ioxynil

Ioxynil and bromoxynil (along with 2,4-DB and MCPB) were patented by Louis Wain as joint-head of the chemistry department at Wye College, and coincidentally discovered independently by May & Baker in England screening spare nitriles for herbicide activity, and by Amchem Products Inc in America doing similar screening, all in 1963. Commercial prospects were promising, as cereals could tolerate large amounts, over 2 lbs/ac;{{cite journal |last1=Holly |first1=K. |title=Ioxynil and Bromoxynil as Herbicides |journal=International Journal of Pest Management: Part C |date=December 1964 |volume=10 |issue=4 |pages=256–258 |doi=10.1080/05331856409432879}} even 4 lbs/ac only temporarily scorches.

Wain theorised ioxynil and bromoxynil, the nitrile (-CN) group herbicides, because of the chemical similarity to a nitro (NO₂) group, and on their success, the -SO₂CH₃ group was explored, leading to the discovery of methylsulphone herbicides.{{Cite journal |author-link=Leslie Fowden |last=Fowden |first=Leslie |date=2002 |title=Ralph Louis Wain, C.B.E. 29 May 1911 – 14 December 2000 |journal=Biographical Memoirs of Fellows of the Royal Society |volume=48 |pages=439–458 |language=en |doi=10.1098/rsbm.2002.0026 |s2cid=70562660 |doi-access=free }}{{rp|page=448-450}}

May & Baker, a subsidiary of Rhone-Poulenc began ioxynil's, and the very similar bromoxynil's, production in Norwich in 1965,{{cite web |title=Briar Chemicals – Company Profile |url=https://www.w2bchemicals.com/briar-chemicals-company-profile/ |website=www.w2bchemicals.com |access-date=10 September 2024}} where it has continued for over 40 years.{{cite web |title=J300 AC Drive Case Study |url=http://www.hid.co.uk/casestudyj300ploughing.html |website=HID Online |publisher=HID Hitachi Ltd |access-date=10 September 2024 |date=2003}} By 1968, ioxynil (as "Buctril") was registered for use in the USA, Canada, the UK, Australia, New Zealand, Japan, the West Indies and most of Europe.

In the 2010s, ioxynil was produced in South Africa to alleviate shortages.{{cite news |last1=Lewis |first1=Sophie |title=South Africa comes to rescue to ease herbicide shortage |url=https://www.weeklytimesnow.com.au/agribusiness/horticulture/south-africa-comes-to-rescue-to-ease-herbicide-shortage/news-story/ad42a89325628ffc9918f9f1245b34a0 |access-date=10 September 2024 |publisher=The Weekly Times |date=31 August 2018}}

Today, ioxynil is banned in the EU and used in Brazil,{{cite news |last1=Russau |first1=Christian |url=https://www.kritischeaktionaere.de/en/bayer-en/pesticides-cancer-double-standards-and-what-bayer-has-to-do-with-the-amazons-rainforest-fires/ |title=Pesticides, Cancer, double standards, and what Bayer has to do with the Amazon's rainforest fires |newspaper=Association of Ethical Shareholders Germany |publisher=Ethical Shareholders |access-date=10 September 2024 |date=27 April 2020}} China (as octanoate),{{cite web |title=Total of 20 Pesticides Initially Registered in China in 2013 |url=https://news.agropages.com/News/NewsDetail---11763.htm |website=Grainews |language=en}} New Zealand,{{cite web |title=DECISION |url=https://www.epa.govt.nz/assets/FileAPI/hsno-ar/APP203454/ac57184486/APP203454-Fianl-decision-Signed.pdf |publisher=Environmental Protection Authority of New Zealand |date=6 May 2018}} Australia, South Africa and Japan.{{Citation needed|date=November 2024}}

The UK followed the EU's ban (taking effect 1 September 2015) to ban ioxynil's sale; the European approval lapsing under Regulation (EC) 1107/2009.{{cite news |title=UK withdraws products containing ioxynil |url=https://news.agropages.com/News/NewsDetail---13564.htm |access-date=10 September 2024 |agency=AgNews |date=19 November 2014}}

India and Columbia raised concerns about the EU's maximum allowed residue for ioxynil (amongst other pesticides), saying the EU's stance was too precautionary and not based on evidence, which is yet inconclusive on their genotoxicity.{{cite news |title=U.S. voices concerns at WTO meeting on pesticides, tariffs, etc. |url=https://www.foodsafetynews.com/2018/11/u-s-voices-concerns-at-wto-meeting-on-pesticides-tariffs-etc/ |access-date=10 September 2024 |agency=Food Safety News |date=November 16, 2018}} Bayer, Syngenta and others launched a lawsuit against the 2022 ban on exporting EU-prohibited pesticides from the EU to nations where they are legal, however the French Constitutional Court has upheld the ban.{{cite news |title=Frankreich: Erfolg gegen Pestizid-Exporte \ Wegweisendes Urteil des höchsten Gerichts |url=https://rdl.de/beitrag/frankreich-erfolg-gegen-pestizid-exporte-wegweisendes-urteil-des-h-chsten-gerichts |access-date=10 September 2024 |publisher=radio dreyeckland 102.3 |date=10 February 2020}}

