International Survey of Herbicide-Resistant Weeds

Countries Weeds Herbicides Mutations Graphs References Researchers
Three Herbicide Site of Action Classification Systems
In an ideal world we would only have one classification system for herbicide sites of action, no such luck. In 1990 whilst at the University of Manitoba I created a classification system based on numbers to assist growers in rotating herbicides, and published it in a fact sheet in 1991  – Group 1 = ACCase inhibitors, Group 2 = ALS inhibitors etc. This was added to and became the basis of the Canadian herbicide classification system, which in turn became the basis of the WSSA (Weed Science Society of America) herbicide classification system first published by Retzinger and Mallory-Smith in 1997.

In the early 1990's Australia created a classification system  based on letters, and HRAC (Herbicide-Resistance Action Committee) also created a classification system  also based on letters … unfortunately not the same letters as the Australian classification system. This was not done deliberately, each group worked independently and thought they were coming up with the “first classification system”. Because growers became use to “their” classification system it became impractical to choose just one system at a later date.

The WSSA classification system is only used in the USA and Canada. The Australian classification system is only used in Australia. The HRAC system is used in all other countries. The good news is that, on the whole, the systems map to each other, for example, HRAC Group O, Australian Group I, and WSSA Group 4 are synthetic auxins and contain the same list of herbicides. There are a few exceptions, particularly where the herbicide site of action is not well understood.

This website primarily uses the HRAC system because it is the classification system used in most countries. In the HRAC classification system there are 25 herbicide Groups. Group Z is unusual in that it represents herbicides with mechanisms that are not well understood. There are 4 Group Z’s. Although the sites of action of Group Z herbicides are not well know, we know that they fall into at least four groups that don’t act at the same site of action. So we have Z – Organoarsenicals, Z - Arylaminopropionic acids, Z – Pyrazoliums, and Z – Uknown.

Scroll down to see the herbicide groups sorted by the HRAC classification system but also listing the WSSA and Australian classifications. Please use the "Comment/Question/Report Error" button in the upper left if you see an error.

Ian Heap.

Cellular targets of herbicide action and herbicide classification by mode of action according to the Herbicide Resistance Action Committee (HRAC). Herbicides target only a few proteins or processes among the tremendous range present in plants.
From: Deciphering the evolution of herbicide resistance in weeds. Christophe Delye et. al. 2013, Trends in Genetics.