Trifluralin is a widely used herbicide which is a suspected carcinogen. In some countries its use is increasing, while in other countries it is banned for its persistence and its threat to ecosystems.
Trifluralin is a selective, pre-sowing or pre-emergence herbicide used to control many annual grasses and broadleaf weeds in a large variety of arable and horticultural crops(1). One of the dinitroanaline herbicides, trifluralin prevents weed growth by inhibiting root development through the interruption of mitosis(2) ¨C early developmental cell division. Trifluralin is applied as a soil-incorporated pesticide, though there are also some surface uses(3).
Chemical Name: a, a, a -Trifluoro-2,6-dinitro-N,N-dipropyl-p-toluidine
Trifluralin was first registered in the US in 1963(4). Patented by Eli Lilly, and still produced by Dow (which took over Eli Lilly), among 24 producers(5). Annual worldwide sales in 1998 were worth US$300 million, and 24,000 tonnes were produced(6).
Trifluralin is registered in more than 50 countries for use on more than 80 crop, vegetable and ornamental uses. The US remains the major current market, where trifluralin ranks as one of the five top-selling herbicides(7), and it is also widely used on cotton and other crops in Africa and other developing countries.
The use of trifluralin is increasing rapidly in the UK in both the volume applied and the area treated. From 1996 to 1998, its use increased by 57% by area treated. From 1996 to 1999, the annual average weight applied totalled 659,904 kilograms. Trifluralin is the eighth most heavily used pesticide on all arable crops: in 1998 it was applied to 736,886 hectares in the UK(8).
Demand in the US for dinitroaniline pesticides, predominantly trifluralin, however, declined an estimated 2.8% from 1993 to 1998(9). In the US, the use of trifluralin on soybeans and cotton accounts for about 75% of overall use. It was the most widely used herbicide on upland cotton in 1998, where it was applied to 57% of the area surveyed(10).
Trifluralin is used on winter wheat and barley, set-aside (arable land temporarily taken out of cultivation), oil-seed rape, brassicas, carrots, lettuce, sugar beet, and beans(11). It is also applied to outdoor bulbs and flowers, fodder crops, glasshouse crops, Christmas trees, herbaceous plants, soft fruit and vegetables(12). In other countries it is used on cotton, soybean, sunflower, canola, turf, alfalfa, tomatoes and vines. Trifluralin is almost exclusively a single-application, ground-applied or soil-incorporated treatment(13).
Trifluralin is classified by the World Health Organisation as unlikely to present an acute hazard in normal use(14).
Although not noted as an acutely toxic pesticide to test animals, the toxicity of certain formulated products containing trifluralin may be more toxic than the technical material itself(15). So, whereas the LD50 dose (the amount of the chemical lethal to one-half of experimental animals) is 10,000 milligrams per kilo (mg/kg) in rats, and greater than 2,000 mg/kg for dogs, rabbits and chickens, the oral LD50 for the product Treflan TR-10 in rats is more than 500 mg/kg(16).
Skin sensitisation (allergies) may occur in some individuals exposed to trifluralin. Inhalation may cause irritation of the lining of the mouth, throat or lungs. The solvent in emulsifiable concentrates of trifluralin may cause irritation to the skin. Most cases of poisoning result from the carrier or solvent in formulated trifluralin products, rather than from the trifluralin itself(17).
Company data states symptoms from inhaling the vapours can include headaches, dizziness and collapse; if ingested, trifluralin can cause nausea, cramps and vomiting, and it may be irritating to the eyes(18).
Prolonged or repeated exposure to trifluralin may cause skin irritation(19). Animal studies have shown consumption of trifluralin at high levels over a long period of time can cause liver and kidney damage(20).
Trifluralin is classified by the US Environmental Protection Agency (EPA) as Group C, possible human carcinogen(21). In a two-year study of rats fed 325 mg/kg per day, malignant tumours developed in the kidneys, bladder and thyroid(22). Because there is a possible increase in the risk of cancer to humans, the EPA's Lifetime Health Advisory level for trifluralin in drinking water of 5 micrograms per litre includes an additional safety margin(23).
A concern about the carcinogenicity risk of occupational exposure to trifluralin is also acknowledged by the US EPA, with the stipulation that workers, particularly mixers, loaders, and applicators, should use personal protective equipment including coveralls, chemical-resistant gloves, shoes and socks. Post-application, workers should observe a 12 hour Restricted Entry Interval(24), a condition which is unlikely to be communicated or observed in developing countries.