Ioxynil is a flammable solid with a weak phenolic smell and decays under UV light.{{GESTIS|ZVG=510262|CAS=3861-47-0|Name=Ioxyniloctanoat|Date=2016-02-01}} Ioxynil's octanoate, ioxynil octanoate, or 4-cyan-2,6-diiodphenyloctanoate, is likewise a colourless insoluble solid and hydrolyses to ioxynil in basic conditions.{{cite book |author=Robert Irving Krieger |title=Handbook of pesticide toxicology |volume=1 |publisher=Academic Press |date=2001 |isbn=978-0-12-426260-7}}

The taste of ioxynil is "slight, not characteristic."

Ioxynil acts via photosynthesis inhibition. It and bromoxynil uncouple oxidative phosphorylation and inhibit photosynthetic phosphorylation. Ioxynil additionally breaks down into iodide ions which inhibit plant growth again. Ioxynil may also inhibit photoreduction of ferricyanide, fixation of carbon dioxide, photoreduction of NAPD or of endogenous plastoquinone. Ioxynil acts as an electron transport inhibitor and uncoupling agent.

Symptoms on weeds appear after a few hours or days. Areas of collapsed tissue appear, rapidly becoming necrotic. In good conditions on small plants, necrosis may complete within two days but some weeds can take up to three weeks to die. Effectiveness is enhanced if any times of high humidity occur 1 or 2 days after application. Light and temperature speed up herbicidal action.

Ioxynil is a Group C, (Australia), C3 (Global) or Group 6, (numeric) resistance class herbicide.{{cite web |title=Australia Herbicide Classification Lookup |url=https://hracglobal.com/tools/australia-classification-lookup/ |website=Herbicide Resistance Action Committee |language=en}}

Ioxynil is toxic to mammals, with an oral LD₅₀ of 110 mg/kg (rats), dermal LD₅₀ of 800 mg/kg, and inhalative LC₅₀ of 0.38 mg/L over four hours. Ioxynil is toxic to fish, with a 96 hour LC₅₀ of 6.8 mg/L, and 3.9 mg/L for daphnia. Plankton and bloodworms are also effected. The oral LD₅₀s in mice, guinea-pigs, rabbits and dogs respectively are 230, 76, 180, > 100 mg/kg.

Ioxynil can affect the human thyroid via binding to transthyretin, a thyroid hormone binding protein which transports thyroid hormone in the blood. It can provoke thyroid tumors in rats, and can disrupt zebrafish's heart development.

Ioxynil is slightly more toxic than the related chloroxynil and bromoxynil due to the heavier iodine halogens.{{cite book |last1=Man |first1=Julia P. |title=New Roles for Old Enzymes: Characterizing Arabidopsis nitrilases |date=June 2016 |publisher=University of Western Australia, School of Chemistry and Biochemistry |url=https://api.research-repository.uwa.edu.au/ws/portalfiles/portal/11627369/THESIS_DOCTOR_OF_PHILOSOPHY_MAN_Julia_Po_Wa_2016.pdf |access-date=8 April 2025}}

Ioxynil is a contact herbicide and has no residual soil activity or translocation, so spray coverage must be thorough as unsprayed weeds will not be controlled; large enough weeds may even contain surviving portions that resprout, and resistance can occur at later growth stages. Translocated chemical may produce chlorosis but is unlikely to be lethal.

Ioxynil bioaccumulates, although it does not linger long in the environment.{{cite journal |last1=Mäenpää |first1=Kimmo A |last2=Sormunen |first2=Arto J |last3=Kukkonen |first3=Jussi V. K |title=Bioaccumulation and toxicity of sediment associated herbicides (ioxynil, pendimethalin, and bentazone) in Lumbriculus variegatus (Oligochaeta) and Chironomus riparius (Insecta) |journal=Ecotoxicology and Environmental Safety |date=1 November 2003 |url=https://www.sciencedirect.com/science/article/pii/S0147651303000101 |volume=56 |issue=3 |pages=398–410 |doi=10.1016/S0147-6513(03)00010-1 |pmid=14575680 |bibcode=2003EcoES..56..398M |access-date=10 September 2024|url-access=subscription }} Ioxynil, bromoxynil, and their octanoate variants, leave negligible residues after use on crops. In all cases, under 0.01 ppm, the limit of detection, though some inactive content may be adsorbed into the soil. Ioxynil breaks down photochemically, first by breaking the halogen iodine free.{{cite journal |last1=Malouki |first1=Moulay A. |last2=Zertal |first2=Abdennour |last3=Lavédrine |first3=Bernadette |last4=Sehili |first4=Tahar |last5=Boule |first5=Pierre |title=Phototransformation of 3,5-dihalogeno-4-hydroxybenzonitriles (ioxynil and chloroxynil) in aqueous solution |journal=Journal of Photochemistry and Photobiology A: Chemistry |date=1 November 2004 |volume=168 |issue=1 |pages=15–22 |doi=10.1016/j.jphotochem.2004.05.007}} It persists for about a month in 20°C aerobic soil, shortened to two weeks in anaerobic soil.{{cite journal |last1=Nolte |first1=J. |last2=Heimlich |first2=F. |last3=Graß |first3=B. |last4=Zullei-Seibert |first4=N. |last5=Preuss |first5=G. |title=Studies on the behaviour of dihalogenated hydroxybenzonitriles in water |journal=Fresenius' Journal of Analytical Chemistry |date=1995 |volume=351 |issue=1 |pages=88–91 |doi=10.1007/BF00324296}}

