Herbicide resistance

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Herbicide resistance is the inherent ability of a weed to survive treatment with a herbicide that would normally kill it. If a resistant plant is allowed to reproduce, the resistance spreads resulting in more and more plants that are resistant to that particular herbicide. This is not the same as poor herbicide performance. Once herbicide resistance occurs it will persist indefinitely.

Herbicides act by interfering with specific processes in plants, known as the herbicide’s mode of action. Different herbicides may have the same mode of action and herbicides with the same modes of action are grouped from Group A to Group Z (see below). The risk of herbicide resistance developing is higher in certain Groups. For example, Groups A and B are more likely to develop resistance than Groups I, L and M.

The best way to manage herbicide resistance is to adopt integrated weed management by combining herbicide use with non-chemical control options (as described in Integrated weed management), as well as:

ensuring any resistant plants do not set seed;
regularly monitoring the results of herbicide treatments and looking out for resistant plants;
undertaking herbicide resistance testing on suspect plants that survive herbicide treatments; and
not relying on the same herbicide Group for regular weed control (rotate treatments using herbicides from different Groups).

Herbicide Group	Risk	Mode of Action	Active ingredient	Example product
A	High	Inhibitors of fat synthesis (ACC'ase [acetyl coA carboxylase] inhibitors)	Haloxyfop	Verdict®
A	High	Inhibitors of fat synthesis (ACC'ase [acetyl coA carboxylase] inhibitors)	Fluazifop-p	Various products
B	High	Inhibitors of acetolactate synthase (ALS inhibitors)	Imazapyr	Various products
B	High	Inhibitors of acetolactate synthase (ALS inhibitors)	Metsulfuron-methyl	Brush-off®
B + M	High + Moderate	Inhibitors of acetolactate synthase (ALS inhibitors) + Inhibitors of EPSP synthase	Metsulfuron-methyl + Glyphosate	Cut-out®
B + M	High + Moderate	Inhibitors of acetolactate synthase (ALS inhibitors) + Inhibitors of EPSP synthase	Imazapyr + Glyphosate	Arsenal Express®
B + I	High + Moderate	Inhibitors of acetolactate synthase (ALS inhibitors) + Disruptors of plant cell growth (synthetic auxins)	Metsulfuron-methyl + Aminopyralid	Stinger™
C	Moderate	Inhibitors of photosynthesis at photosystem II (PS II inhibitors)	Atrazine	Various products
C	Moderate	Inhibitors of photosynthesis at photosystem II (PS II inhibitors)	Diuron	Various products
C	Moderate	Inhibitors of photosynthesis at photosystem II (PS II inhibitors)	Bromacil	Various products
C	Moderate	Inhibitors of photosynthesis at photosystem II (PS II inhibitors)	Simazine	Various products
C	Moderate	Inhibitors of photosynthesis at photosystem II (PS II inhibitors)	Hexazinone	Velpar® L
C	Moderate	Inhibitors of photosynthesis at photosystem II (PS II inhibitors)	Bromoxynil	Bromicide®
C	Moderate	Inhibitors of photosynthesis at photosystem II (PS II inhibitors)	Tebuthiuron	Graslan®
C + F	Moderate	Inhibitors of photosynthesis at photosystem II (PS II inhibitors) + Bleachers: inhibitors of carotenoid biosynthesis at the phytoene desaturase step (PDS inhibitors)	Bromoxynil + Diflufenican	Jaguar
D	Moderate	Inhibitors of microtubule assembly	Propyzamide	Kerb
F	Moderate	Bleachers: inhibitors of carotenoid biosynthesis at the phytoene desaturase step (PDS inhibitors)	Diflufenican	Various products
G	Moderate	Inhibitors of protoporphyrinogen oxidase (PPOs)	Oxyfluorfen	Goal Herbicide
G	Moderate	Inhibitors of protoporphyrinogen oxidase (PPOs)	Flumioxazin	Terrain
G	Moderate	Inhibitors of protoporphyrinogen oxidase (PPOs)	Carfentrazone-ethyl	Shark™ Aquatic Herbicide
I	Moderate	Disruptors of plant cell growth (synthetic auxins)	2,4-D	Affray 300®
I	Moderate	Disruptors of plant cell growth (synthetic auxins)	Dichlorprop	Lantana 600®
I	Moderate	Disruptors of plant cell growth (synthetic auxins)	Triclopyr	Garlon® 600
I	Moderate	Disruptors of plant cell growth (synthetic auxins)	Fluroxypyr	Starane™ Advanced
I	Moderate	Disruptors of plant cell growth (synthetic auxins)	2,4-D amine	Various products
I	Moderate	Disruptors of plant cell growth (synthetic auxins)	2,4-D ester	Various products
I	Moderate	Disruptors of plant cell growth (synthetic auxins)	2,4-D LV ester	Various products
I	Moderate	Disruptors of plant cell growth (synthetic auxins)	Dicamba	Kamba® 500
I	Moderate	Disruptors of plant cell growth (synthetic auxins)	Clopyralid	Lontrel®
I	Moderate	Disruptors of plant cell growth (synthetic auxins)	MCPA	Various products
I	Moderate	Disruptors of plant cell growth (synthetic auxins)	Picloram	Tordon® Granules
I	Moderate	Disruptors of plant cell growth (synthetic auxins)	Triclopyr + Picloram	Tordon® DSH
I	Moderate	Disruptors of plant cell growth (synthetic auxins)	2,4-D + Picloram	Tordon® 75-D
I	Moderate	Disruptors of plant cell growth (synthetic auxins)	MCPA + Dicamba	Kamba® M
I	Moderate	Disruptors of plant cell growth (synthetic auxins)	Picloram + Aminopyralid	Vigilant II ®
I	Moderate	Disruptors of plant cell growth (synthetic auxins)	Picloram + Triclopyr + Aminopyralid	Grazon Extra®
J	Moderate	Inhibitors of fat synthesis (Not ACCase inhibitors)	Flupropanate	Tussock®
J	Moderate	Inhibitors of fat synthesis (Not ACCase inhibitors)	2,2-DPA	Propon®
L	Moderate	Inhibitors of photosynthesis at photosystem I (PSI inhibitors)	Diquat	Reglone®
M	Moderate	Inhibitors of EPSP synthase	Glyphosate	Various products
M	Moderate	Inhibitors of EPSP synthase	Glyphosate-trimesium	Glyphosate Trimesium 480
M + B	Moderate + High	Inhibitors of EPSP synthase + Inhibitors of acetolactate synthase (ALS inhibitors)	Glyphosate + Metsulfuron-methyl	Trounce®
N	Moderate	Inhibitors of glutamine synthetase	Glufosinate-ammonium	Basta®
O	Moderate	Inhibitors of cell wall (cellulose) synthesis	Dichlobenil	Casoron G®
Q	Moderate	Bleachers: Inhibitors of carotenoid biosynthesis unknown target	Amitrole	Amitrole T®
Q	Moderate	Bleachers: Inhibitors of carotenoid biosynthesis unknown target	Amitrole + Ammonium thiocyanate	Various products
Z	Moderate	Herbicides with unknown and probably diverse sites of action	MSMA	Armada 720 SL