HRAC classification
{{Short description|Herbicide classification system}}
The Herbicide Resistance Action Committee (HRAC) classifies herbicides by their mode of action (MoA) to provide a uniform way for farmers and growers to identify the agents they use and better manage pesticide resistance around the world.{{cite web |url= https://ipm.ifas.ufl.edu/pdfs/Appendix7-HRAC.pdf |title= Appendix 7. Classification of Herbicides According to Mode of Action |website= University of Florida IFAS Extension |accessdate= November 20, 2024 }}{{cite web |url= https://ewrs.org/en/info/Blog/92/HRAC-MoA-Classification-Update-Webinar-Training-6th-May-2022 |title= HRAC MoA Classification Update Webinar Training 6th May 2022 |website= European Weed Research Society |accessdate= November 20, 2024 }} It is run by CropLife International{{Cite journal |last1=Sievernich |first1=Bernd |last2=Belvaux |first2=Xavier |last3=Hunt |first3=Barrie |date=February 2024 |others=Bundesbehörden Und Einrichtungen Im Geschäftsbereich Des Bundesministeriums Für Ernährung Und Landwirtschaft (BMEL) |title=HRAC Europe – Partner on Weed Resistance Management |url=https://www.openagrar.de/receive/openagrar_mods_00093013 |journal=Julius-Kühn-Archiv |language=en |volume=478 |issue=31 |pages=94 |doi=10.5073/20240109-073031-0}} in conjunction with the Weed Science Society of America (WSSA).{{Citation |last1=Hirai |first1=Kenji |title=Major Synthetic Routes for Modern Herbicide Classes and Agrochemical Characteristics |date=2002 |work=Herbicide Classes in Development |pages=179–289 |editor-last=Böger |editor-first=Peter |url=http://link.springer.com/10.1007/978-3-642-59416-8_10 |access-date=2024-11-21 |place=Berlin, Heidelberg |publisher=Springer Berlin Heidelberg |language=en |doi=10.1007/978-3-642-59416-8_10 |isbn=978-3-642-63972-2 |last2=Uchida |first2=Atsushi |last3=Ohno |first3=Ryuta |editor2-last=Wakabayashi |editor2-first=Ko |editor3-last=Hirai |editor3-first=Kenji|url-access=subscription }}
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.{{cite web |title=Fact sheet HRAC Mode of Action Updates |url=https://wssa.net/wp-content/uploads/HRAC-FACT-SHEET-FINAL-1.pdf |publisher=Weed Science Society of America |access-date=20 September 2024}} Cross-resistant and multiply resistant weeds resist multiple MoAs,{{cite web |title=Overview |url=https://hracglobal.com/herbicide-resistance/overview |website=Herbicide Resistance Action Committee |access-date=20 September 2024 |language=en}} and are particularly difficult to control.
There is limited evidence of resistance undoing other resistances. For example, prosulfocarb and trifluralin: 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.{{cite journal |last1=Busi |first1=Roberto |last2=Goggin |first2=Danica E |last3=Onofri |first3=Andrea |last4=Boutsalis |first4=Peter |last5=Preston |first5=Christopher |last6=Powles |first6=Stephen B |last7=Beckie |first7=Hugh J |title=Loss of trifluralin metabolic resistance in Lolium rigidum plants exposed to prosulfocarb recurrent selection |journal=Pest Management Science |date=December 2020 |volume=76 |issue=12 |pages=3926–3934 |doi=10.1002/ps.5993|pmid=32638493 }}
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.{{cite journal |last1=Shino |first1=Mamiko |last2=Hamada |first2=Takahiro |last3=Shigematsu |first3=Yoshio |last4=Hirase |first4=Kangetsu |last5=Banba |first5=Shinichi |title=Action mechanism of bleaching herbicide cyclopyrimorate, a novel homogentisate solanesyltransferase inhibitor |journal=Journal of Pesticide Science |date=2018 |volume=43 |issue=4 |pages=233–239 |doi=10.1584/jpestics.D18-008|pmc=6240781 }}
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 could be an impediment for international growers.
Herbicides that act through multiple modes have multiple classifications, corresponding to each MoA.{{Cite journal |last1=Oršolić |first1=Davor |last2=Pehar |first2=Vesna |last3=Šmuc |first3=Tomislav |last4=Stepanić |first4=Višnja |date=2021-06-01 |title=Comprehensive machine learning based study of the chemical space of herbicides |journal=Scientific Reports |language=en |volume=11 |issue=1 |page=11479 |doi=10.1038/s41598-021-90690-w |issn=2045-2322 |pmc=8169684 |pmid=34075109}} For example, Quinmerac 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).