7.5 Herbicide Resistance

Any weed population may contain or develop plants resis­tant to an herbicide or class of herbicides (herbicides with the same mode of action or the same target site of action). Weed species with acquired resistance may eventually dominate the weed population if herbicides from the same class are used repeatedly in the same field or in successive years as the primary method of weed control. This may result in partial or total loss of control of those species by the group of herbicides. In some species, repeated use of one herbicide class may lead to the selection of naturally occurring biotypes that are resistant to these products.

If poor performance occurs and cannot be attributed to ad­verse weather or application conditions, a resistant biotype may be present. This is most likely to occur in fields where other control strategies such as crop rotation, mechanical removal, and other classes of herbicides are not used from year to year.

To delay herbicide resistance:

·            Use best practices, including IPM (integrated pest management) to develop an overall weed management strategy,

·            Monitor weed populations, especially treated weeds for loss of herbicide efficacy,

·            Avoid the consecutive use of the same herbicide or herbicides from the same class,

·            Use tank mixtures or premixes with herbicides from different classes (as long as all products are registered for the same use and are effective at the mix rate on the weed of concern).