Herbicide modes of action

The accompanying table lists herbicide active ingredients currently available for crop protection in New Zealand in groups according to shared risk from resistance development in target weeds. The group codes are intended for use on herbicide labels to help in the prevention and management of herbicide resistance in New Zealand. Information used in constructing these tables came from publicly available information from the Herbicide Resistance Action Committee and CropLife Australia.

Note that in May 2021, it was decided to move from a mode of action code system developed specifically for New Zealand based on the old HRAC system to a new code system that has recently been adopted not only by HRAC but all other countries in the world including USA and Australia. This will be phased in gradually over the next 3 years as labels get renewed on herbicide containers. In the interim, both systems will be shown in the table below.

The New Zealand Committee on Pesticide Resistance (NZCPR) is a subcommittee of the New Zealand Plant Protection Society (NZPPS). Neither NZCPR or NZPPS guarantees that the information in these tables is accurate or up-to-date, or accepts any liability for loss or damage arising from the use information contained in these tables.

Use of Group Codes on pesticide product labels

Pesticide product labels should include a statement about resistance prevention or management. This statement should begin with the mode of action group(s) of the active ingredient(s), as in the following example using atrazine:

Resistance prevention or management
Product_name contains the GROUP 5 herbicide, atrazine, and continuous use of herbicides in this group may lead to a build-up of weeds resistant to it. To prevent or delay resistance, alternate with a herbicide from another group if possible and adhere to label instructions. [More detailed resistance guidelines for this product could then follow if they exist from strategies on the NZPPS web-site]

**Herbicide Mode of Action Groups, February 2022**

* not registered or no longer registered in New Zealand
Old Group Code	New Group Code	Mode of Action	Chemical Family	Active Ingredient
A	1	Inhibition of acetyl CoA carboxylase (ACCase)	Aryloxyphenoxypropionates "FOPs"	clodinafop-propargyl fenoxaprop-P-ethyl fluazifop-P-butyl haloxyfop-P quizalofop-P-ethyl
			Cyclohexanediones "DIMs"	clethodim sethoxydim* tepraloxydim tralkoxydim*
			Phenylpyrazoline "DEN"	pinoxaden
B	2	Inhibition of acetolactate synthase ALS (acetohydroxyacid synthase AHAS)	Sulfonylureas	chlorimuron-ethyl chlorsulfuron flazasulfuron foramsulfuron* halosulfuron-methyl iodosulfuron mesosulfuron-methyl metsulfuron-methyl nicosulfuron primisulfuron-methyl thifensulfuron-methyl tribenuron-methyl
			Imidazolinones	imazapyr imazethapyr
			Triazolopyrimidines	florasulam flumetsulam pyroxsulam
C1	5	Inhibition of photosynthesis at photosystem II	Triazines	ametryn* atrazine cyanazine desmetryn* prometryn propazine simazine terbumeton* terbuthylazine terbutryn
			Triazinones	hexazinone metamitron metribuzin
			Uracils	bromacil terbacil
			Pyridazinones	chloridazon
			Phenyl-carbamates	desmedipham phenmedipham
C2	5	Inhibition of photosynthesis at photosystem II	Ureas	diuron isoproturon linuron methabenzthiazuron
C3	6	Inhibition of photosynthesis at photosystem II	Nitriles	bromofenoxim* bromoxynil ioxynil
			Benzothiadiazinone	bentazone
			Phenyl-pyridazines	pyridate*
D	22	Photosystem-I-electron diversion	Pyridiniums	diquat paraquat
E	14	Inhibition of protoporphyrinogen oxidase (PPO)	Diphenyl ether	oxyfluorfen
			N-phenyl-imide	flumioxazin fluthiacet-methyl saflufenacil
			N-phenyl-oxadiazolone	oxadiazon
			N-phenyl-triazolinone	carfentrazone-ethyl sulfentrazone
F1	12	Bleaching: Inhibition of carotenoid biosynthesis at the phytoene desaturase step (PDS)	N-phenyl heterocycle	norflurazon*
			Pyridinecarboxamides	diflufenican
			Diphenyl heterocycle	fluridone*
F2	27	Bleaching: Inhibition of 4-hydroxyphenylpyruvate-dioxygenase (4-HPPD)	Pyrazoles	mesotrione topramezone
F3	34	Bleaching: Inhibition of carotenoid biosynthesis (unknown target)	Triazoles	amitrole
F3	13		Isoxazolidinones	clomazone
G	9	Inhibition of EPSP synthase	Glycines	glyphosate
H	10	Inhibition of glutamine synthetase	Phosphinic acids	glufosinate-ammonium
I	18	Inhibition of DHP (dihydropteroate) synthase	Carbamates	asulam
K1	3	Microtubule assembly inhibition	Dinitroanilines	oryzalin pendimethalin prodiamine trifluralin
			Benzamide	propyzamide
			Benzoic acid	chlorthal-dimethyl
K2	23	Inhibition of mitosis / microtubule organisation	Carbamates	chlorpropham propham* carbetamide*
K3	15	Inhibition of cell division	Alpha-chloroacetamides	acetochlor alachlor dimethenamid-P s-metolachlor propachlor
			Isoxazoline	pyroxasulfone
			Alpha-oxyacetamides	flufenacet
L	29	Inhibition of cellulose biosynthesis	Alkylazines	indaziflam
L	29	Inhibition of cellulose biosynthesis	Nitriles	dichlobenil
N	15	Inhibition of lipid synthesis - not ACCase inhibition	Thiocarbamates	prosulfocarb triallate
	15		Benzofuran	ethofumesate
	0		Chloro-carbonic-acids	TCA flupropanate 2,2‑dichloropropionic acid
O1	4	Action like indole acetic acid (auxin mimics)	Phenoxy-carboxylic-acids	2,4-D 2,4-DB dichlorprop-P MCPA MCPB mecoprop mecoprop-P
O2	4		Benzoate	dicamba
O3/4	4		Pyridine carboxylates	aminopyralid clopyralid halauxifen-methyl picloram
O3/4	4		Pyridyloxy-carboxylates	fluroxypyr triclopyr
	4			aminocyclopyrachlor
	31	Inhibitor of serine threonine protein phosphatase	Unknown subgroup	endothal
Z	0	Unknown	Arylaminopropionic acids	flamprop-M-isopropyl
			Organoarsenicals	MSMA*
			Other	fatty acids palm oil fatty acids pine oil