. Pesticide labels include useful information about any special characteristics of the product related to off-target movement.  
Use pesticides safely! 
  • Wear protective clothing and safety devices as recommended on the label.
  • Bathe or shower after each use.
  • Read the pesticide label--even if you've used the pesticide before.
  • Follow closely the instructions on the label (and any other directions you have).
  • Be cautious when you apply pesticides.
  • Know your legal responsibility as a pesticide applicator. You may be liable for injury or damage resulting from pesticide use.

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.
  • Avoid using high pressure. Higher pressures create fine droplets; 40 PSI should be considered the maximum for conventional broadcast spraying.
  • Use a drift-reducing nozzle where practical. They produce larger droplets and operate at lower pressure than the equivalent flat-fan nozzle.
  • Many drift-reducing spray additives which can be used with regular spray equipment are available today.
  • Use wide angle nozzles and keep the boom stable and as close to the crop as possible.
  • Spray when wind speeds are less than 10 mph and when wind direction is away from sensitive crops.
  • Do not spray when the air is completely calm or when an inversion exists.
  • Use a shielded spray boom when wind conditions exceed prime pesticide application conditions.

Drift Prevention and Sprayer Calibration

The amount of chemical solution applied per acre depends on forward speed, system pressure, size of nozzle, and nozzle spacing on the boom. A change in any one of these will change the application rate. The application rate is usually expressed as Gallons Per Acre or GPA. The forward speed and pressure must be adjusted correctly to set the sprayer for any given rate per acre. The nozzle size should be changed to make a large change in application rates, and all nozzles should discharge an equal amount of spray. If any of these adjustments are incorrect, poor results will be obtained.The first thing to do with sprayer calibration is select the type and size nozzle for your spraying job. You can base the nozzle type decision on spraying conditions and guidelines as recommended in the following tables.Once you've selected the type of nozzle, the next step is to calculate the nozzle size.Nozzle selection should not be based on "Gallons Per Acre" or GPA as advertised by some manufacturers. Certain GPA standards are usually required by some pesticide labels . A nozzle that is identified as a 10-gallon nozzle will deliver this amount per acre only for one condition, such as when the nozzle spacing is 20 inches on the boom, the sprayer is traveling at 4 mph and the boom pressure is 30 psi. If the spacing, speed or pressure varies from these set values, the nozzle will not deliver the specified gallons per acre. If the pressure is too high, then small droplets will be produced resulting in drift. Choice of nozzle size should be based on a gallons-per-minute calculation rather than a gallons-per-acre calculation. Basing calculations on gpm allows the operator to make the spraying decisions based on the crop and field conditions.