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.

HRAC Group: A : ACCase inhibitors

Inhibition of Acetyl CoA Carboxylase

WSSA Group : 1 Aussie Group : A

HRAC Group: B : ALS inhibitors

Inhibition of Acetolactate Synthase

WSSA Group : 2 Aussie Group : B

HRAC Group: C1 : Photosystem II- Serine 264 Binders

Inhbition of Photosynthesis at PSll - Serine 264 Binders

WSSA Group : 5 Aussie Group : C

HRAC Group: C2 : PSII inhibitors - Serine 264 Binders

Inhbition of Photosynthesis at PSll - Serine 264 Binders

WSSA Group : 7 Aussie Group : C

HRAC Group: C3 : PSII inhibitors - Histidine 215 Binders

Inhbition of Photosynthesis at PSll - Histidine 215 Binders

WSSA Group : 6 Aussie Group : C

HRAC Group: D : PSI Electron Diverter

PSI Electron Diversion

WSSA Group : 22 Aussie Group : L

HRAC Group: E : PPO inhibitors

Inhibition of Protoporphyrinogen Oxidase

WSSA Group : 14 Aussie Group : G

HRAC Group: F1 : Phytoene Desaturase inhibitors

Inhibition of Phytoene Desaturase

WSSA Group : 12 Aussie Group : F

HRAC Group: F2 : HPPD inhibitors

Inhibition of Hydroxyphenyl Pyruvate Dioxygenase

WSSA Group : 27 Aussie Group : H

HRAC Group: F3 : Lycopene Cyclase inhibitors

Inhibition of Lycopene Cyclase

WSSA Group : 11 Aussie Group : Q

HRAC Group: F4 : DOXP inhibitors

Inhibition of Deoxy-D-Xyulose Phosphate Synthase

WSSA Group : 13 Aussie Group : Q

HRAC Group: G : EPSP synthase inhibitors

Inhibition of EPSP synthase

WSSA Group : 9 Aussie Group : M

HRAC Group: H : Glutamine Synthetase inhibitors

Inhibition of Glutamine Synthetase

WSSA Group : 10 Aussie Group : N

HRAC Group: I : DHP Synthase inhibitors

Inhibition of Dihydropteroate Synthase

WSSA Group : 18 Aussie Group : R

HRAC Group: K1 : Microtubule Assembly inhibitors

Inhibition of Microtubule Assembly

WSSA Group : 3 Aussie Group : D

HRAC Group: K2 : Microtubule Organization inhibitors

Inhibition of Microtubule Organization

WSSA Group : 23 Aussie Group : E

HRAC Group: K3 : Very Long-Chain Fatty Acid Synthesis inhibitors

Inhibition of Very Long-Chain Fatty Acid Synthesis

WSSA Group : 15 Aussie Group : K

HRAC Group: L : Cellulose Synthesis inhibitors

Inhibition of Cellulose Synthesis

WSSA Group : 20, 21, 26, 29 Aussie Group : I, O, Z

HRAC Group: M : Uncouplers


WSSA Group : 24 Aussie Group : Z

HRAC Group: N : Lipid Inhibitors

Inhibition of lipid synthesis - not ACCase inhibition

WSSA Group : 16, 26, 8 Aussie Group : J

HRAC Group: O : Auxin Mimics

Auxin Mimics

WSSA Group : 4 Aussie Group : I

HRAC Group: P : Auxin Transport inhibitors

Inhibition of Auxin Transport

WSSA Group : 19 Aussie Group : P

HRAC Group: Z : Nucleic acid inhibitors


WSSA Group : 17 Aussie Group : Z

HRAC Group: Z : Antimicrotubule mitotic disrupter


WSSA Group : 25 Aussie Group : Z

HRAC Group: Z : Unknown


WSSA Group : 27 Aussie Group : Z

HRAC Group: Z : Cell elongation inhibitors


WSSA Group : 8 Aussie Group : Z

Site of Action Descriptions from WSSA

Amrhein, N. et al. 1980. Plant Physiol. 66:830.

Ashtakala, S. S. et al. 1989. J. Plant Physiol. 135:86.

Ashton, F. M. and A. S. Crafts. 1981. Mode of Action of Herbicides, 2nd ed. John Wiley & Sons, New York.

Barry and Pallett. 1990. Z. Naturforsch. 45c:492.

Bartels and Watson. 1978. Weed Sci. 26:198.

Burton, J.D. et al. 1989. Pestic. Biochem. Physiol. 34:76.

Casida, J. E. et al. 1974. Science 184:573.

Croteau, R. 1992. Plant Physiol. 98:1515.

Devine, M., S. O. Duke, and C. Fedtke. 1993. Physiology of Herbicide Action. Prentice Hall, New Jersey.

Dodge, A. D. 1982. Pages 57-77 in D. E. Moreland, J. B. St. John, and F. D. Hess, eds., Biochemical

Responses Induced by Herbicides. Am. Chem. Soc. Symp. Ser. No. 181, Washington D.C.

Duke and Hoagland. 1978. Plant Sci. Lett. 11:185.

Duke, S. O. et al. 1991. Weed Sci. 39:465.

Fedtke, C. 1982. Biochemistry and Physiology of Herbicide Action. Springer-Verlag, New York.

Focke and Lichtenthaler. 1987. Z. Naturforsch. 42c:1361.

Fuerst, E. P. 1987. Weed Technol. 1:270.

Gronwald, J. W. 1991. Weed Sci. 39:435.

Heim, D. R. et al. 1990. Plant Physiol. 93:695.

Heim and Larrinua. 1989. Plant Physiol. 91:1226.

Husted, R. F. et al. 1966. Proc. North Cent. Weed Control Conf. 44.

Böger P. et al. 2000. Pest. Managmt. Sci. 56:497-508.

Kidd, B. R. et al. 1982. Plant Sci. Lett. 26:211.

LaRossa and Schloss. 1984. J. Biol. Chem. 259:8753.

Lea, P.J. et al. 1984. Phytochemistry 23:1.

Lee and S.O. Duke. 1994. Abstr. Weed Sci. Soc. Am. 34:52.

Lee, T. T. 1980. Weed Res. 20:365.

Mallory-Smith, C.A. and E.J. Retzinger, Jr. 2003. Revised classification of herbicides by site of action for weed

resistance management strategies. Weed Technol. 17:605-619.

Myers, D. F. et al.2009. Indaziflam/BCS-AA10717-A new Herbicide for Pre-Emergent Control of Grasses and

Broadleaf Weeds for Turf and Ornamentals. WSSA Abstracts #386.

Sandmann and Böger. 1989. Pages 25-44 in P. Böger and G. Sandmann, eds., Target Sites of Herbicide

Action. CRC Press, Boca Raton, FL.

Sauer, H. et al. 1987. Z. Naturforsch. 42c:270.

Sterrett and Fretz. 1975. HortScience 10:161.

Stoltenberg, D.E. et al. 1989. Weed Sci. 37:512.

Tachibana, K. et al. 1986. J. Pestic. Sci. 11:33.

Vaughn and Lehnen. 1991. Weed Sci. 39:450.

Veerasekaran, P. et al. 1981. Pestic. Sci. 12:325.


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Heap, I.  The International Herbicide-Resistant Weed Database.  Online.  Sunday, April 18, 2021 .  Available Copyright © 1993- 2021 All rights reserved. Fair use of this material is encouraged. Proper citation is requested.