How are herbicides classified based on modes of action?

Back

A herbicide mode of action describes the way a herbicide works to control weeds. In general, herbicides interrupt certain biological processes, often by disrupting enzyme activity or other plant functions. The numerical mode of action system groups herbicides and provides a reference for growers and ag professionals to rotate modes of action and help design integrated weed management programs (Figure 10). Currently, herbicides are grouped into 25 different modes of action (plus an additional group of herbicides with unknown modes of action for 26 total groups). Each class is identified by a Group Number.

The herbicide site of action, or target site, is the location of the primary interruption/disruption and is often a specific plant enzyme. Enzymes are proteins that function like stairsteps in a biological process required for plant growth. In most cases, herbicides within the same mode of action group have the same site of action. For example, all Group 1 herbicides inhibit the enzyme acetyl CoA carboxylase (ACCase inhibitors) and all the Group 2 herbicides inhibit the enzyme acetolactate synthase (ALS inhibitors) (Figure 10).

Figure 10. Take Action Herbicide Classification by Mode of Action Poster. In this chart, herbicides are grouped by mode of action. This visible portion shows three groups (1, 2, and 9), the chemical family, active ingredient, and branded product examples. Premix products and their active ingredients and trade names are also shown.

The herbicide mode of action describes the way a herbicide works to control weeds by interrupting a certain biological process. Herbicides with the same mode of action share a Group Number and have similar properties (Figure 10). For example, ACCase inhibitors are the mode of action of the postemergence grass-specific herbicides and includes three different chemical families. All three families have the same mode of action and belong to Group 1. Clethodim and quizalofop are example Group 1 active ingredients. The Group 2 herbicides are the ALS inhibitors and include five different chemical families. Imazamox and chlorimuron are example Group 2 active ingredients. Glyphosate is a Group 9 herbicide. The Group 9 mode of action is the inhibition of EPSP synthase. Of the 26 different herbicide groups, 17 are commonly used in U.S. agriculture. For ag professionals and farmers, knowing the Group Number classification is important for using multiple effective modes of action and for herbicide rotation.

The chemical family is based on the chemical structure of the herbicide molecule. There are three chemical families in the Group 1 herbicides including the aryloxyphenoxypropionate (FOP), cyclohexanedione (DIM), and phenylpyrazolin (DEN) (Figure 10). The active ingredient, also known as the common name for the herbicide, is listed on the herbicide label. Examples of Group 1 common names include quizalofop, clethodim, and pinoxaden. Finally, labels also list herbicide trade names, the brand names the herbicides are sold under. For example, herbicide trade names include Assure II (quizalofop), Select (clethodim), and Axial XL (pinoxaden). Many herbicide products contain more than one herbicide and are sold as prepackaged mixtures (premixes). Product labels contain the herbicide group number for all the active ingredients. Axial Star is a premix of pinoxaden (Group 1) and fluroxypyr (Group 4) (Figure 11).

Figure 11. This herbicide label cover page displays the herbicide group numbers as well as active ingredients. Axial Star contains herbicide active ingredients with two different modes of action and thus, two different group numbers. (Image credit: https://www.syngenta-us.com/current-label/axial_star, accessed online, Nov. 2, 2023)

The global Herbicide Resistance Action committee, (HRAC) is an international body founded by the agrochemical industry, which has developed the most current mode of action classification scheme with corresponding group numbers. The 25 known herbicide modes of action fall into three broad categories based on their mode of action: light activation, cellular metabolism, and cell division and growth. HRAC has published a MAP in poster form that shows how the different herbicide modes of actions are grouped (Figure 12). Herbicide groups target different physiological processes in the plant cell based on mode of action. These processes can include photosynthesis, fatty acid synthesis, amino acid synthesis, and other processes (Figure 13).

 

Figure 12. The HRAC Mode of Action Classification Map provides more detail on herbicide mode of action and the herbicide target site in the plant cell. (Image credit: https://hracglobal.com/tools/hrac-mode-of-action-classification-2022-map)

Figure 13. This illustration shows the target of different herbicide groups in the plant cell based on mode of action. Herbicides disrupt key physiological processes within the plant cells, including photosynthesis, fatty acid synthesis, amino acid synthesis, and other processes. (Illustration adapted by Lourdes Rubione and William Curran from Delye et al. 2013 Trends in Genetics)