The Herbicide Resistance Action Committee (HRAC) classifies herbicides by their mode of action (MoA) to provide a uniform way for farmers and growers to identify the agents they use and better manage pesticide resistance around the world.^[1][2] It is run by CropLife International^[3] in conjunction with the Weed Science Society of America (WSSA).^[4]

Systems

There are three classifications: Australian, WSSA (also called Global) and Numeric. They all classify by mode of action, and their categories map mostly 1-to-1. For example, the Australian Group G is the WSSA's Group E, is the numeric Group 14.

These two systems were developed independently, and farmers got used to it. The WSSA's system took hold in Canada and the US, and is also called the "global" system, as opposed to the Australian system, which stayed in Australia.^[5]

In 2021, a numeric system was added, to make codes globally more consistent. This classification also added or reclassified some herbicides. It came due to confusion and reduced regional concerns that using the English alphabet could be too difficult for international growers.^[6]

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.^[6] Cross-resistant and multiply resistant weeds resist multiple MoAs,^[7] 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.^[8]

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.^[9]

Naming

Group Z is for an unknown mode of action until it can be grouped exactly. Groups J and Q are skipped (but not in the Australian HRAC) for confusion with I and O.^[10]

Herbicides that act through multiple modes have multiple classifications, corresponding to each MoA.^[11] For example, Quinmerac is classified as Group 4/29 (O/L) because it is both an Auxin mimic (Group 4 or O) and inhibits cellulose synthesis (Group 29 or L).^[12]

In the WSSA classification, there are related classes, with different mechanisms to achieve similar effect:^[10]

• C1, C2, C3: Photosynthesis inhibitors. Subclasses differ by behaviour at the binding protein.
• F1, F2, F3: Bleaching herbicides
• K1, K2, K3: Growth inhibitors

Groups

Note: Several Group N (WSSA) / Group 8 herbicides have been rolled into Group 15, for example prosulfocarb, which has been a Group N / Group 8^[10] but as of 2025, Group 8 does not appear on the global HRAC's list (linked here), and prosulfocarb is listed in a Group 15. (Australian Group J)^[18]

Note: Sources disagree on some classifications. This might be due to separation between the HRAC and the WSSA, updates and reclassifications.

See also

• Insecticide Resistance Action Committee
• Fungicide Resistance Action Committee

References

External links

• Global Herbicide Classification Lookup