Pesticide Drift

Reeves Petroff, Pesticide Education Specialist, Montana State University Extension Service

Drift is the movement of spray droplets or pesticide vapors out of the sprayed area. Herbicide spray drift, the focus of this factsheet, 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 also can cause non-uniform application in a field, with possible crop damage and/or poor weed control. Insecticide spray drift can damage beneficial insect populations especially bees and natural predators of Montana agricultural pests. Drift also wastes money.

Factors Affecting Spray Drift

Spray Particle Size

Since wind moves small droplets farther than large droplets, spray drift can be reduced by increasing droplet size (Table 1). Droplet size can be increased by reducing spray pressure, increasing nozzle orifice size, special drift reduction nozzles, additives that increase spray viscosity, and rearward nozzle orientation in aircraft.

Some postemergence herbicides such as fluazifop-P (Fusilade 2000), fenoxaprop (Option II), quizalofop-P (Assure II), phenmedipham or/or desmedipham (Betanex, Betamix), sethoxydim (Poast), bentazon (Basagran) and bromoxynil (Buctril) require small droplets for optimum performance, so techniques which increase droplet size may reduce weed control. Weed control from herbicides that readily translocate such as 2,4-D, MCPA, dicamba (Banvel), clopyralid (Stinger) and picloram (Tordon) are affected little by droplet size within a normal droplet size range, so drift control techniques generally will not reduce weed control with these herbicides. Glyphosate (Roundup and others) is also readily translocated, so droplet size generally has minimal effect on weed control. (Small droplets may be retained better than large droplets on hard to wet grasses). Glyphosate can be partially inactivated by increased water volume, so spray volume recommendations on the label should be followed.

Method of Application

Low-pressure ground sprayers are commonly used for herbicide application and are normally operated at 30 to 50 pounds per square inch (psi) with 5 to 20 gallons of water per acre. Low-pressure ground sprayers generally produce larger spray droplets that are released from the nozzle closer to the target than with aerial sprayers.

Distance Between Nozzle and Target (Boom Height)

Less distance between the nozzle and the target means less distance for a droplet to travel before drift occurs. Small spray droplets have little inertial energy, so a short distance from nozzle to target increases the chance that the small droplets can reach the target. Check nozzle specifications for the correct height above the target.

Herbicide Volatility

All herbicides can drift as spray droplets, but some herbicides are sufficiently volatile to cause plant injury from drift of vapor (fumes). For example, 2,4-D or MCPA esters may produce damaging vapors, while 2,4-D or MCPA amines are essentially non-volatile and can drift only as droplets or dry particles. Vapor drift occurs when a volatile herbicide changes from solid or liquid into a gaseous state and moves from the target area. Herbicide vapor may drift farther and over a longer time than spray droplets. However, spray droplets can move over two miles under certain environmental conditions so crop injury a long distance from the intended target is not necessarily due to vapor drift. A wind blowing away from a susceptible crop during application will prevent damage from droplet drift, but a later wind shift could move damaging vapors from the treated field into the susceptible crop.

Relative Humidity and Temperature

Low relative humidity and/or high temperature will cause more rapid evaporation of spray droplets between the spray nozzle and the target. Evaporation also reduces droplet size, which in turn increases the potential drift of spray droplets. However, low humidity may reduce the effectiveness of herbicides because rapid drying of a spray droplet will reduce herbicide penetration into a plant. Also, plants growing in low humidity produce a thicker cuticle than in high humidity, resulting in greater resistance to herbicide penetration. In general, total drift movement of herbicide out of the target area will be greater with low relative humidity and high temperatures.

Temperature also influences the volatility of herbicides. Research indicates that vapor formation from a high volatile ester of 2,4-D approximately tripled with a temperature increase from 60 to 80 degrees Fahrenheit. At 80 F, 2,4-D vapor formation was about 24 times greater from a high volatile than a low volatile ester.

Wind Direction and Wind Velocity

Herbicides should not be applied when the wind is blowing toward an adjoining susceptible crop or a crop in a vulnerable stage of growth. All feasible drift control techniques should be used if herbicide must be applied while the wind is blowing toward a susceptible crop.

The amount of herbicide lost from the target area and the distance the herbicide moves will increase as wind velocity increases, so greater wind velocity generally will cause more drift. However, severe crop injury from drift can occur with low wind velocities, especially under conditions that result in vertically stable air.

Air stability

Wind is generally recognized as an important factor affecting drift, but vertical air movement often is overlooked. Normally, air near the soil surface is warmer than higher air. Warm air will rise while cooler air will sink which provides vertical mixing of air. Small spray droplets suspended in the warm air near the soil surface will be carried aloft and away from susceptible plants by the vertical air movement. A temperature inversion occurs when air near the soil surface is cooler than the higher air. Small spray droplets can be suspended longer, move laterally in a light wind and impact plants two miles or more downwind. Temperature inversions are most common near sunrise and are generally associated with low wind and clear skies.

Temperature inversions are identified by observing chimney smoke or dust in the air. Also, fog and dew formation generally indicate the presence of temperature inversions.

Spray pressure

Spray pressure influences the size of droplets formed from the spray solution. The spray solution emerges from the nozzle in a sheet, and droplets form at the edge of that sheet. Increased nozzle pressure causes the sheet to be thinner, and this thinner sheet will break into smaller droplets. Also, larger orifice nozzles with high delivery rates produce a thicker sheet of spray solution and larger droplets than smaller nozzles.

Nozzle spray angle

Spray angle is the angle formed between the edges of the spray pattern from a single nozzle. Nozzles with wider spray angles (110o) will produce a thinner sheet of spray solution, and smaller spray droplets than a nozzle with the same delivery rate but narrower spray angle (80o). However, wide-angle nozzles are placed closer to the target for proper overlap than narrow angle nozzles and the benefits of lower nozzle placement offsets the disadvantage of slightly smaller droplets for drift reduction.

Also See:

Understanding Spray Drift
Spray Drift Management
Avoiding Pesticide Drift
Selecting Nozzle Tips and Strainers