Trifluralin is a common pre-emergent selective herbicide, a dinitroaniline. With about 14e6lb used in the United States in 2001,[1] and NaNe6lb in 2012,[2] it is one of the most widely used herbicides. Trifluralin is also used in Australia, and New Zealand,[3] previously in the EU. Introduced in 1964, Trifluralin was the first organofluorine compound used as an agrochemical.
Trifluralin is generally applied to the soil to control annual grass and broadleaf weed species. It inhibits root development by interrupting mitosis and controls weeds as they germinate.[4] Trifluralin moves very little inside the plant, remaining in the roots.[5]
Selective herbicides were unavailable in the 1950s to protect soybean and cotton (2,4-DNP could have been used but had to be exactingly applied lest it destroy the crops), so Lilly Research Laboratories screened ~2000 compounds from 1958 to 1980 blindly looking for a result. Trifluralin was initially thought a failure, yet the plots stayed free of weeds weeks later.[6] Application by incorporation into the top soil instead was eight times more potent.[7] Pre-plant soil incorporation was a new technique at the time. It is unclear why trifluralin's exotic 4-trifluoromethyl was tested so early (1960), before more common candidates such as fluoro, bromo, or iodo.[6]
By 1968, trifluralin was internationally available, including Australia and New Zealand,[3] and trifluralin was the 5th most used herbicide in the US, at 22960000lbs by 1974.[8]
Related compounds show similar herbicidal properties. In a study of 16, trifluoromethyl (as trifluralin is) compounds proved more active pre-emergence, and methyl compounds more active post-emergence. Replacing trifluralin's two propyl groups (with ethyl, allyl or butyl) yielded lower pre-emergent activity in all cases; post-emergence activity was highest in ethyl, allyl combination analogs.[9]
Nitralin replaces the trifluroromethyl group with a methylsulfonyl. Benfluralin replaces the propyl-propyl groups with ethyl-butyl. Profluralin replaces one propyl group with cyclopropylmethyl. Profluralin and nitralin are mostly obsolete, but benfluralin is commercially used, though less so than trifluralin.
Trifluralin inhibits tubulin formation, by binding to tubulin, and when the resulting herbicide complex is built into the growing micro-tubule, it blocks further tubulin binding, halting growth.[10] It also depolymerises (splits) the microtubules.
Dinitroanilines hit microtubules in plants and protists, but not animals, nor fungi, nor carrots, whose microtubules, even in purified form in laboratory work, are unaffected.[10]
Resistance, where evolved, can be through mutated α- or β-tubulin, particularly common in protists. This resistance is especially hard to evolve for weeds to tubulin disrupting herbicides because both α-tubulin and β-tubulin must mutate, as imbalance between their expressions is potentially lethal. Non-target-site resistance is usually though increased metabolism of trifluralin. Mobility-related mechanisms are not effective as minimal movement in the weed is needed to prevent germination.[10]
Resistance has been shown to devolve under repeated application of prosulfocarb on lolium rigidum (ryegrass). Supposedly, the mechanism of prosulfocarb-resistance is inverse to trifluralin resistance, requiring lower metabolism of herbicide, rather than greater. Therefore, when growing resistance for with one mechanism, the weeds undo their resistance to the other.[11] Some resistance mechanisms impose severe fitness cost on weeds, such as much reduced growth rate.[10]
Trifluralin is a Group D resistance class, (Aus), K1 or 3. (global or numeric)[12] [13] Other Group D herbicides will experience resistance near identically.
Wheat and triticale, if affected by trifluralin, display reduced root extension, increased number of seminal roots, increased root diameter and decreased root dry weight.[14]
Trifluralin has been banned in the European Union since 20 March 2008, primarily due to high toxicity to aquatic life.[15] The United Kingdom banned it under the same legislation.[16] With IPU banned at the same time, few options are left for controlling black-grass.[17]
Trifluralin is on the United States Environmental Protection Agency list of Hazardous Air Pollutants as a regulated substance under the Clean Air Act.[18]
Trifluralin is typically sold as emulsifiable concentrate[19] or granules. Application rates vary, such as 0.8-3.0 L of 480 g/L formulation per hectare, typically diluted with water,[20] and other compatible herbicides, e.g. isoproturon, to be sprayed in one go.
