# HRAC classification

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Herbicide classification system

The **Herbicide Resistance Action Committee** (**HRAC**) classifies [herbicides](/source/Herbicide) by their [mode of action](/source/Mode_of_action) (MoA) to provide a uniform way for farmers and growers to identify the agents they use and better manage [pesticide resistance](/source/Pesticide_resistance) around the world.[1][2] It is run by [CropLife International](/source/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](/source/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](/source/Prosulfocarb) and [trifluralin](/source/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](/source/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

HRAC Classification Groups[12][10][13] Group (AUS) Group (WSSA) Group (Numeric) Mode of action Example herbicides Example chemical families A A 1 Inhibits acetyl CoA carboxylase Haloxyfop-methyl, Clethodim, Sethoxydim, Pinoxaden Aryloxyphenoxypropionates, Cyclohexanediones, Phenylpyrazolines B B 2 Inhibits acetolactate synthase Imazamox, Chlorsulfuron, Pyrithiobac-sodium, Florasulam Imidazolinones, Pyrimidinyl benzoates, Sulfonylureas, Triazolopyrimidines C C1 / C2 5 Inhibits photosynthesis at PSII - serine 264 binders Atrazine, Simazine, Propanil, Amicarbazone, Bromacil, Diuron Triazines, Amides, Phenlcarbamates, Pyridazinones, Triazinones, Triazolinones, Uracil, Ureas C C2 7 (merged into 5) Inhibits photosynthesis at PSII[10] Isoproturon,[14] Chlorotoluron, Tebuthiuron[10] Urea C C3 6 Inhibits photosynthesis at PSII - histidine 215 binders/uncouplers Bentazon, Bromoxynil, Ioxynil Benzothiadiazinones, Nitriles D K1 3 inhibits microtubule assembly Trifluralin, Pendimethalin, Propyzamide, Dithiopyr,[13] butamiphos, chlorthal-dimethyl[15] Dinitroanilines, Benzoic acids, Pyridines,[13] Phosphoroamidates, Benzoic acids[15] E K2 23 Inhibits microtubule organization Carbetamide Carbamates F F1 12 Inhibits phytoene desaturase Diflufenican, Norflurazon Phenyl ethers, N-Phenyl heterocycles G E 14 Inhibits protoporphyrinogen oxidase Butafenacil, Carfentrazone-ethyl, Oxyfluorfen N-Phenyl-imides. Diphenyl ethers, N-Phenyl-oxadiazolones, Phenylpyrazoles H F2 27 Inhibits hydroxyphenyl pyruvate dioxygenase Isoxaflutole, Pyrasulfotole, Mesotrione Soxazoles, Pyrazoles, Triketones I O 4 Auxin mimic Dicamba, Halauxifen, Picloram, 2,4-D, MCPA, Triclopyr, Quinclorac 6-Arylpicolinates, 6-Chloropicolinates, Benzoates, Phenoxy-carboxylates, Pyridyloxy-carboxylates, Quinoline-carboxylates J N 8 (merged into 15) Inhibition of lipid synthesis Butylate, EPTC, Ethofumesate, Dalapon, Thiobencarb[16] Thiocarbamates, Benzofurans, Chloro-Carbonic-acids K K3 15 Inhibits very long chain fatty acid synthesis Metolachlor, Pyroxasulfone, Chloroacetamides, Isoxazolines, Thiocarbamates L D 22 PS I electron diversion Diquat, Paraquat Pyridinium M G 9 Inhibits 5-enolpyruvylshikimate 3-phosphate synthase Glyphosate Glycine N H 10 Inhibits glutamine-synthetase Glufosinate-ammonium Phosphinic acid O L 29 inhibition of cellulose synthesis Isoxaben, Dichlobenil, Indaziflam Nitrile, Benzamide, Alkylazine P P 19 Auxin transport inhibitor Naptalam Aryl-carboxylates Q F3 / F4 13 Inhibits deoxy-D-xylulose phosphate synthesis Clomazone, Bixlozone Isoxazolidinone - F3 11 Inhibits carotenoid biosynthesis Amitrole[17] Triazole R I 18 Inhibits dihydropteroate synthase Asulam carbamate - - 28 Inhibition of dihydroorotate dehydrogenase Tetflupyrolimet T Q 30 Inhibits fatty acid thioesterase Cynmethylin Benzyl-ether - - 31 Inhibits serine-threonine protein phosphatase Endothal Other - M 24 Uncouplers Dinoseb Dinitrophenol - S / F3 32 Inhibition of Solanesyl Diphosphate Synthase Aclonifen Diphenyl ether - T 33 Inhibition of Homogentisate Solanesyltransferase Cyclopyrimorate Phenoxypyridazine Z Z 0 Unknown Flamprop-m, DSMA, MSMA, Pelargonic acid, Bensulide, Delapon, Napropamide Arylaminopropionic acid, Others

Note: Several Group N (WSSA) / Group 8 herbicides have been rolled into Group 15, for example [prosulfocarb](/source/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](https://hracglobal.com/tools/classification-lookup)), 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](/source/Insecticide_Resistance_Action_Committee)

- [Fungicide Resistance Action Committee](/source/Fungicide_Resistance_Action_Committee)

## References

1. **[^](#cite_ref-1)** ["Appendix 7. Classification of Herbicides According to Mode of Action"](https://ipm.ifas.ufl.edu/pdfs/Appendix7-HRAC.pdf) (PDF). *[University of Florida IFAS Extension](/source/Institute_of_Food_and_Agricultural_Sciences)*. Retrieved November 20, 2024.

