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Herbicides are used to kill or damage plants and are the most rapidly growing type of pesticide. Prior to the 1930s, herbicides were nonspecific and often very toxic to humans as well as other animals. In the 1930s, researchers discovered several chemicals that selectively killed plants while developing new insecticides. These chemicals are now widely used to increase food production by killing weeds that choke out or compete with food crops.

The most well known herbicides are the chlorophenoxy compounds that include 2,4-D and 2,4,5-T. This herbicide mixture, sometimes called Agent Orange in the 1960s, was widely used to kill broadleaf plants in agricultural fields, along roadsides, and on rights of way for power lines. It was also extensively used as a chemical warfare agent to kill unwanted vegetation, for example in jungles. The mechanism of action of this class of chemicals is poorly understood, but the herbicides appear to interact with plant growth hormones. (See Pesticides - History for discussion of the contamination of 2,4,5-T with dioxin.)

Paraquat and the related chemical diquat are nonselective herbicides that are also toxic to mammals. Occupational or accidental exposure to paraquat can occur by ingestion, skin exposure, or inhalation, all of which can cause serious illness or death. While seldom used in the United States at this time, paraquat is still widely used in developing countries. At one time it was used in marijuana plant eradication programs, but it was discontinued when a number of fatalities were observed in smokers of paraquat-contaminated marijuana.

There are many other herbicides in widespread use, such as alachlor, glyphosate, and atrazine, and they have a range of actions on plants and animals.

Herbicides have become an essential part of the agriculture business and are thought by some to be necessary for high crop yields. However, a serious limitation of many herbicides is their lack of specificity; in other words, herbicides can damage the crops of interest. The manufacturers of herbicides are working to address this problem and are increasingly turning to biotechnology to create genetically modified crops that are herbicide resistant. For example, Monsanto produces the glyphosate-based herbicide RoundUp. The company also manufactures a genetically modified soybean that is resistant to RoundUp. This allows farmers to use RoundUp herbicide with the RoundUp Ready soybean plants and not have to worry about killing the soybean plants. The genetically modified RoundUp Ready soybean is now widely planted, though the practice has generated considerable controversy internationally.
(Photo: weeds sprayed with herbicide)

Next: Biological Properties of Fungicides

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