2,4-D

2,4-D (or 2,4-dichlorophenoxyacetic acid) 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 crops.

2,4-D is sold in various formulations under a wide variety of brand names. It continues to be used for its low cost, despite the availability of more selective, effective, and less toxic products.

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 broadleaf trees in conifer plantings)
• Grass hayfields and pastures
• Cereal grains
• Corn and sorghum (occasionally)

Physical Properties

Note: properties given are for the acid. Commercial formulations used today use the trimethylamine salt or one of several esters, for which properties differ.

• Relative molecular mass: 221.04
• Appearance: Colourless, white to yellow powder when pure
• Melting point: 140.5 C
• Water Solubility: 900 mg/L @ 25 C
• Relative vapour density: 7.6
• Vapour pressure: 0.02 mPa @ 25 C, 52.3 Pa @ 160 C
• CAS number: 94-75-7

Toxicity

The LD₅₀ in rats has been reported as 375 and 666 mg/kg in two different studies. 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.

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 2,4 dichlorophenol, which is itself produced by chlorination of phenol. The production process creates several contaminants including isomers, monochlorophenol , and other polychorophenols and their acids.

The powerful defoliant and herbicide Agent Orange, used extensively throughout the Vietnam War, contained 2,4-D. However, the controversies associated with the use of Agent Orange were associated with a contaminant (dioxin) in the 2,4,5-T component.

External links

For some widely divergent views on 2,4-D, compare the industry's web site

• http://www.24d.org

to the "fact sheet" from an anti-pesticide group:

• http://www.beyondpesticides.org/pesticides/factsheets/2,4-D.pdf

2,4-D has been evaluated by the European Union and included on its list of approved pesticides. The evaluation report contains a large quantity of data on the properties (phys-chem, toxicity, environmental fate) of 2,4-D and is available at:

• http://europa.eu.int/comm/food/plant/protection/evaluation/exist_subs_rep_en.htm