Application Technology and Best Management Practices for Minimizing Herbicide Runoff

Abstract

The fate of field-applied herbicides, including losses in surface runoff with water and sediment, is highly dependent on herbicide properties. The two most important properties are soil adsorption and persistence. Adsorption affects the potential for a herbicide to be lost primarily with sediment, runoff water, or possibly leaching water. Solubility, often though not always inversely correlated with adsorption, is of secondary importance, although low solubility can limit transport with water. Persistence affects the time available to be lost in runoff. Studies have shown that for soil-applied herbicides; extraction into runoff water or movement with sediment takes place from a thin soil layer 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 the concentration in that carrier; therefore, practices that reduce either, or both, can reduce losses. Rate of application has been directly related to concentration and therefore loss. Reducing rate, such as by banding, soil incorporation, and avoidance of application to crop residue reduce losses. The choice of herbicide and herbicide formulation, in conjunction with application technology, as they affect properties, rate, and placement, play a large role in determining runoff loss. Runoff losses of herbicides that are strongly adsorbed and therefore transported mainly with sediment can be reduced by erosion control; runoff volume reduction can reduce losses with water of moderately to weakly adsorbed herbicides. Conservation tillage has potential to reduce both runoff and erosion. Timing of application relative to expected intense storms, both in the short and long term, can reduce the potential for runoff. If possible to determine thresholds, herbicide use could be avoided if weed infestation is below the economic effect level. Buffer or filter strips have the potential to reduce transport of herbicides lost from fields to surface water resources.