Key Summary Points:
Spray drift, sometimes referred to as primary, or physical, drift, is the movement of spray droplets outside the target site at the time of application (before deposition has occurred). Spray drift can be successfully managed through good application practices. It is primarily the interaction between droplet size, nozzle height (application height), and weather conditions that affect the potential for spray drift.
Droplet size is the most important factor affecting the potential for spray drift. Factors that affect droplet size include nozzle type and capacity, spray pressure, temperature, and relative humidity. For aerial application, nozle orientation and aircraft speed are also important. Spray drift potential is based on the physics of droplet movement. Applicator knowledge of the factors that affect spray drift and prudent decisions concerning equipment set-up and application technique determine the incidence and impact of spray drift.
The national Spray Drift Task Force (SDTF) is a consortium of the U.S. crop protection product producers and formulators that was formed in response to spray drift data requirements imposed by the Environmental Protection Agency (EPA). The SDTF quantified spray drift from a range of applications for aerial, ground, airblast, and chemigation applications. The studies and resulting computer models quantify the various factors that can affect spray drift. The results and conclusions are consistent with previous scientific studies.
The SDTF studies confirm that active ingredients do not differ in the potential for spray drift. SDTF results conclusively demonstrate that sulfonylurea herbicides have no greater potential for spray drift than other classes of active ingredients. The liquid physical properties of the final spray preparation can affect spray drift potential by affecting the droplet size spectrum. However, it is primarily additives and certain formulation ingredients that have this effect. The vast majority of spray preparations consist of multiple ingredients that combine to produce similar physical properties and similar drift potential.
A good spray drift management program includes:
Secondary drift refers to the off-site movement of an active ingredient through the air after the droplets have deposited onto plant or soil surfaces. Vapor drift, the evaporation of volatile active ingredients from plant and soil surfaces, is the most frequently documented form of secondary drift.
Unlike spray drift, the potential for vapor drift is very dependent on the chemical/physical properties of the active ingredient. The potential for off-site damage from vapor drift is only sign)ficant for relatively volatile (high vapor pressure) active ingredients. Sulfonylurea herbicides are compounds with extremely low volatility and, therefore, will not cause damage via vapor drift. Because vapor drift and other forms of secondary drift occur after spray application deposition, application related factors (droplet size, application height, etc.) tend to be less important in affecting secondary drift potential.