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, November 2023
     *no longer available in New Zealand

New Group Code Old Group Code Mode of Action Chemical Family Active Ingredient
1   A 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*
Phenylpyrazoline "DEN" pinoxaden
2   B Inhibition of acetolactate synthase (ALS) Sulfonylureas chlorimuron-ethyl
chlorsulfuron
flazasulfuron
foramsulfuron
halosulfuron-methyl
iodosulfuron
mesosulfuron-methyl
metsulfuron-methyl
nicosulfuron
primisulfuron-methyl
thifensulfuron-methyl
tribenuron-methyl
triflusulfuron-methyl
Imidazolinones imazapyr
imazethapyr
Triazolinone thiencarbazone-methyl
Triazolopyrimidines florasulam
flumetsulam
pyroxsulam
3   K1 Microtubule assembly inhibition Dinitroanilines oryzalin
pendimethalin
prodiamine
trifluralin
Benzamide propyzamide
Benzoic acid chlorthal-dimethyl
4 O1 Auxin mimics Phenoxy-carboxylates 2,4-D
2,4-DB
dichlorprop*
MCPA
MCPB
mecoprop
O2 Benzoate dicamba
O3/4 Pyridine-carboxylates aminopyralid
clopyralid
halauxifen
picloram
O3/4 Pyridyloxy-carboxylates fluroxypyr
triclopyr
  Pyrimidine-carboxylates aminocyclopyrachlor
5 C1 Inhibition of photosynthesis at photosystem II (serine 264 binders) Triazines atrazine
cyanazine
prometryn
propazine
simazine
terbuthylazine
terbutryn
Triazinones hexazinone
metamitron
metribuzin
Uracils bromacil
terbacil
Pyridazinones chloridazon
Phenyl-carbamates desmedipham
phenmedipham
C2 Ureas diuron
isoproturon
linuron
methabenzthiazuron
metobromuron
6 C3 Inhibition of photosynthesis at photosystem II (histidine 215 binders) Nitriles bromoxynil
ioxynil
Benzothiadiazinone bentazone
Phenyl-pyridazine pyridate*
9 G Inhibition of EPSP synthase Glycine glyphosate
10 H Inhibition of glutamine synthetase Phosphinic acid glufosinate-ammonium
12 F1 Inhibition of phytoene desaturase N-phenyl heterocycle norflurazon*
Phenyl ether diflufenican
13 F3 Inhibition of deoxy-D-xylulose phosphate synthase Isoxazolidinone clomazone
14      E 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
15   K3 Inhibition of very long-chain fatty acid synthesis Alpha-chloroacetamides acetochlor
alachlor
dimethenamid
metolachlor
propachlor
Isoxazoline pyroxasulfone
Alpha-oxyacetamide flufenacet
N Thiocarbamates prosulfocarb
triallate
Benzofuran ethofumesate
18 I Inhibition of DHP (dihydropteroate) synthase Carbamate asulam
22 D Photosystem-I electron diversion Pyridiniums diquat
paraquat
23 K2 Inhibition of mitosis / microtubule organisation Carbamates chlorpropham
propham
27 F2 Inhibition of 4-hydroxyphenylpyruvate-dioxygenase (4-HPPD) Pyrazoles mesotrione
topramezone
29  L Inhibition of cellulose biosynthesis Alkylazine indaziflam
Nitrile dichlobenil*
31   Inhibitor of serine threonine protein phosphatase Unknown subgroup endothal
34 F3 Inhibition of lycopene cyclase Triazole amitrole
0 N Unknown Chloro-carbonic-acids TCA*
flupropanate
2,2‑DPA (dalapon)*
Z Arylaminopropionic acids flamprop-M-isopropyl
Other fatty acids
nonanoic acid
palm oil fatty acids
pine oil