2,4-Dichlorophenoxyacetic acid is a common systemic herbicide used in the control of
broadleaf weeds. It is one of the most widely used herbicides in the world, and is the third
most commonly used herbicide in North America. 2,4-D is a synthetic auxin, and as such it
is often used in laboratories for plant research and as a supplement in plant cell culture
media such as MS medium. 2,4-D was one of the ingredients in Agent
Orange, the herbicide widely used during the Vietnam war. According to the US National
Pesticide Information Center, "the controversy regarding health effects centered around the
2,4,5-T component of the herbicide and its contaminant, dioxin."
History 2,4-D was first published by Robert Pokorny
in 1941, who was working in the US; the related compound, MCPA, was discovered at about the
same time by scientists in UK. Both compounds were developed afterwards as part of a clandestine
wartime effort to create chemical warfare agents for use in World War II; 2,4-D was
not used this way in WWII. The first publication of 2,4-D's herbicidal
activity came in 1944. 2,4-D was brought to market as an herbicide
called "Weedone" starting in 1945 by the American Chemical Paint Company. It revolutionized
weed control, as it was the first compound that, at low doses, could selectively control
dicots, but not most monocots - narrow leaf crops like wheat, maize, rice, and similar
cereal grass crops. At a time when labor was scarce and there was a huge need for increased
food production, it literally "replaced the hoe".
2,4-D was one of the ingredients in Agent Orange, the herbicide widely used during the
Vietnam war. According to the US National Pesticide Information Center, "the controversy
regarding health effects centered around the 2,4,5-T component of the herbicide and its
contaminant, dioxin." Genetically modified crops
Dow has demonstrated soybean resistance to 2,4-D due to insertion of a bacterial aryloxyalkanoate
dioxygenase gene. Using a combination of the choline salt of 2,4-D and glyphosate in the
Enlist Weed Control System on 2,4-D and glyphosate resistant transgenic crops, Dow intends its
use as an alternative to Roundup Ready crops due to the increasing prevalence of glyphosate
resistant weeds. Department of Agriculture approval is pending for a 2,4-D resistant
corn. Mode of action
2,4-D is a synthetic auxin, which is a class of plant hormones. 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.
Manufacture 2,4-D is a member of the phenoxy family of
herbicides. 2,4-D is manufactured from chloroacetic acid
and 2,4-dichlorophenol, which is itself produced by chlorination of phenol. Alternatively,
it may be produced by the chlorination of phenoxyacetic acid. The production processes
create several contaminants including isomers, monochlorophenol, and other polychlorophenols
and their acids.
Applications 2,4-D is primarily used as an herbicide. It
is sold in various formulations under a wide variety of brand names. 2,4-D can be found
in commercial lawn herbicide mixtures. These products often contain other active ingredients
including mecoprop and dicamba. Over 1,500 herbicide products contain 2,4-D as an active
ingredient. 2,4-D is most commonly used for:
Weed control in lawns and other turf No-till burndown
Control of weeds and brush along fences and highway and railroad rights of way
Conifer release Grass hayfields and pastures
Cereal grains Corn and sorghum
As a synthetic auxin analog 2,4-D continues to be used, where legal, for
its low cost. However, where municipal lawn pesticide bylaws exist, such as in Canada,
alternatives must be used. A 2010 monitoring study conducted in the US
and Canada found that "current exposures to 2,4-D are below applicable exposure guidance
values." Toxicity
Acute toxicity The LD50 determined in acute toxicity rat
studies is 639 mg/kg. While urinary alkalinisation has been used
in acute poisonings, evidence to support its use is poor.
Contaminants It was assumed that because of improved manufacturing
processes that there were no longer any dangerous dioxins in 2,4-D; however, a July 2013 Four
Corners investigation found "elevated levels" of dangerous dioxins in a generic version
of 2,4-D, one of Australia's most widely used herbicides. One scientist said the product
tested by Four Corners, which was imported from China, had "one of the highest dioxin
readings for 2,4-D in the last 10 to 20 years, and could pose potential health risks."
Cancer risk On August 8, 2007, the U.S. Environmental
Protection Agency issued a ruling that stated 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. Much older data had previously used in a 1987
report by the International Agency for Research on Cancer which concluded that some chlorphenoxy
herbicides including 2,4-D, MCPA and 2,4,5-T as a group were classified as a class 2B carcinogen
- "possibly carcinogenic to humans". Environmental behavior
Owing to the longevity and extent of use, 2,4-D is among the most thoroughly studied
herbicides with respect to environmental properties. 2,4-D amine salts and esters are not persistent
under most environmental conditions. The degradation of 2,4-D is rapid in aerobic mineral soils.
2,4-D is broken down by microbes in soil, in processes that involve hydroxylation, cleavage
of the acid side-chain, decarboxylation, and ring opening. The ethyl hexyl form of the
compound is rapidly hydrolyzed in soil and water to form the 2,4-D acid. 2,4-D has a
low binding affinity in mineral soils and sediment, and in those conditions is considered
intermediately to highly mobile, and therefore likely to leach if not degraded.
In aerobic aquatic environments, the half life is 15 days, while in anaerobic aquatic
environments, 2,4-D was moderately persistent to persistent. 2,4-D has been detected in
streams and shallow groundwater at low concentrations, in both rural and urban areas. Breakdown is
pH dependent. Microbial breakdown
A number of 2,4-D-degrading bacteria have been isolated and characterized from a variety
of environmental habitats. Metabolic pathways for the compound’s degradation have been
available for many years, and genes encoding 2,4-D catabolism have been identified for
several organisms. As a result of the extensive metadata on environmental behavior, physiology
and genetics, 2,4-D was the first herbicide for which the bacteria actively responsible
for in situ degradation was demonstrated. This was accomplished using the technique
of DNA-based stable isotope probing, which enables a microbial function, such as degrading
a chemical, to be linked with the organism’s identity without the need to culture the organism
involved. This advancement has been particularly beneficial for the study of soil microorganisms,
since only a very small fraction of the thousands of species present in highly diverse soil
bacterial communities can be isolated in pure culture.
Legal issues Minimum residue levels were re-evaluated by
the European Union and the results were published in 2011. Concern over 2,4-D is such that it
is currently not approved for use on lawns and gardens in Denmark, Norway, Kuwait and
the Canadian provinces of Québec and Ontario. 2,4-D use is severely restricted in the country
of Belize. In 2005, the United States Environmental Protection Agency approved the continued use
of 2,4-D. On July 10, 2013 the Pest Management Regulatory Agency in Canada updated the re-evaluation
notice of 2,4-D stating that the 2,4-D registrants provided the PMRA with required data submissions
and deemed such data acceptable. On April 18, 2012, EPA denied the petition filed November
6, 2008 by the Natural Resources Defense Council to revoke all tolerances and to cancel all
registrations of 2,4-D. EPA stated that recent new study and EPA’s comprehensive review
confirmed EPA’s previous finding that the 2,4-D tolerances are safe.
On 21 August 2013 the Australian Pesticides and Veterinary Medicines Authority banned
selected 2,4-D high volatile ester products due to their environmental hazards. HVE 2,
4-D products had already been banned in Europe and North America for twenty years; low volatile
ester products continue to be available in Australia and worldwide. In July 2013 APVMA
published their report findings. See also
Genetically modified crops# Herbicide resistance Genetic pollution
References
External links CDC - NIOSH Pocket Guide to Chemical Hazards
Overview of the toxic effects of 2,4-D