Sulfonylurea Herbicides Resistance to ALS Inhibitors Fact Sheet

Key Summary Points:

• Resistance is a common problem with virtually all classes of herbicides. Those that work by acetolactate synthase (ALS) inhibition, such as sulfonylureas, are no different.
  
• Resistance is a manageable problem.

     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:

• Reliance year after year on a single, highly effective herbicide or on herbicides with the same mode of action for weed control.
  
• Use of longer-residual herbicides (or multiple applications of shorter residual herbicides with the same of mode of action) in the same season.
  
• Exclusive reliance on chemical methods of weed control, rather than including other practices such as tillage or cultivation.

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:

• Use shorter-residual herbicides versus longer-residual herbicides.
• Use tank mixes or sequential treatments of herbicides that have different modes of action, which will provide different modes of action to control the weed.
• Where possible, use tillage or cultivation in conjunction with herbicides as part of any overall weed control program.
• Use crop rotation where possible since the risk of herbicide resistance is greatest in a monoculture situation.
• If a weed escapes herbicide treatment or tillage, do not let it go to seed.

     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 sign)ficant 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.