The increasing emergence of herbicide-resistant weed species prompted research and extension specialists to develop guidelines for growers to use to minimize the possibility of inadvertently helping to spread resistant species. This approach is based, in part, on how various herbicides work, i.e., their sites of action within the plants they contact.

Recently, E.J. Retzinger, Jr., and C. Mallory-Smith published "Classification of Herbicides by Site of Action for Weed Resist ance Management Strategies," listing the characteristics for num erous herbicidal products. According to the authors, the listing was "developed to help maintain the usefulness of herbicides as a tool in crop production" and to also slow the emergence of herbicide-resistant weeds.

In their paper, the authors refer to a set of guidelines pub lished by the Weed Science Society of America (WSSA) for promot ing awareness of the problem and recommending steps that growers should follow. The following points are broadly based on the WSSA guidelines.

1. Scout fields (I): before applying a herbicide; determine weed species present.
2. Evaluate economics: is herbicide application justified by increased returns?
3. Consider use of alternatives: cultivation, delayed planting, using weed-free crop seeds.
4. Rotate crops: avoid growing the same crop in the same location season after season.
5. Limit herbicide application: minimize usage of a single herb icide, or herbicides with the same site of action, during a growing season.
6. Vary herbicides: use mixtures or sequential applications.
7. Scout fields (II): after applying herbicides, note weed escapes or species shifts.
8. Eliminate weed seed transfer: clean off equipment and clothing before moving from an infested area.