The Herbicide Resistance Action Committee (HRAC) groups herbicides based on how they work, called their mode of action (MoA). This helps farmers and growers recognize the herbicides they use and manage pesticide resistance globally. HRAC is managed by CropLife International in partnership with the Weed Science Society of America (WSSA).

Systems

There are three classification systems: Australian, WSSA (also called Global), and Numeric. All three systems group herbicides based on how they work, and their categories usually match closely. For example, the Australian Group G is the same as the WSSA Group E and the Numeric Group 14.

These systems were created separately, and farmers became familiar with them over time. The WSSA system became widely used in Canada and the United States, and it is called the "global" system. The Australian system remained used only in Australia.

In 2021, a numeric system was introduced to make herbicide codes more consistent worldwide. This system also added or changed the classification of some herbicides. It was created to solve confusion and make it easier for growers in different countries to use the same system.

Resistance overview

A weed that becomes resistant to one herbicide often also becomes resistant to other herbicides that work in the same way. To manage this, farmers must use herbicides that work differently, or belong to different resistance groups. Rotating herbicides with different modes of action prevents weeds from developing resistance to the same type of herbicide. When this happens, future weed generations do not gain an advantage from resistance traits that appeared in earlier generations. Weeds that are resistant to multiple herbicides, either through cross-resistance or multiple resistance, are very hard to control.

There is little evidence that resistance to one herbicide can remove resistance to another. For example, prosulfocarb and trifluralin have opposite resistance mechanisms. Evolving resistance to one herbicide may cause the weed to lose resistance to the other, especially through metabolic resistance. Prosulfocarb requires weeds to break it down very slowly to survive, while trifluralin must be broken down quickly to avoid damage.

Herbicide resistance became a major issue in the 1970s and 1980s. Weeds have now developed resistance to 23 out of 26 known herbicide sites of action, and to more than 163 different herbicides. The creation of new herbicides has slowed since around 2000, and no new ways of acting have been discovered by 2020.

Naming

Group Z is for herbicides whose way of working is not yet fully understood and needs more study to be placed in the correct group. Groups J and Q are not used in some systems, like the Australian HRAC, because they can be confused with Groups I and O.

Herbicides that work in more than one way are listed in multiple groups, each showing a different way they act. For example, Quinmerac is in Group 4/29 (O/L) because it both mimics a plant hormone called auxin (Group 4 or O) and stops plants from making a material called cellulose (Group 29 or L).

In the WSSA classification, some groups have similar purposes but work through different methods:

• C1, C2, C3: Herbicides that stop plants from making food through photosynthesis. These groups differ based on how they interact with proteins in the plant.
• F1, F2, F3: Herbicides that cause plants to lose their green color (bleaching).
• K1, K2, K3: Herbicides that slow plant growth.

Groups

Note: Some herbicides previously classified as Group N (WSSA) / Group 8 have been reclassified into Group 15. For example, prosulfocarb was once listed in Group N / Group 8. However, as of 2025, Group 8 is no longer on the global HRAC list (linked here), and prosulfocarb is now listed in Group 15 (Australian Group J).

Note: Different sources may provide conflicting classifications. This could be due to differences in how the HRAC and WSSA categorize herbicides, as well as updates and changes in classifications over time.