# Ioxynil

Ioxynil is a post-emergent selective nitrile herbicide. It is used in Australia, New Zealand and Japan to control broadleaf weeds via the inhibition of photosynthesis. It is used notably on onion crops, among others, normally at 300–900 g/Ha. It was introduced in 1966. The supply of ioxynil is decreasing, as of 2019 but the herbicide remains effective.

## History

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; 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 (NO$_2$) group, and on their success, the -SO$_2$CH$_3$ group was explored, leading to the discovery of methylsulphone herbicides.$^{:448-450}$
May & Baker, a subsidiary of Rhone-Poulenc began ioxynil's, and the very similar bromoxynil's, production in Norwich in 1965, where it has continued for over 40 years. 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.

## Regulations

Today, ioxynil is banned in the EU and used in Brazil, China (as octanoate), New Zealand, Australia, South Africa and Japan.
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.
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. 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.

## Properties

Ioxynil is a flammable solid with a weak phenolic smell and decays under UV light. Ioxynil's octanoate, ioxynil octanoate, or 4-cyan-2,6-diiodphenyloctanoate, is likewise a colourless insoluble solid and hydrolyses to ioxynil in basic conditions.
The taste of ioxynil is "slight, not characteristic."

## Mechanism and effect

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.

## Toxicology

Ioxynil is toxic to mammals, with an oral LD$_{50}$ of 110 mg/kg (rats), dermal LD$_{50}$ of 800 mg/kg, and inhalative LC$_{50}$ of 0.38 mg/L over four hours. Ioxynil is toxic to fish, with a 96 hour LC$_{50}$ of 6.8 mg/L, and 3.9 mg/L for daphnia. Plankton and bloodworms are also effected. The oral LD$_{50}$s 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.

## Environmental fate

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. 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.

## Lists

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.
Ioxynil has been sold in formulations also containing bromoxynil and isoproturon.
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, 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 and prickly paddy melon.
Crops situations which ioxynil has been used on include: onions, spring onions, welsh onions, garlic onions, cereals, leeks, garlic, shallots, flax, sugarcane, forage grasses, lawns and turf. Peas, oats, maize, sorghum and rice show high tolerance. Limited resistance is seen in lucerne, clover and carrot.