APPLICATION TECHNOLOGY AND BEST MANAGEMENT-PRACTICES FOR MINIMIZING HERBICIDE RUNOFF

The fate of field-applied herbicides, including losses in surface runo ff with water and sediment, is highly dependent on herbicide propertie s. The two most important properties are soil adsorption and persisten ce. Adsorption affects the potential for a herbicide to be lost primar ily with sediment, runoff water, or possibly leaching water. Solubilit y, often though not always inversely correlated with adsorption, is of secondary importance, although low solubility can limit transport wit h water. Persistence affects the time available to be lost in runoff S tudies have shown that for soil-applied herbicides, extraction into ru noff water or movement with sediment takes place from a thin soil laye r at the surface. In addition, for herbicides studied, there is little interaction between surface crop residue and applied herbicides, and washoff from the residue readily occurs with small amounts of rainfall . Runoff loss equals the volume of carrier (water or sediment) times t he concentration in that carrier; therefore, practices that reduce eit her, or both, can reduce losses. Rate of application has been directly related to concentration and therefore loss. Reducing rate, such as b y banding, soil incorporation, and avoidance of application to crop re sidue reduce losses. The choice of herbicide and herbicide formulation , in conjunction with application technology, as they affect propertie s, rate, and placement, play a large role in determining runoff loss. Runoff losses of herbicides that are strongly adsorbed and therefore t ransported mainly with sediment can be reduced by erosion control; run off volume reduction can reduce losses with water of moderately to wea kly adsorbed herbicides. Conservation tillage has potential to reduce both runoff and erosion. Timing of application relative to expected in tense storms, both in the short and long term, can reduce the potentia l for runoff. If possible to determine thresholds, herbicide use could be avoided if weed infestation is below the economic effect level. Bu ffer or filter strips have the potential to reduce transport of herbic ides lost from fields to surface water resources.