1. **[^](#cite_ref-2)** ["HRAC MoA Classification Update Webinar Training 6th May 2022"](https://ewrs.org/en/info/Blog/92/HRAC-MoA-Classification-Update-Webinar-Training-6th-May-2022). *European Weed Research Society*. Retrieved November 20, 2024.

1. **[^](#cite_ref-3)** Sievernich, Bernd; Belvaux, Xavier; Hunt, Barrie (February 2024). ["HRAC Europe – Partner on Weed Resistance Management"](https://www.openagrar.de/receive/openagrar_mods_00093013). *Julius-Kühn-Archiv*. **478** (31). Bundesbehörden Und Einrichtungen Im Geschäftsbereich Des Bundesministeriums Für Ernährung Und Landwirtschaft (BMEL): 94. [doi](/source/Doi_(identifier)):[10.5073/20240109-073031-0](https://doi.org/10.5073%2F20240109-073031-0).

1. **[^](#cite_ref-4)** Hirai, Kenji; Uchida, Atsushi; Ohno, Ryuta (2002), Böger, Peter; Wakabayashi, Ko; Hirai, Kenji (eds.), ["Major Synthetic Routes for Modern Herbicide Classes and Agrochemical Characteristics"](http://link.springer.com/10.1007/978-3-642-59416-8_10), *Herbicide Classes in Development*, Berlin, Heidelberg: Springer Berlin Heidelberg, pp. 179–289, [doi](/source/Doi_(identifier)):[10.1007/978-3-642-59416-8_10](https://doi.org/10.1007%2F978-3-642-59416-8_10), [ISBN](/source/ISBN_(identifier)) [978-3-642-63972-2](https://en.wikipedia.org/wiki/Special:BookSources/978-3-642-63972-2), retrieved 2024-11-21{{[citation](https://en.wikipedia.org/wiki/Template:Citation)}}: CS1 maint: work parameter with ISBN ([link](https://en.wikipedia.org/wiki/Category:CS1_maint:_work_parameter_with_ISBN))

1. **[^](#cite_ref-5)** ["Herbicide Site of Action Classification Systems in Australia, Canada, and the HRAC Global system"](https://www.weedscience.org/Summary/SOADescription.aspx). *www.weedscience.org*. Retrieved 3 July 2025.

1. ^ [***a***](#cite_ref-fax_6-0) [***b***](#cite_ref-fax_6-1) ["Fact sheet HRAC Mode of Action Updates"](https://wssa.net/wp-content/uploads/HRAC-FACT-SHEET-FINAL-1.pdf) (PDF). Weed Science Society of America. Retrieved 20 September 2024.

1. **[^](#cite_ref-7)** ["Overview"](https://hracglobal.com/herbicide-resistance/overview). *Herbicide Resistance Action Committee*. Retrieved 20 September 2024.

1. **[^](#cite_ref-8)** Busi, Roberto; Goggin, Danica E; Onofri, Andrea; Boutsalis, Peter; Preston, Christopher; [Powles, Stephen B](/source/Stephen_Powles); Beckie, Hugh J (December 2020). "Loss of trifluralin metabolic resistance in Lolium rigidum plants exposed to prosulfocarb recurrent selection". *Pest Management Science*. **76** (12): 3926–3934. [doi](/source/Doi_(identifier)):[10.1002/ps.5993](https://doi.org/10.1002%2Fps.5993). [PMID](/source/PMID_(identifier)) [32638493](https://pubmed.ncbi.nlm.nih.gov/32638493).

1. **[^](#cite_ref-9)** Shino, Mamiko; Hamada, Takahiro; Shigematsu, Yoshio; Hirase, Kangetsu; Banba, Shinichi (2018). ["Action mechanism of bleaching herbicide cyclopyrimorate, a novel homogentisate solanesyltransferase inhibitor"](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6240781). *Journal of Pesticide Science*. **43** (4): 233–239. [doi](/source/Doi_(identifier)):[10.1584/jpestics.D18-008](https://doi.org/10.1584%2Fjpestics.D18-008). [PMC](/source/PMC_(identifier)) [6240781](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6240781).

1. ^ [***a***](#cite_ref-wssa_10-0) [***b***](#cite_ref-wssa_10-1) [***c***](#cite_ref-wssa_10-2) [***d***](#cite_ref-wssa_10-3) [***e***](#cite_ref-wssa_10-4) [***f***](#cite_ref-wssa_10-5) ["Classification of Herbicides According to Site of Action"](https://www.weedscience.org/Documents/ShowDocuments.aspx?DocumentID=1193). *www.weedscience.org*. WSSA. Retrieved 3 July 2025.

