Resistance overview
A weed that develops resistance to one herbicide typically has resistance to other herbicides with the same mode of action (MoA), so herbicides with different MoAs, or different resistance groups, are needed. Preventative weed resistance management rotates herbicide types to prevent selective breeding of resistance to the same mode of action. By rotating MoAs, successive generations gain no advantage from any resistant mutations of the last generation. ''Cross-resistant'' and ''multiply resistant'' weeds resist multiple MoAs, and are particularly difficult to control. There is limited evidence of resistance undoing other resistances. For example, prosulfocarb and trifluralin: their inverse mechanisms of resistance contradict, and so by evolving to one the weed loses resistance to the other, at least by metabolic resistance. Prosulfocarb requires a weed to metabolise it very slowly to survive; trifluralin on the other hand must be metabolised quickly before it can deal damage to the weed. Resistance first became problematic in the 1970s and 1980s, and herbicide resistant weeds have developed against 23 of 26 known herbicide sites of action, and over 163 different herbicides. Herbicide development has slowed down significantly, with no new mechanisms being discovered from circa 2000 to 2020.Naming types
The HRAC give a letter based class to each active constituent herbicide. The Australian HRAC code is separately assigned, though is often the same as the global code. In 2021, alternative numeric classes were added, to make codes globally more consistent. This set of classification changes also added or moved a few herbicides that had been misclassified, and reduced regional concerns that using theGroups
See also
*References
{{Herbicides Herbicides