2,4-Dichlorophenoxyacetic acid
2,4-Dichlorophenoxyacetic acid is an organic compound with the chemical formula. It is usually referred to by its ISO common name 2,4-D. It is a systemic herbicide that kills most broadleaf weeds by causing uncontrolled growth, but most grasses such as cereals, lawn turf, and grassland are relatively unaffected.
2,4-D is one of the oldest and most widely available herbicides and defoliants in the world, having been commercially available since 1945, and is now produced by many chemical companies since the patent on it has long since expired. It can be found in numerous commercial lawn herbicide mixtures, and is widely used as a weedkiller on cereal crops, pastures, and orchards. Over 1,500 herbicide products contain 2,4-D as an active ingredient.
History
2,4-D was first reported in 1944 by Franklin D. Jones at the C. B. Dolge Company in Connecticut. The biological activity of 2,4-D as well as the similar hormone herbicides 2,4,5-T, and MCPA were discovered during World War II, a case of multiple discovery by four groups working independently under wartime secrecy in the United Kingdom and the United States: William G. Templeman and associates at Imperial Chemical Industries in the UK; Philip S. Nutman and associates at Rothamsted Research in the UK; Franklin D. Jones and associates at the American Chemical Paint Company; and Ezra Kraus, John W. Mitchell, and associates at the University of Chicago and the United States Department of Agriculture. All four groups were subject to wartime secrecy laws and did not follow the usual procedures of publication and patent disclosure. In December 1942, following a meeting at the Ministry of Agriculture the Rothamsted and ICI workers pooled resources and Nutman moved to Jealott's Hill to join the ICI effort. The first scientific publication describing the 2,4-D structure and plant growth regulating activity was by Percy Zimmerman and Albert Hitchcock at the Boyce Thompson Institute, who were not the original inventors. The precise sequence of early 2,4-D discovery events and publications has been discussed.William Templeman found that when indole-3-acetic acid, a naturally occurring auxin, was used at high concentrations, it could stop plant growth. In 1940, he published his finding that IAA killed broadleaf plants within a cereal field. MCPA was discovered at about that time by his ICI group.
In the United States, a similar search for an acid with a longer half life, i.e., a metabolically and environmentally more stable compound, led to 2,4-dichlorophenoxyacetic acid and 2,4,5-trichlorophenoxyacetic acid, both phenoxy herbicides and analogs of IAA. Robert Pokorny, an industrial chemist for the C.B. Dolge Company in Westport, Connecticut, published their synthesis in 1941.
2,4-D was not used as a chemical warfare agent during the war. The Allies of World War II were looking for a chemical to starve Nazi Germany and Japan into submission by killing their potato and rice crops, but 2,4-D was found to be ineffective for that purpose, because both crops tolerate it. Within a year after the war ended, 2,4-D was commercially released as an herbicide to control broadleaf weeds in grain crops such as rice and wheat, and in the 1950s it was registered in the United States to control size and enhance skin color in potatoes without affecting yields.
The first publication of 2,4-D's use as a selective herbicide came in 1944. The ability of 2,4-D to control broadleaf weeds in turf was documented soon thereafter, in 1944. Starting in 1945, the American Chemical Paint Company brought 2,4-D to market as an herbicide called "Weedone". It revolutionized weed control, as it was the first compound that, at low doses, could selectively control dicotyledons, but not most monocotyledons — narrowleaf crops, such as wheat, maize, rice, and similar cereal grass crops. At a time when labor was scarce and the need for increased food production was large, it literally "replaced the hoe". Its effectiveness, together with its non-corrosive, non-explosive and seemingly non-irritating or poisonous qualities, led to it being considered the best all-around herbicide by 1946.
2,4-D is one of the ingredients in Agent Orange, an herbicide that was widely used during the Malayan Emergency and the Vietnam War. However, 2,3,7,8-tetrachlorodibenzodioxin, a contaminant in the production of another ingredient in Agent Orange, 2,4,5-T, was the cause of the adverse health effects associated with Agent Orange.
In the 2000s, Dow AgroSciences developed a new choline salt version of 2,4-D that Dow included in its "Enlist Duo" herbicide along with glyphosate and an agent that reduces drift; the choline salt form of 2,4-D is less volatile than 2,4-D.