Trifluralin must be incorporated into soil within 24 hours of sowing, or in some cases sooner. Various methods achieve this; most involve machinery set to 5-13 cm deep.[20] This is to minimise volatilisation losses from trifluralin's relatively high vapour pressure.[21] Selectivity is possible even on susceptible crops, by sowing below the herbicide band, and shallower germinating weeds will be controlled.[20]
Trifluralin breaks down into many products as it degrades, ultimately being incorporated into soil-bound residues or converted to carbon dioxide (mineralized). Among the more unusual behaviors of trifluralin is inactivation in wet soils. This has been linked to transformation of the herbicide by reduced soil minerals, which in turn had been previously reduced by soil microorganisms using them as electron acceptors in the absence of oxygen. This environmental degradation process has been reported for many structurally related herbicides (dinitroanilines) as well as a variety of explosives such as TNT and picric acid.[22]
Trifluralin has a long half-life in soil of ~180 days, but it is accepted at high application rates because of its low soil mobility and high volatility.[6] It is extremely resistant to leaching, and shows little lateral soil movement. Repeated annual application shows steady and continuous decline in soil and does not accumulate, even applied well in excess of recommended rates.[23]
Ultraviolet light can cause degradation. Trifluralin is stable to hydrolysis.
Trifluralin is safe for mammals and chickens, even in large amounts.[24] Mammals eliminate 85% after oral consumption within 72 hours. It is toxic to fish though: LC50 for rainbow trout is 10-40 μg/L. Metabolism involves the thyroid; heavy and continuous exposure in rats can stress it via overstimulation.[25]
There is discussion of trifluralin being carcinogenic. Some studies have shown links, such as a 1986 study of three non-hodgkin lymphoma cases. A later, larger study found no significant relation. A review study examined trifluralin against kinds of cancer, finding no link except to colon cancer, which was found in only one studied cohort. Research on humans remains unconvincing, but EPA animal toxicity data "supports the possible carcinogenicity" of trifluralin.[26] No association exists with lung cancer.[27] Trifluralin exposure can reduce cell apoptosis.[28]
Trifluralin on mammalian ovaries (tested in mice, at 150 mg/kg/day) showed no effect on oocyte quality, but may induce a stress response in ovarian somatic cells. Fertility was unaffected. Levels of pRb stayed unchanged, though trifluralin raised levels of p53, a tumor suppressing gene, by 2.5 times. The additional p53 appeared not to increase rates of apoptosis.[29]
Due to trifluralin's high vapour pressure, food residue is reduced in processing, especially in high temperature processes, such as in the mashing of beer.[30]
Trifluralin can be used as ointment to treat Leishmaniasis welts on the skin. It, and other dinitroanilines, are tubulin-binding agents with selective antileishmanial properties, leishmania being the parasite causing the disease, which killed 60,000 people in 2001. Research into expanding's trifluralin's medical use is stymied by its low water solubility and easy sublimation. Trifluralin analogues have been tried, and some show greater efficacy than miltefosine; all trifluralin analogues have the benefits of being non-hemolytic and lower cell toxicity.[31]
Trifluralin also has anti-malarial properties and accumulates in parasite-infected erythrocytes, though low solubility makes effective administration of trifluralin difficult.[32] Treatment of Toxoplasma gondii and cryptosporidiosis is effective but limited due to solubility.[33] [34] Liposome-administered trifluralin has been used to treat leishmania in dogs successfully.[35]
Commercial formulations have included trifluralin mixtures with: linuron, napropamide, metribuzin, clomazone,tebutam, bromoxynil and ioxynil, isoproturon, terbutryn, trietazine, neburon and isoxaben.
Crops trifluralin is used in include: Wheat, barley, cotton, triticale, rye, sunflowers, sugar cane, peas, canola, safflower, peanuts, tobacco, pigeon peas, lupins, lucerne, linseed, legume seed, strawberry, lentils, faba beans, chickpeas, cowpeas, lablab, mung beans, borlotti beans, red beans, adzuki beans,[20] citrus fruit, lettuce, capsicums, tomatoes, artichokes, onions, garlic, brassicas, sugar beet, parsnips, carrots or soya.