2,4-Dichlorophenoxyacetic acid

2,4-Dichlorophenoxyacetic acid (2,4-D) is a common systemic herbicide used in the control of broadleaf weeds. It is the third-most widely used herbicide in North America and the most widely used herbicide in the world.

2,4-D was developed during World War II by a British team at Rothamsted Experimental Station, under the leadership of Judah Hirsch Quastel, aiming to increase crop yields for a nation at war. When it was commercially released in 1946, it became the first successful selective herbicide and allowed for greatly enhanced weed control in wheat, maize (corn), rice, and similar cereal grass crop, because it only kills dicots, leaving behind monocots.

2,4-D is a synthetic auxin, which is a class of plant growth regulators. 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.

2,4-D is sold in various formulations under a wide variety of brand names. 2,4-D can be found in lawn herbicide mixtures such as "PAR III", "Trillion", "Tri-Kil", "Killex" and "Weedaway Premium 3-Way XP Turf Herbicide" (all of these mixtures typically contain three active ingredients: 2,4-D, Mecoprop and Dicamba -- all synthetic herbicides).

2,4-D continues to be used, where legal, for its low cost. However, where municipal lawn pesticide bylaws exist (as in Canada ^[1] ) alternatives such as Corn gluten meal and vinegar based products like Weed Pharm are increasingly being used to combat weeds.

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Major uses

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 (control of broad-leaf trees in conifer plantings)
• Grass hayfields and pastures
• Cereal grains
• Corn and sorghum (occasionally)
• As a synthetic auxin analogue

Over 1,500 herbicide products contain 2,4-D as an active ingredient

Toxicity

The LD₅₀ The acute toxicity,rats study) according to US EPA 2,4-D Reregistration Eligiblity Decision, 2006,is 639 mg/kg. Single oral doses of 5 and 30 mg/kg body weight did not cause any acute toxic effects in human volunteers.

The amine salt formulations can cause irreversible eye damage, ester formulations are considered non-irritating to the eyes.

On August 8, 2007, the United States Environmental Protection Agency issued a ruling which stated existing data do not support a conclusion that links human cancer to 2,4-D exposure.^[2] However, the International Agency for Research on Cancer (IARC) has classified 2,4-D among the phenoxy acid herbicides MCPA and 2,4,5-T as a class 2B carcinogen-possibly carcinogenic to humans. ^[3]

Manufacture

2,4-D is a member of the phenoxy family of herbicides, which include:

• 2,4,5-trichlorophenoxyacetic acid (2,4,5-T)
• 2-methyl-4-chlorophenoxyacetic acid (MCPA)
• 2-(2-methyl-4-chlorophenoxy)propionic acids (mecoprop, MCPP)
• 2-(2,4-dichlorophenoxy)propionic acid (dichloroprop, 2,4-DP)
• (2,4-dichlorophenoxy)butyric acid (2,4-DB)

2,4-D is manufactured from chloroacetic acid and 2,4-dichlorophenol, which is itself produced by chlorination of phenol. The production process creates several contaminants including isomers, monochlorophenol, and other polychlorophenols and their acids.

The powerful defoliant and herbicide Agent Orange, used extensively throughout the Vietnam War, contained 2,4-D. The controversies associated with the use of Agent Orange were associated with a contaminant (dioxin) in the 2,4,5-T component. However, 2,4-D is still contaminated to some extent with dioxins, predominately those with 2 or 3 chlorine atoms. Another form of dioxin, 2,7-dichlorodibenzo-p-dioxin (DCDD), an inevitable by product of 2,4-D manufacturing, was found to be "equipotent" to dioxin TCDD in its toxic effect on the immunity of mice. TCDD received all the publicity while the DCDD component was largely forgotten.^[4] To this day DCDD is not regulated or monitored by the EPA and PMRA, even though DCDD levels could be at much higher levels than TCDD. The typical smell of 2,4-D is the first break-down product - the very toxic 2,4-dichlorophenol.

2,4-D has been evaluated by the European Union and included on its list of approved pesticides.^[5]

Concern over 2,4-D is such that it is currently not approved for use on lawns and gardens in Sweden, Denmark, Norway, Kuwait and the Canadian province of Québec. 2,4-D use is severely restricted in the country of Belize. In Canada, well over 135 municipalities have placed bylaws that restrict the cosmetic use of pesticides, including the use of herbicides containing 2,4-D.^[6]

In 2005, the United States Environmental Protection Agency approved the continued use of 2,4-D.^[7]

In Canada, the Pest Management Regulatory Agency (PMRA) has placed a condition of registration on 2,4-D such that the 2,4-D registrant(s) must provide the PMRA with a required developmental neurotoxicity study by September 20, 2009.^[8]

References

1. ^ http://www.flora.org/healthyottawa/BylawList.pdf
2. ^ http://www.epa.gov/fedrgstr/EPA-PEST/2007/August/Day-08/p15109.htm
3. ^ IARC monographs on the evaluation of carcinogenic risks to humans: An updating of IARC Monographs volumes 1 to 42. Supplement 7, WHO, Lyon, France 1987.
4. ^ http://pwp.lincs.net/sanjour/monsanto.htm
5. ^ http://europa.eu.int/comm/food/plant/protection/evaluation/exist_subs_rep_en.htm
6. ^ http://www.flora.org/healthyottawa/BylawList.pdf
7. ^ http://www.epa.gov/oppsrrd1/reregistration/24d/
8. ^ http://www.pmra-arla.gc.ca/english/pdf/pacr/pacr2007-06-e.pdf

The industry's web site

• http://www.24d.org

Health and Environmental References:

• http://www.beyondpesticides.org/pesticides/factsheets/2,4-D.pdf
• http://www.flora.org/healthyottawa/fs-5.htm
• http://www.sierraclub.ca/national/programs/health-environment/pesticides/2-4-D-overview.pdf