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).
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.[4] Cross-resistant and multiply resistant weeds resist multiple MoAs,[5] 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.[6]
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.[4]
Herbicides that act through multiple modes have multiple classifications, corresponding to each MoA.[7] 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).
| HRAC (AUS) | HRAC (Global) | HRAC (Numeric) | class=unsortable | Mode of action | class=unsortable | Example herbicides | class=unsortable | Example chemical families |
|---|---|---|---|---|---|---|---|---|
| A | A | 1 | Inhibits acetyl coa carboxylase | Aryloxyphenoxy-propionates, Cyclohexanediones, Phenylpyrazolines | ||||
| B | B | 2 | Inhibits acetolactate synthase | Imidazolinones, Pyrimidinyl benzoates, Sulfonylureas, Triazolopyrimidines | ||||
| C | C1 / C2 | 5 | Inhibits photosynthesis at PSII - serine 264 binders | Triazines, Amides, Phenlcarbamates, Pyridazinones, Triazinones, Triazolinones, Uracil, Ureas | ||||
| C | C3 | 6 | Inhibits photosynthesis at PSII - histidine 215 binders/uncouplers | Benzothiadiazinones, Nitriles | ||||
| - | C2 | 7 | Isoproturon[10] | Urea | ||||
| D | K1 | 3 | inhibits microtubule assembly | Dinitroanilines, Benzoic acids, Pyridines, Phosphoroamidates, Benzoic acids[11] | ||||
| E | K2 | 23 | Inhibits microtubule organisation | Carbamates | ||||
| F | F1 | 12 | Inhibits phytoene desaturase | Phenyl ethers, N-Phenyl heterocycles | ||||
| G | E | 14 | Inhibits protoporphyrinogen oxidase | N-Phenyl-imides. Diphenyl ethers, N-Phenyl-oxadiazolones, Phenylpyrazoles | ||||
| H | F2 | 27 | Inhibits hydroxyphenyl pyruvate dioxygenase | Soxazoles, Pyrazoles, Triketones | ||||
| I | O | 4 | 6-Arylpicolinates, 6-Chloropicolinates, Benzoates, Phenoxy-carboxylates, Pyridyloxy-carboxylates, Quinoline-carboxylates | |||||
| J | K3 / N | 15 | Inhibits very long chain fatty acid synthesis | Thiocarbamates, Benzofurans | ||||
| J | - | Unknown | Chlorocarbonic acids | |||||
| K | K3 | 15 | Inhibits very long chain fatty acid synthesis | α-Chloroacetamides, Isoxazolines | ||||
| K | - | Unknown | Acetamide | |||||
| L | D | 22 | PS I electron diversion | Pyridinium | ||||
| M | G | 9 | Inhibits enolpyruvil shikimate phosphate synthase | Glycine | ||||
| N | H | 10 | Phosphinic acid | |||||
| O | L | 29 | inhibition of cellulose synthesis | Nitrile, Benzamide, Alkylazine | ||||
| P | P | 19 | Auxin transport inhibitor | Aryl-carboxylates | ||||
| Q | F3 / F4 | 13 | Inhibits deoxy-d-xylulose phosphate synthesis | Isoxazolidinone | ||||
| Q | - | - | Unknown | Triazole | ||||
| R | I | 18 | Inhibits dihydropteroate synthase | carbamate | ||||
| - | - | 28 | Inhibition of dihydroorotate dehydrogenase | |||||
| T | Q / Z | 30 | Inhibits fatty acid thioesterase | Benzyl-ether | ||||
| Z | R | 31 | Inhibits serine-threonine protein phosphatase | Other | ||||
| Z | Z | - | Unknown | Arylaminopropionic acid, Others | ||||
| - | M | 24 | Uncouplers | Dinitrophenol | ||||
| - | S / F3 | 32 | Inhibition of Solanesyl Diphosphate Synthase | Diphenyl ether | ||||
| - | T | 33 | Inhibition of Homogentisate Solanesyltransferase | Phenoxypyridazine |