File:Ioxynil variants.png

Ioxynil has several variants: ioxynil octanoate, ioxynil lithium and ioxynil sodium.

Ioxynil is or has been sold under these tradenames: Ioxynil, Unyunox, Totril, Toxynil, Hawk, Hocks, Sanoxynil, Iotril, Certrol, Actril, Actrilawn, Bentrol, Belgran, Bronx, Cipotril, Dantril, Oxytril, Mextrol-Biox, Sanoxynil, Shamseer-2, Stellox, Iotox, Iconix and Trevespan. Some products include multiple active ingredients.{{Citation needed|date=November 2024}}

Ioxynil has been sold in formulations also containing bromoxynil and isoproturon.{{Citation needed|date=November 2024}}

It is used to control these weeds: bellvine, burr medic, capeweed, chickweed, climbing buckwheat, common heliotrope, common sowthistle, corn gromwell, dandelion, dead nettle, fat-hen, fumitory, green amaranth, green crumbleweed, bittercress, ox tongue, pigweed, potato weed, saffron thistle, scarlet pimpernel, shepherd's purse, slender celery, smallflower mallow, stagger weed, threecornered Jack, three flowered nightshade, turnip weed, Ward's weed, wild radish, wild turnip, wireweed,{{cite web |title=Unyunox 250 EC |url=https://agnova.com.au/content/custom/products/downloads/079%20Unyunox_AgNote_2022%20General_Web.pdf |publisher=AgNova}} annual sowthistle, cornbind, musky storksbill, willow weed, buttercup, field pansy, groundsel, plantain, speedwell, stinking mayweed, the knotweed family broadly, in particular tartary buckwheat, the composite or sunflower family, chamomile, mayweed, some borages, fiddlenecks, gromwells{{cite journal |title=Technical Bulletin Ioxynil and Bromoxynil Selective Herbicides |journal=International Journal of Pest Management: Part C |date=September 1968 |volume=14 |issue=3 |pages=301–322 |doi=10.1080/05331856809438464 |url=https://www.tandfonline.com/doi/epdf/10.1080/05331856809438464?needAccess=true|url-access=subscription }} and prickly paddy melon.

Crops situations which ioxynil has been used on include: onions, spring onions, welsh onions,{{cite web |title=Iconix Label |url=https://www.imtrade.com.au/wp-content/uploads/2020/04/Website-Imt-ICONIX®-500-EC-Herbicide-Label-Leaflet-GHS.pdf |publisher=Imtrade Australia Pty Ltd}} garlic onions, cereals, leeks, garlic, shallots, flax, sugarcane, forage grasses, lawns and turf.{{cite web |title=Ioxynil |url=http://sitem.herts.ac.uk/aeru/ppdb/en/Reports/402.htm |publisher=Pesticide Properties DataBase |access-date=8 September 2024}} Peas,{{cite journal |last1=Holly |first1=K. |last2=Holroyd |first2=J. |title=3,5-Di-iodo-4-hydroxyhenzonitrile. A progress report on experimental work by the A.R.C. Weed Research Organisation May - October 1963 |journal=Africultural Research Council |date=November 1963 |url=https://www.bcpc.org/wp-content/uploads/2024/05/WRO-Technical_Report_Number_2.pdf |publisher=Weed Research Organisation |location=Begbroke Hill, Kidlington, Oxford}} oats, maize, sorghum and rice show high tolerance. Limited resistance is seen in lucerne, clover and carrot.

{{reflist}}

• {{PPDB|402}} (Ioxynil)
• {{PPDB|1063}} (Ioxynil octanoate)
• {{PPDB|2215}} (Ioxynil lithium)
• {{PPDB|2216}} (Ioxynil sodium)

{{Herbicides}}

{{Short description |Herbicide Chemical Compound}}

Category:Herbicides

Category:Nitriles

Category:Products introduced in 1966

Category:Phenols

Category:Iodoarenes

Category:Group 6 herbicides