The Herbicide Resistance Action Committee (HRAC) classifies
herbicide
Herbicides (, ), also commonly known as weed killers, are substances used to control undesired plants, also known as weeds.EPA. February 201Pesticides Industry. Sales and Usage 2006 and 2007: Market Estimates. Summary in press releasMain page f ...
s by their
mode of action
In pharmacology and biochemistry, mode of action (MoA) describes a functional or anatomical change, resulting from the exposure of a living organism to a substance. In comparison, a mechanism of action (MOA) describes such changes at the molecul ...
(MoA) to provide a uniform way for farmers and growers to identify the agents they use and better manage
pesticide resistance
Pesticide resistance describes the decreased susceptibility of a pest population to a pesticide that was previously effective at controlling the pest. Pest species evolve pesticide resistance via natural selection: the most resistant specimens su ...
around the world. It is run by
CropLife International
CropLife International is an international trade association of agrochemical companies founded in 2001. It was previously known as ''Global Crop Protection Federation'' and started out as ''International Group of National Associations of Manufact ...
in conjunction with the Weed Science Society of America (WSSA).
Resistance overview
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.
''Cross-resistant'' and ''multiply resistant'' weeds resist multiple MoAs, and are particularly difficult to control.
There is limited evidence of resistance undoing other resistances. For example,
prosulfocarb and
trifluralin
Trifluralin is a common pre-emergent selective herbicide, a dinitroaniline. With about used in the United States in 2001, and in 2012, it is one of the most widely used herbicides. Trifluralin is also used in Australia, New Zealand, Brazil an ...
: 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.
Resistance first became problematic in the 1970s and 1980s, and herbicide resistant weeds have developed against 23 of 26 known herbicide sites of action, and over 163 different herbicides. Herbicide development has slowed down significantly, with no new mechanisms being discovered from circa 2000 to 2020.
Naming types
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
Modern English is written with a Latin-script alphabet consisting of 26 Letter (alphabet), letters, with each having both uppercase and lowercase forms. The word ''alphabet'' is a Compound (linguistics), compound of ''alpha'' and ''beta'', t ...
could be an impediment for international growers.
Herbicides that act through multiple modes have multiple classifications, corresponding to each MoA.
For example,
Quinmerac
Quinmerac is a chemical herbicide first manufactured by BASF in 1993. Its formula is , and it is a quinolinemonocarboxylic acid that includes chlorine and methyl groups as substituents.
Use
Quinmerac is used as a herbicide to control various ...
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).
Groups
See also
*
Insecticide Resistance Action Committee The Insecticide Resistance Action Committee (IRAC) was formed in 1984 and works as a specialist technical group of the industry association CropLife to be able to provide a coordinated industry response to prevent or delay the development of insec ...
References
{{Herbicides
Herbicides