1. **[^](#cite_ref-11)** Oršolić, Davor; Pehar, Vesna; Šmuc, Tomislav; Stepanić, Višnja (2021-06-01). ["Comprehensive machine learning based study of the chemical space of herbicides"](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8169684). *Scientific Reports*. **11** (1): 11479. [doi](/source/Doi_(identifier)):[10.1038/s41598-021-90690-w](https://doi.org/10.1038%2Fs41598-021-90690-w). [ISSN](/source/ISSN_(identifier)) [2045-2322](https://search.worldcat.org/issn/2045-2322). [PMC](/source/PMC_(identifier)) [8169684](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8169684). [PMID](/source/PMID_(identifier)) [34075109](https://pubmed.ncbi.nlm.nih.gov/34075109).

1. ^ [***a***](#cite_ref-excel_12-0) [***b***](#cite_ref-excel_12-1) ["2024 HRAC GLOBAL HERBICIDE MOA CLASSIFICATION MASTER LIST"](https://hracglobal.com/tools/2024-hrac-global-herbicide-moa-classification). *Herbicide Resistance Action Committee*.

1. ^ [***a***](#cite_ref-hrac_list_13-0) [***b***](#cite_ref-hrac_list_13-1) [***c***](#cite_ref-hrac_list_13-2) ["Australia Herbicide Classification Lookup"](https://hracglobal.com/tools/australia-classification-lookup). *Herbicide Resistance Action Committee*. Retrieved 20 September 2024.

1. **[^](#cite_ref-14)** ["Pesticide Properties Database"](http://sitem.herts.ac.uk/aeru/ppdb/en/index.htm). *sitem.herts.ac.uk*.

1. ^ [***a***](#cite_ref-moa_table_15-0) [***b***](#cite_ref-moa_table_15-1) ["2008 Herbicide Mode of Action Table"](http://www.weedscience.org/Documents/ShowDocuments.aspx?DocumentID=1192). *weedscience.org*.

1. **[^](#cite_ref-16)** Hertfordshire, University of. ["Thiobencarb (Ref: IMC 3950)"](https://sitem.herts.ac.uk/aeru/ppdb/en/Reports/636.htm). *sitem.herts.ac.uk*.

1. **[^](#cite_ref-alberta_17-0)** ["Herbicide Group Classification by Mode of Action"](https://open.alberta.ca/dataset/26c577bd-78c9-4b54-a410-9b0e9aa465f6/resource/6feb3363-6f7d-4119-ae69-4908cb888baf/download/herbicide-group.pdf) (PDF). *open.alberta.ca*. Government of Alberta.

1. **[^](#cite_ref-18)** ["Global Herbicide Classification Lookup"](https://hracglobal.com/tools/classification-lookup). *Herbicide Resistance Action Committee*. Retrieved 3 July 2025.

## External links

- [Global Herbicide Classification Lookup](https://hracglobal.com/tools/classification-lookup)

v t e Pest control: herbicides Anilides/anilines acetochlor alachlor asulam benfluralin butachlor diethatyl diflufenican dimethenamid flamprop metazachlor metolachlor oryzalin pendimethalin pretilachlor propachlor propanil trifluralin Aromatic acids aminopyralid chloramben clopyralid dicamba picloram pyrithiobac quinclorac quinmerac Arsenicals cacodylic acid copper arsenate DSMA MSMA HPPD inhbitors flurochloridone isoxaflutole leptospermone mesotrione nitisinone sethoxydim sulcotrione Nitriles DCBN bromoxynil ioxynil Organophosphorus bensulide bialaphos ethephon fosamine glufosinate glyphosate piperophos Phenoxy Auxins 2,4-D 2,4-DB dichlorprop fenoprop MCPA MCPB mecoprop 2,4,5-T ACCase inhibitors FOP herbicides chlorazifop cyhalofop diclofop fenoxaprop fluazifop haloxyfop quizalofop DIM herbicides alloxydim clethodim cycloxydim butroxydim sethoxydim Protox inhibitors Nitrophenyl ethers acifluorfen aclonifen bifenox fluorodifen fomesafen lactofen nitrofen oxyfluorfen Pyrimidinediones butafenacil saflufenacil Triazolinones carfentrazone sulfentrazone Pyridines dithiopyr fluroxypyr imazapyr thiazopyr triclopyr Quaternary Photosystem I inhibitors cyperquat diquat paraquat Thiocarbamates Benthiocarb Butylate S-Ethyl dipropylthiocarbamate prosulfocarb Triazines cellulose biosynthesis inhibitors indaziflam Photosystem II inhibitors ametryn atrazine cyanazine hexazinone metamitron metribuzin prometon prometryn propazine simazine simetryn terbuthylazine terbutryn Ureas Photosystem II inhibitors chlortoluron DCMU difenoxuron isoproturon linuron monuron monolinuron tebuthiuron ALS inhibitors chlorsulfuron flazasulfuron metsulfuron-methyl sulfometuron methyl tribenuron Others 3-AT aminocyclopyrachlor Bentazon clomazone dinoseb dinoterb juglone methazole metam sodium pyribenzoxim

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