Weed Control Guide for Ohio Field Crops

Weed Control Principles

Off-Target Movement of Herbicides

Spray drift is the downwind movement of spray particles from the target site to areas with sensitive plant species. The extent of spray drift increases as (1) the size of spray droplets decreases, (2) the height above the ground from which the droplets are released increases, and (3) wind speed increases. Drift can be minimized by following these guidelines.

• Spray when wind speed is low.
• Use the maximum nozzle orifice size without distorting spray pattern.
• Reduce spray pressure to the lowest setting without distorting spray pattern.
• Using nozzles that minimize drift, such as Floodjet, Turbo Floodjet and Flat Fan DriftGuard nozzles.
• Use drift control agents when permitted by the label.
• Follow label precautions for drift reduction measures.

Volatility and vapor drift is the tendency of a herbicide to vaporize and drift through the air as a gas. A herbicide with a high vapor pressure has a greater tendency to volatilize than one with a low vapor pressure. Eradicane must be incorporated to eliminate volatility that results in reduced weed control. Application of Command is prohibited near sensitive plants because of the phytotoxicity of spray particles and vapors. Banvel and the ester formulations of 2,4-D may vaporize at temperatures as low as 70°F and be moved by wind to areas with sensitive plants, including soybeans and vegetable crops. Amine formulations of 2,4-D are essentially nonvolatile.

The rate of herbicide volatilization increases with increasing temperature. In the summer, temperatures at the soil surface may exceed 140°F on a clear day, greatly enhancing conditions for volatility. Vapors drift farther and over a longer time than spray droplets. Changes in temperature and wind direction following application can move damaging vapors to sensitive plants. To avoid vapor drift, carefully observe label precautions when applying a volatile herbicide.

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