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 CodeNew Group CodeMode of ActionChemical FamilyActive 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
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
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