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2,4-D
72%SL   86%SL
2,4-D, a chlorinated phenoxy compound, functions as a systemic herbicide and is used to control many types of broadleaf weeds. There are many forms or derivatives (esters, amines, salts) of 2,4-D and these vary in solubility and volatility. Unless otherwise specified, this document will refer to the acid form of 2,4-D. This compound is used in cultivated agriculture and in pasture and rangeland applications, forest management, home and garden situations and for the control of aquatic vegetation. 2,4-D was a major component (about 50%) of the product Agent Orange used extensively throughout Vietnam. However most of the problems associated with the use of Agent Orange were associated with a contaminant (dioxin) in the 2,4,5-T component of the defoliant. The association of 2,4-D with Agent Orange has prompted a vast amount of study on the herbicide.
 
ACUTE TOXICITY
While the LD50 of 2,4-D suggests that it is only moderately toxic, the product carries the DANGER signal word on the label indicating that it is highly toxic. This is because 2,4-D has produced serious eye and skin irritation among agricultural workers.
 
The oral LD50 of 2,4-D in the rat ranges from 375 to 666 mg/kg; 370 mg/kg in the mouse; and less than 320 to 1,000 mg/kg in the guinea pig. The rat and rabbit have dermal LD50 values of 1,500 mg/kg and 1,400 mg/kg, respectively. In humans, prolonged breathing of 2,4-D causes coughing, burning, dizziness, and temporary loss of muscle coordination.
 
Symptoms of poisoning can be fatigue and weakness with perhaps nausea. On rare occasions there can be inflammation of the nerve endings with muscular effects following high levels of exposure. Symptoms vary with the different commercial products because of the specific amounts and types of additives such as surfactants and solvents.
 
CHRONIC TOXICITY
 
Rats given moderate amounts (50 mg/kg) of 2,4-D in the diet for two years had no adverse effects. Some dogs fed lower amounts of the compound in their food for two years died, probably because dogs do not excrete organic acids efficiently. A human given a total of 16.3 grams in 32 days as "desperation therapy" lapsed into a stupor, showed signs of incoordination, weak reflexes, and urinary incontinence.
 
ENVIRONMENTAL FATE
 
2,4-D applied at 1.16 lb/acre to bluegrass turf in a laboratory experiment had a half-life of ten days. Other half-life figures for the herbicide in soil are seven days (15-25 degree C with 65% moisture) and ten days in non-sterile soil and 1.5 to 16 days in other studies. Soil microbes are primarily responsible for its disappearance in soil. Studies in Alaska and Canada failed to detect leaching in 22 weeks or from spring to fall, but 2,4-D has been included on the EPA list of compounds that are likely to leach from soil.
 

In aquatic environments microorganisms readily degrade 2,4-D and breakdown by sunlight is not a major reason for loss. Rates of breakdown increase with increased nutrients, sediment load and dissolved organic carbon. Under oxygenated conditions the half-life can be short, in the order of one week to several weeks. 2,4-D interferes with normal plant growth processes. Uptake of the compound is through leaves, stems and roots; however, it is generally nonpersistent. In one study when 2,4-D was applied to grass, there were 80 ppm at day zero, 45 ppm at 14 days, and 6 ppm at 56 days. Breakdown in plants is by a variety of biological and chemical pathways.

       



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