Trifluralin is an endocrine-disrupting chemical, according to both the UK Environment Agency and the World Wide Fund for Nature(25). These chemicals have adverse, ¡®gender-bender' effects by interfering with the body's hormones, or chemical messengers, and are active at even miniscule levels (see PAN UK briefing No.2 Mixed messages: pesticides that confuse hormones ).
Loss of appetite and weight loss followed by miscarriages were observed when pregnant rats were fed 224 or 500 mg/kg per day. Foetal weight decreased and there was an increase in the number of foetal runts at 500 mg/kg per day dosage(26).
Fate in the environment:
The persistence of trifluralin in agricultural soils following incorporation is highly variable, depending on several factors including depth of incorporation, soil moisture and temperature. Its persistence is categorised as ¡®moderate' to ¡®persistent'(27). Several field dissipation studies in northern latitudes in Canada indicated half-lives ranging from 126 to 190 days(28).
Trifluralin residues in the atmosphere of remote, non-use regions have been reported, suggesting its potential for long-range transport. Scientists found traces of trifluralin in the Canadian Arctic, which were believed to be from from Asia, probably China, in 1991(29).
Trifluralin pollutes the atmosphere and is carried long distances in dust and air. There is a general lack of understanding concerning mechanisms controlling the potential for such long distance transport. Further work is needed to understand the interactions and fate of trifluralin in the atmosphere(30).
Trifluralin is volatile, especially in wet conditions. A study found that when surface-applied to a wet fallow field, trifluralin losses through volatilisation range from 50% to 90% of the amount applied within a few hours or days(31).
Trifluralin is highly toxic to aquatic animals (fish and invertebrates) and it poses high risks to endangered species. Sediment-feeding organisms are particularly at risk because of the tendency of trifluralin to bioaccumulate. Studies also suggest exposure-related abnormalities in the vertebral development of aquatic animals, at low concentrations(32).
Residues of trifluralin have been found to concentrate in peppermint oil and spearmint oil(33). No UK or EU MRLs have been set. Trifluralin was found in UK carrots in the period 1991-99 but its use in this crop has since declined greatly(34).
Resistance in blackgrass to dinitroanilines was first recorded in 1987 in the UK. Common British weeds now resistant to trifluralin include shepherd's needle, smooth sow-thistle, common couchgrass, creeping thistle and cleavers(35). Weeds in US cotton crops are now commonly resistant to trifluralin(36).
The manufacturers stipulate that containers which have had trifluralin in them should not be re-used(37). A study to find out whether pesticide containers were safe to recycle found that trifluralin was the pesticide released in the highest quantities from recycled plastic(38).
Trifluralin is currently under review as a priority substance under the European Commission's Water Framework Directive process(39), and is on the OSPAR 1998 list of candidate substances(40), comprising hazardous chemicals in the marine environment.
It was brought under Special Review in 1979 by the US EPA because of the presence of a N-nitrosamine contaminant which had been shown to cause tumours and to have mutagenic effects in animals. The review was concluded in 1982, with the requirement that this contaminant should not exceed 0.5 parts per million, a level which the EPA believes will have no toxic effects(41).
The only two products containing trifluralin were withdrawn from the Swedish market in 1990, because the chemical enters aquatic ecosystems under normal use conditions, and it was considered to pose an unacceptable risk to aquatic organisms due to effects at low concentrations and a high bioaccumulating potential; the substance is also persistent with a half-life of at least six months or longer(42). The chemical was withdrawn from the Danish market in 1997, because fieldwork in four different soil-types showed its half-life was 201, 165, 143 and 289 days respectively; in Denmark pesticides with a half-life of more than three months cannot be registered(43). Trifluralin is not authorised in the Netherlands(44) or in Norway, where it was banned in 1993(45).
It is a matter of concern that the use of the herbicide trifluralin, an endocrine-disrupting chemical and possible human carcinogen, is increasing significantly in the UK. As there is evidence that the pesticide has chronic health effects, even at very low doses, its use in agriculture should concern regulators, public health professionals and the public.
The use of trifluralin is unsustainable because of growing resistance problems. As it is highly toxic to aquatic life, it could jeopardise vulnerable populations of protected species. Its known tendency to volatilise, and to travel great distances in the atmosphere, mean that trifluralin could become a widespread contaminant.
Herbicides such as trifluralin are used in increasing amounts against the pest problems created by monocultural agriculture. An alternative approach, which is not dependent on chemical pest control, builds soil fertility and uses crop rotations, creating natural resilience against pests. The uncontrolled dispersal of these chemicals in the environment imposes costs to human health, and to wildlife, which are unacceptably high. ( AC )