Action against weed resistance

Types of herbicide resistance

Can weeds be resistant to more than one herbicide or herbicide Group?

Resistant weeds may be resistant to only one herbicide Group or to two or more herbicide Groups. And two terms commonly used to describe how weeds resist herbicide activity include "target-site resistance" and "metabolic resistance":

Herbicides work like a lock and key mechanism: by de-activating important enzymes in weeds, they essentially "lock" them out. However, if the enzyme changes its shape, the herbicide may not be able to bind and deactivate it. And that’s when the herbicide no longer works anymore on that weed.
  • Resistance
    Single Herbicide Resistance
    A biotype that is resistant to only one active ingredient. (Example: weed is resistant to active ingredient X which is a Group A, but is still susceptible to active ingredient Y, which is also a Group A).
  • Cross-Resistance
    Cross Herbicide Resistance
    A biotype with resistance to two or more active ingredients belonging to the same herbicide Group. (Example: weed is resistant to multiple or all active ingredients in Group A, but is still susceptible to other herbicide modes of action).
  • Multiple resistance
    Multiple Herbicide Resistance
    A biotype with resistance to two or more herbicide Groups. This may be the result of two or more different resistance mechanisms. (Example: weed is resistant to Group A and Group B herbicides).

Metabolic resistance prevents herbicides from reaching their target sites (usually enzymes) by:

  • Reducing herbicide absorption or translocation,
  • Detoxifying a herbicide's active ingredient,
  • "Storing" the herbicide in a cellular site that is not vulnerable to the active ingredient, or
  • Repairing damaged tissue.

Do herbicide-resistant weeds still show symptoms of herbicide damage, or would they appear untouched?
Unfortunately, it can be both. Typically,you might not identify resistant populations until there have been multiple failures with the same product or products in the same herbicide Group, identified by patches of “weed escapes.” For example, if you have observed that a herbicide has become less effective over the past three years, you’ve selected for resistant biotypes.