Several drift control methods are available that will reduce drift to an acceptable level if they are applied. Zero drift is impossible with present day equipment. One drift control method that is becoming very popular is the use of shields. If wind and weather cannot be controlled, it may be advisable to control conditions where chemicals are being applied with the use of a shield. Several manufacturers provide shields for nozzle spray pattern protection. Some cover the entire spray boom with a solid cover, others are perforated shields while others are individual nozzle spray shields.

Several research reports are available comparing the drift. The following is a summary of the research completed.

This study was completed in 1992 and consisted of measuring the relative drift from a shielded sprayer, wind cones, a shield constructed in the Ag Engineering Department and an unshielded spray boom. This study was done with an 8001 nozzle operating at 40 psi, travel speed of 4 mph with a nozzle spacing of 20 inches. This gave an application rate of 7.4 gpa. Trials were completed in varying wind conditions. A fluorescent dye was added to the

spray tank and spray drift was collected on a cotton string which was analyzed with a fluorometer. The collection site was laid out with 100 feet of string placed horizontal above ground plus an 18 foot vertical section at the far end of the horizontal. This was placed downwind of the sprayer and parallel with the wind. A sketch of the test area is shown in Figure 1.

Table I shows the relative difference in the downwind drift for both the horizontal and vertical sections of string. Each trial was replicated 12 times to allow for variable wind conditions.

It was found in this study that all shields significantly reduce the drift as compared to an unshielded boom (Table 1). There is no significant difference among shields but it looks like full boom shields reduce the drift slightly better than windcones. All shields reduce the drift by at least 50%.

It is to be noted that shields only reduce the amount of drift and DO NOT ELIMINATE drift completely. Care must be used when spraying near susceptible crops as some drift still occurs.

Figure 2 is an example of the drift collected on both 100 foot horizontal string and the 18 foot vertical string. Most ofthe drift drops fall out in the first 2O feet downwind of the sprayer, but significant spray again shows up on the l8 foot vertical section in variable places and amounts. This tells us that fine spray drops stay suspended in the air and tend to move up as they move horizontally. This is caused by turbulence from wind. Spray moving downwind can damage shelterbelts and will not come down to the earth until they are picked up in rain.

Figure 1. Field Test Layout.

Summer 1992

V. Hofman, NDSU

Figure 2. Example graphs of downwind drift on horizontal and vertical strings.

This research project used a full metal shield (Renn-Vertec, now Blumhardt) with 8002 and 800025 nozzles. It was found that the use of a hood over a spray boom greatly reduced the amount of drift when using the 8002 nozzle. Drift was reduced about two-thirds over an unshielded boom. It was also found that drift from hooded sprayers is highly dependent upon the droplet spectrum. Decreasing drop size from 320 microns when using an 8002 nozzle to 130 microns when using the 800025 nozzle produced a three-fold increase in drift. A shield can significantly reduce drift but if smaller nozzles are installed to reduce application rates, drift will again rise.

This research study used 8002 and 11002 nozzles on a shielded and unshielded spray boom. Brandt Windcones were used on the shielded trials. Results of the trials are shown in Tables 2 and 3.

A summary of this study indicates that individual nozzle shields on a ground sprayer will significantly reduce drift.

Also, drift is reduced more with shields at low wind speeds than in stronger winds. This is indicated in Table 3. Variation of spray patterns over the swath width does not change significantly with the use of shields as compared to an unshielded boom.

When applying pesticides there is always a chance that some spray will escape from the target area. Drift is of concern because it removes the chemical from the intended field, making it less effective and depositing it where it is not needed and often not wanted. The second concern is generally the most critical because the pesticide becomes an environmental pollutant in the off target area. Costly problems can result when carelessly applied pesticides, especially herbicides, drift and cause damage to trees and important crops. Although drift cannot be completely eliminated, the use of proper equipment and spraying techniques will maintain drift deposits within acceptable limits.

The primary recommendation for drift control is to read the pesticide label. Instructions are given to insure the safe and effective use of pesticides with minimal risk to the environment. Chemical company surveys indicate that approximately two-thirds of the drift complaints involved application procedures known to be "off-label."

There are two ways that herbicides move downwind: 1) particle drift and 2) vapor drift. Particle drift is the off- target movement of spray particles formed during application. Vapor drift is associated with the volatilization of herbicide molecules and the movement of these molecules off-target, making it independent of the application. The vapor drift potential of a herbicide can be predicted by its vapor pressure in relation to air temperature, the size of the treated area, and climatic conditions. Most investigations show that the distances that vapor can travel are much greater than the distances traveled by particle drift of nonvolatile herbicides. Because the volatilities of herbicides generally are known, formulations can be used that will not produce unacceptable off-target effects.

The amount of particle drift depends mainly on the number of small "driftable" particles produced by the nozzle. Although excellent coverage can be achieved with extremely small droplets, decreased deposition and increased drift potential limit the minimum size that will provide effective weed control.

With water carriers, spray droplets decrease in size during application due to evaporation. Even in a 1 to 2 mph wind, droplets less than 100 microns in size obtain a horizontal trajectory in a very short time and the water in the droplet rapidly disappears. The pesticide in these droplets become very small aerosols most of which will not fall out until picked up in falling rain. For example, water droplets less than 20 microns in diameter will evaporate in less than one second at a distance of less than one inch from the nozzle (Table 2). Droplets over 10 microns in size resist evaporation much more than smaller droplets due to their larger ratio of volume to surface area.

