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The Herbicide 2, 4-D

  Herbicide effects on broadleaf weed  
To most Americans, "2, 4-D" is probably just a mish mash of numbers and letters. But to farmers and gardeners, it is the most widely used herbicide in the world and the third most commonly used in North America.

Chemically, the compound is known as 2,4-Dichlorophenoxyacetic acid and has a molecular formula – for those understand those kinds of things – of C8H6Cl2O3.

It was actually developed as part of the World War II war effort by a British team intent on increasing crop yields by suppressing weeds. It was introduced commercially in 1946 and quickly shot to the top of the usage charts. A 1996 study by the U.S. Department of Agriculture estimated that if 2, 4-D was taken off the market, it would result in $1.6 million in increased food and fiber costs to the consumer.

It was the first "selective" herbicide, meaning that it suppressed "dicots" (plants with two seed leaves, also known as broadleaf plants) while leaving "monocots" alone (plants with one seed leaf or thin leaves). In other words, the herbicide can be sprayed on grasses (like wheat, corn, rice and other cereal crops) – it will leave them alone while it kills broadleaf weeds.

Alex Martin InterviewIn a sense, 2, 4-D gives a broadleaf plant a form of symbolic cancer – it causes it to grow uncontrollably. According to retired UNL agronomy professor Alex Martin, "The plant grows itself to death. Now, that may not be technically true, but it's a sort of unregulated growth that results in the destruction of vital tissues." Alex says the herbicide affects broadleaf weeds and not grasses because the two classes of plants transport chemicals differently through their leaves. Broadleaf weeds have their growing point at the tips of the leaves. Grasses grow from their base.

One of the reasons that 2, 4-D is used so extensively is that it's cheap. There are over 1,500 herbicide products that contain 2. 4-D, and it's often mixed with other active ingredients like mecoprop and dicamba. Some of the (imaginative) brand names include Weed B Gone, PAR III, Trillion, Tri-Kil, Killex and Weedaway.

As with any pesticide, there are widespread concerns about the toxicity of 2, 4-D and its chemical cousins in the phenoxy family of herbicides.

The studies seem to be contradictory. A 1990 study of farmers in Nebraska found that, even when adjusting for exposure to other chemicals, 2, 4-D exposure substantially increased the risk of Non-Hodgkin's lymphoma (NHL). But a 2000 study of former Dow Chemical Company employees who manufactured the herbicide found no significant increase of risk for NHL. However it did find an increased risk for mortality due to Amyotrophic Lateral Sclerosis (ALS or Lou Gehrig's disease).

Studies on the cancer risk of 2, 4-D have been conflicted, as well. In 2007, the EPA ruled that existing data does not support a link between cancer and 2, 4-D exposure. But, the International Agency for Research on Cancer has classified 2, 4-D among the chemicals that could "possibly" cause cancer.

Other studies have linked small exposures to 2, 4-D to liver damage, and there has been concern about the possibility of cirrhosis of the liver in exposed golfers.

2, 4-D was also one of the chemicals in the infamous Agent Orange defoliant used in Vietnam. That use has been linked to a host of health problems in returning veterans and citizens in Vietnam, but most of the studies conclude that the chemical dioxin was probably the one that caused the most damage.

Despite the concerns, 2, 4-D remains one of the most widely used herbicides around the world.

Written by Bill Ganzel, the Ganzel Group. First published in 2009. A partial bibliography of sources is here.


 

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