On the other hand, herbicide resistance is a well-understood phenomenon, and a variety of strategies for managing resistance have been successfully applied. Herbicide resistance rarely occurs when growers rotate crops and use herbicides with different modes of action on the same weed species.
Herbicides are valuable anywhere weeds need to be controlled, even in areas where weed resistance has occurred. Most biotypes resistant to one herbicide remain sensitive to herbicides with different modes of action. Weed biotypes resistant to herbicides that normally control the species exist naturally as a tiny fraction of the weed population.
Herbicide resistance was first reported in the 1960s and has since been confirmed in more than 100 weed species worldwide. Plants have the ability to develop resistance to all classes of herbicides. This has occurred with most classes and has the potential to happen to all. Resistance to ALS inhibitors was first confirmed in 1987. Biotypes of 38 species (principally broadleaf weeds but some grass and sedge species as well) are now reported to be resistant to ALS inhibitors.
Conditions that can promote herbicide resistance include:
Note: The following section relates to resistance to ALS inhibitors specifically in the U.S. and Canada. Other regions of the world need to adapt the information to their specific situations.
Cases of resistance to ALS inhibitors have occurred principally in cereal growing areas of North America because of farming practices such as not rotating crops or using only one herbicide mode of action. For example, kochia, a weed that affects cereal crops in the plains of the U.S. and Canada, has the largest number of biotypes with confirmed resistance to ALS inhibitors.
Resistance to ALS inhibitors is proving to be similar to resistance to other herbicide classes, and implementation of recommended management practices will help maintain the value of these herbicides for the foreseeable future. Specific recommendations include:
When used properly, ALS-inhibiting herbicides provide effective, easy-to-use, and economical control of weeds. Good management practices will help growers ensure that resistant biotypes do not affect the quality or yield of their crops.
As a leading world producer of herbicides, the DuPont Agricultural Enterprise considers resistance an important industry issue and conducts significant research on how to manage its effects. DuPont also works with several private, public, and academic organizations to develop and provide guidelines for minimizing resistance to herbicides.