HRAC classification

The Herbicide Resistance Action Committee (HRAC) classifies herbicides by their mode of action (MoA) to provide a uniform way for farmers and growers to identify the agents they use and better manage pesticide resistance around the world.^[1][2] It is run by CropLife International^[3] in conjunction with the Weed Science Society of America (WSSA).^[4]

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.^[5] Cross-resistant and multiply resistant weeds resist multiple MoAs,^[6] and are particularly difficult to control.

There is limited evidence of resistance undoing other resistances. For example, prosulfocarb and 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.^[7]

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.^[8]

Naming types

The HRAC give a letter based class to each active constituent herbicide. The Australian HRAC code is separately assigned, though is often the same as the global code. In 2021, alternative numeric classes were added, to make codes globally more consistent. This set of classification changes also added or moved a few herbicides that had been misclassified, and reduced regional concerns that using the English alphabet could be an impediment for international growers.^[5]

Herbicides that act through multiple modes have multiple classifications, corresponding to each MoA.^[9] For example, 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).^[10]

Groups

HRAC Classification Groups^[11][10]

HRAC (AUS)	HRAC (Global)	HRAC (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	C3	6	Inhibits photosynthesis at PSII - histidine 215 binders/uncouplers	Bentazon, Bromoxynil, Ioxynil	Benzothiadiazinones, Nitriles
-	C2	7		Isoproturon[12]	Urea
-	N	8		Thiobencarb[13]
D	K1	3	inhibits microtubule assembly	Trifluralin, Pendimethalin, Propyzamide, Dithiopyr,[11] butamiphos, chlorthal-dimethyl[14]	Dinitroanilines, Benzoic acids, Pyridines,[11] Phosphoroamidates, Benzoic acids[14]
E	K2	23	Inhibits microtubule organisation	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	K3 / N	15	Inhibits very long chain fatty acid synthesis	Prosulfocarb, Ethofumesate	Thiocarbamates, Benzofurans
J		-	Unknown	Bensulide, Delapon	Chlorocarbonic acids
K	K3	15	Inhibits very long chain fatty acid synthesis	Metolachlor, Pyroxasulfone	α-Chloroacetamides, Isoxazolines
K		-	Unknown	Napropamide	Acetamide
L	D	22	PS I electron diversion	Diquat, Paraquat	Pyridinium
M	G	9	Inhibits enolpyruvil shikimate 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[15]	Triazole
R	I	18	Inhibits dihydropteroate synthase	Asulam	carbamate
-	-	28	Inhibition of dihydroorotate dehydrogenase	Tetflupyrolimet
T	Q / Z	30	Inhibits fatty acid thioesterase	Cynmethylin	Benzyl-ether
Z	R	31	Inhibits serine-threonine protein phosphatase	Endothal	Other
Z	Z	0	Unknown	Flamprop-m, DSMA, MSMA, Pelargonic acid	Arylaminopropionic acid, Others
-	M	24	Uncouplers	Dinoseb	Dinitrophenol
-	S / F3	32	Inhibition of Solanesyl Diphosphate Synthase	Aclonifen	Diphenyl ether
-	T	33	Inhibition of Homogentisate Solanesyltransferase	Cyclopyrimorate	Phenoxypyridazine

See also

• Insecticide Resistance Action Committee