High
Yield Grass Seed Production and Water Quality Protection HandbookHigh
Yield Grass Seed Production and Water Quality Protection Handbook| Nutrient Management | Residue
Management | Weed Control | Herbicide
Drift Management |
| Disease Management | Management
of Insects, Slugs and Related Pests | Vole control
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 Above: By changing to drift-reduction
nozzles, growers can minimize drift and improve product performance. |
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Below: Multiple nozzle bodies enable
growers to vary from drift-reduction or standard nozzles depending on conditions.  |
Herbicide spray drift is a result of the movement of spray droplets or herbicide vapors from the target site.
Several factors contribute to the potential for drift of herbicide spray droplets. Drift management strategies often focus on weather conditions as the primary culprit in herbicide movement. However, the influence of weather conditions such as wind, temperature and humidity is based on spray droplet size. The larger the spray droplet, the lower the risk of herbicide drift.
Low relative humidity and high temperature increase the risk for herbicide drift by increasing evaporation of spray droplets between the spray boom and application target. In turn, smaller spray droplets are more prone to drift off-target. Additionally in low relative humidity and high temperature, plants tend to produce thicker cuticles that resist herbicide penetration and reduce weed control.
The risk of herbicide drift increases with wind speed when the velocity is greater than 3 mph. However, off-target herbicide movement is often a result of applications that are made in "dead calm" or light wind conditions less than 3 mph. Dead calm conditions are often indicative of vertically stable air as a result of a temperature inversion.
Temperature inversions are most common after clear nights when radiant ground cooling is greatest. The release of a smoke bomb or observation of a nearby chimney or fog layer are good indicators of a temperature inversion.
While herbicide drift is most often associated with the movement of spray particles at the time of herbicide application, some herbicides also volatilize and cause injury from movement of vapor or fumes. Volatilization is the change from a solid or liquid into a gas. Herbicide volatilization may occur several days after application and vapors may travel as far as a few miles. Volatility differs by herbicide and formulation. For example, 2,4-D ester is volatile and prone to vapor drift, but 2,4-D amine is relatively non-volatile. Both 2,4-D ester and 2,4-D amine may drift as spray droplets.
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Translocated herbicides that are readily moved in the plant may be applied with a larger droplet size, around 350-450 microns. Examples of such herbicides include glyphosate, 2,4-D and clopyralid. Contact herbicides, however, are not translocated well in the plant and require thorough coverage for effective control. Examples include paraquat, bromoxynil and glufosinate. Droplet size for contact herbicides should range from 200-350 microns. As a general rule for herbicides, spray droplet size should be greater than 200 microns.
Spray particle size increases as spray pressure decreases (Table 2). The spray solution emerges from the nozzle in a sheet, and droplets form at the edge of the sheet. As spray pressure increases, the sheet becomes thinner and breaks into smaller droplets.
Larger orifice nozzles with high delivery rates produce a thicker sheet of spray solution and larger droplets than smaller nozzles. Therefore, increasing nozzle size and applied gallons per acre may reduce the potential for herbicide drift.
Droplet sizes produced at various spray pressures will differ depending on nozzle types (Table 3). Several drift-reduction nozzles are now available. The optimum spray pressure differs by nozzle type. Spraying at a pressure greater than that recommended for a particular nozzle can increase the risk for herbicide drift, so follow manufacturer's recommendations for each nozzle.