From these research results, we can conclude that there is a rapid decrease in drift potential of droplets as they increase to about 150 or 200 microns. The size where drift potential decreases depends on wind speed, but generally the size lies in the range of 150 to 200 microns for wind speeds of 1 to 7 miles per hour. For typical ground application of pesticides with water carriers, droplets of 50 microns or less will completely evaporate to a residual core of pesticide before reaching the target (Table 2). Droplets greater than 150 microns will have no significant reduction in size before deposition on the target. Evaporation of droplets between 50 and 150 microns is significantly affected by temperature, humidity and other climatic considerations.

Most hydraulic nozzles produce a wide range of droplet sizes, from less than 10 microns to over 1000 microns depending on the type and size of nozzle being used. The actual size distribution of droplets produced by a nozzle needs to be known in order to make adjustments concerning coverage, deposition, and spray drift potential.

To estimate the drift potential from spray nozzles, the percentage of the spray volume that is contained in droplets having a diameter less than 100 microns frequently is used to represent the "driftable" fraction.of spray produced by a nozzle. Table 1 shows a summary of droplet sizes for typical nozzles used to apply herbicides. As shown in the table, there is a wide range of spray volume contained in droplets less than 100 microns. The spray volume contained in small droplets is affected by nozzle type, nozzle size, and spray pressure. For each application, these operating parameters must be selected to provide the coverage required while maintaining the drift potential within acceptable limits.

Techniques used when applying herbicides greatly determine the amount of spray drift that occurs (Table 3). The type of nozzle, pressure, height, and spray volume all affect the off-target movement. The ability to reduce drift is no better than the weakest component in the spraying procedure.

The potential for drift must be considered when selecting a nozzle type. Of the many nozzle types available for applying pesticides, a few are specifically designed for reducing drift. The Raindrop nozzle, for example, has been designed to effectively reduce the exit pressure at the spray tip, resulting in a reduction of small droplets. Some nozzles are designed to operate effectively at low pressures. Extended range flat-fan nozzles provide uniform spray patterns at pressures down to 15 psi, thereby reducing the amount of small driftable spray particles in the spray pattern. Higher pressures produce finer spray droplets.

Spray height is an important factor in reducing spray drift losses. Drift can be reduced by mounting the boom closer to the ground. Correct spray height for each nozzle type is determined by nozzle spacing and spray angle. Wide-angle nozzles can be placed closer to the ground than nozzles producing narrow spray angles. On the other hand, wide-angle nozzles also produce smaller droplets. When this occurs, the advantages of lower boom height are negated to some extent.

Using larger nozzle sizes is a means of minimizing drift. Increasing the spray volume by using higher capacity spray tips results in larger droplets that are less likely to move off- target. The only effective means of reducing drift by increasing spray volume is to increase the nozzle size.

Weather conditions can have a major impact on the amount of off-target drift. Factors affecting drift include wind speed and direction, temperature, relative humidity, and atmospheric stability. Wind speed is usually the most critical factor of all meteorological conditions affecting drift. The greater the wind speed, the farther off-target small droplets will be carried.

Determining the wind direction relative to sensitive crops is important in attempting to minimize damage from drift. The presence of sensitive crops downwind often is overlooked by applicators. Leaving a buffer zone at the downwind edge of a spray area will greatly reduce damage to sensitive plants. After the wind has died down or changed direction, the buffer zone can be safely sprayed.

Temperature and humidity also affect the amount of drift that occurs through evaporation of spray particles. Although some evaporative loss of spray occurs under all atmospheric conditions, these losses are usually less pronounced in cool and damp conditions. Temperature also influences atmospheric air turbulence, stability, and inversions.

A stable atmosphere or "inversion" can be recognized by observing a column of smoke. If the smoke does not dissipate or if it moves downwind without vertical mixing, conditions are not good for spraying. The best way to avoid drift associated with atmospheric conditions is to eliminate the formation of small particles from the spray. Once this is done, weather stability factors essentially can be ignored.

One of the best tools available for minimizing drift damage is the use of drift control additives to increase the spray drop size. Tests indicate that downwind drift deposits are reduced from 50 to 80 percent with the use of drift control additives. Drift control additives make up a specific class of chemical adjuvants and should not be confused with products such as surfactants, wetting agents, spreaders, and stickers. Drift control additives are formulated to produce a droplet spectrum with fewer small droplets.

A number of drift control additives are commercially available, but they must be mixed and applied according to label direction in order to be effective. Some products are recommended to be used at a rate of two to eight ounces per 100 gallons of spray solution. Increased rates may further reduce drift but also may cause nozzle distribution patterns to be nonuniform. Drift control additives will vary in cost from as low as 10 cents to over one dollar per acre. They do not eliminate drift, however, and common sense must still remain the primary factor in reducing drift damage.

When drift control practices are used, many result in the production of larger drops. Be sure the pesticide you are using allows for use of larger drops. BE SURE TO READ THE CHEMICAL LABEL.

Table 1. Comparison of dropmet spectrums for various nozzle tpes, sizes and pressures

DROPLET SIZE COMPARISONS OF NOZZLE TYPE AT 40 PSI

DROPLET SIZE COMPARISON OF NOZZLE SIZE AT 40 PSI

DROPLET SIZE COMPARISONS OF PRESSURE

Table 2. Evaporation and Deceleration o Various Size Droplets ¹

Conditions assumed: 90°F, 36% Relative Humidity, 25 psi, 3.75% pesticide solution.

Obtained from an article written by Loren Bode and Robert Wolfe, Agriculture Engineering Department, University of Illinois, Urbana, Illinois.

Table 3. Summary of recommended procedures for reducing drift damage.