Manufacture
2,4-D is a member of the phenoxy family of herbicides. It is manufactured from chloroacetic acid and 2,4-dichlorophenol, which is itself produced by chlorination of phenol. Alternatively, it is produced by the chlorination of phenoxyacetic acid. The production processes may create several contaminants including di-, tri-, and tetrachlorodibenzo-p-dioxin isomers and N-nitrosamines, as well as monochlorophenol.Mode of action
2,4-D is a synthetic auxin that induces uncontrolled growth and eventually death in susceptible plants. It is absorbed through the leaves and is translocated to the meristems of the plant. Uncontrolled, unsustainable growth ensues, causing stem curl-over, leaf withering, and eventual plant death. 2,4-D is typically applied as an amine salt, but more potent ester versions exist, as well.Applications
2,4-D is primarily used as a selective herbicide that kills many terrestrial and aquatic broadleaf weeds, but not grasses. 2,4-D can be found in commercial lawn herbicide mixtures, which often contain other active ingredients including mecoprop and dicamba. Over 1,500 herbicide products contain 2,4-D as an active ingredient.A variety of sectors use products containing 2,4-D to kill weeds and unwanted vegetation. In agriculture, it was the first herbicide for selective killing of weeds but not crops. It has been used since 1945 to control broad-leafed weeds in pastures, orchards, and cereal crops such as corn, oats, rice, and wheat. Cereals, in particular, have excellent tolerance to 2,4-D when it is applied before planting. 2,4-D is the cheapest way for farmers to control winter annual weeds by spraying in the fall, often at the lowest recommended rate. This is particularly effective before planting beans, peas, lentils, and chickpeas. The estimated use of 2,4-D in US agriculture is mapped by the US Geological Survey. In 2019, the latest date for which figures are available, this reached annually.
In domestic lawn and garden maintenance, 2,4-D is commonly used. In forestry, it is used for stump treatment, trunk injection, and selective control of brush in conifer forests. Along roadways, railways, and power lines, it is used to control weeds and brush which might interfere with safe operation and damage equipment. Along waterways, it is used to control aquatic weeds that might interfere with boating, fishing, and swimming or clog irrigation and hydroelectric equipment. It is often used by government agencies to control the spread of invasive, noxious, and non-native weed species and prevent them from crowding out native species, and also to control many poisonous weeds such as poison ivy and poison oak.
A 2010 monitoring study conducted in the US and Canada found that "current exposures to 2,4-D are below applicable exposure guidance values."
2,4-D has been used in laboratories for plant research as a supplement in plant cell culture media such as MS medium since at least 1962. 2,4-D is used in plant cell cultures as a dedifferentiation hormone. It is classified as an auxin plant hormone derivative.
Health effects
Men who work with 2,4-D are at risk for abnormally shaped sperm and thus fertility problems; the risk depends on the amount and duration of exposure and other personal factors.Acute toxicity
According to the U.S. Environmental Protection Agency, "The toxicity of 2,4-D depends on its chemical forms, including salts, esters, and an acid form. 2,4-D generally has low toxicity for humans, except certain acid and salt forms can cause eye irritation. Swimming is restricted for 24 hours after application of certain 2,4-D products applied to control aquatic weeds to avoid eye irritation." the median lethal dose or LD50 determined in acute toxicity rat studies was 639 mg/kg.Urinary alkalinisation has been used in acute poisoning, but evidence to support its use is poor.
Cancer risk
The International Agency for Research on Cancer classifies 2,4-D as a possible carcinogen to humans while the United States Environmental Protection Agency does not. The International Agency for Research on Cancer, said 2,4-D was classified as "possibly carcinogenic to humans, based on inadequate evidence in humans and limited evidence in experimental animals".In June 2015 the World Health Organization's International Agency for Research on Cancer confirmed its 1987 classification of 2,4-D as a possible carcinogen.
On August 8, 2007, the EPA issued a ruling that existing data do not support a link between human cancer and 2,4-D exposure.
A 1995 panel of 13 scientists reviewing studies on the carcinogenicity of 2,4-D had divided opinions. None of the scientists thought the weight of the evidence indicated that 2,4-D was a "known" or "probable" cause of human cancer. The predominant opinion indicated that it is possible that 2,4-D can cause cancer in humans, although not all of the panelists believed the possibility was equally likely: one thought the possibility was strong, leaning toward probable, and five thought the possibility was remote, leaning toward unlikely. Two panelists believed it unlikely that 2,4-D can cause cancer in humans.
In a prior 1987 report the IARC classified some chlorphenoxy herbicides including 2,4-D, MCPA and 2,4,5-T as a group as class 2B carcinogens - "possibly carcinogenic to humans".
2,4-D has been linked to lymphoma and bladder cancer in dogs.