Herbicides

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Herbicides are chemicals that kill plants. Herbicides are widely used in modern agriculture to control weeds, reduce competition, and increase productivity of crop plants. They are also used by homeowners to control lawn weeds and by turf grass managers, foresters, and other professionals. Herbicides are used not only on land, but also in lakes, rivers, and other aquatic environments to control aquatic weeds.

The modern use of herbicides began in the 1940s, with the development of 2,4-D (2, 4-dichlorophenelyacetic acid). By the end of that decade, herbicide use had grown from a few thousand acres to several million. There are now approximately four hundred different herbicides registered for use in the United States. While the rates of application vary by crop, the vast majority of commercial agricultural crop acreage receives at least one application of herbicide every year.

Herbicides may be applied directly to the soil or to the leaves of the target plant. Soil applications may be targeted at preventing seed germination, to affect root growth, or to be absorbed and to work systemically (within the whole plant body). Foliar (leaf) applications may target the leaves or be absorbed. In addition to directly killing the target weed, herbicides can, over time, reduce the number of weed seeds in the soil, decreasing the need for continued intensive applications in the future.

Herbicides kill plants by interfering with a fundamental process within their cells. 2,4-D is a synthetic auxin. It promotes cell elongation (rather than cell division), and in effective concentrations kills the target plant by causing unregulated growth. Plants treated with 2,4-D display misshapen stems, inappropriate adventitious root growth, and other aberrant effects (growing in an unusual location on the plant). The excessive growth exhausts food reserves, and the combination of effects eventually causes the death of the plant. 2,4-D is often used to kill dicot weeds growing among monocot crops, since monocots are more resistant to its effects. 2,4-D and a related compound, 2,4,5-T were combined in Agent Orange, the defoliant used in the Vietnam War. Health effects from exposure to Agent Orange are believed to be due to contamination with dioxin, and not to the herbicides themselves.

Glyphosphate (marketed as Roundup^®) interferes with an enzyme involved in amino acid synthesis, thereby disrupting plant metabolism in a variety of ways. It is one of the most common herbicides and is available for homeowner use as well as for commercial operators. Glyphosphate is a non-selective herbicide, killing most plants that it contacts. However, it is fairly harmless to animals, including humans, since amino acid metabolism is very different in animals. A gene for glyphosphate resistance has now been introduced into a number of important crop plants, allowing increased use of glyphosphate to control weeds on these crops.

Atrazine interferes with photosynthesis. Atrazine is taken up by roots and transported to chloroplasts, where it binds to a protein in the Photo-system II reaction center. This prevents the normal flow of electrons during photosynthesis and causes chloroplast swelling and rupture.

Paraquat also interferes with photosynthesis, but through a different mechanism. This herbicide accepts electrons from photosystem I and then donates them to molecular oxygen. This forms highly reactive oxygen free radicals, which are immediately toxic to the surrounding tissue. Paraquat is also toxic to humans and other animals.

As with any agent that causes death in a group of organisms, herbicides cause natural selection among weed species. Evolution of herbicide resistance is a serious problem and has spurred research on new herbicide development and a deeper understanding of mechanisms of action. These concerns have joined with environmental and health concerns to promote a more integrated approach to weed management, combining tillage practices, selection for weed-tolerant varieties, better understanding of weed biology, and better timing of herbicide application. This integrated approach requires more time and attention from the farmer but can also offer significant benefits.

Agriculture, Modern; Dicots; Hormones; Monocots; Photosynthesis, Light Reactions And.

Aldrich, R. J., and R. J. Kremer. Principles in Weed Management, 2nd ed. Ames, IA: Iowa State University Press, 1997.

Devine, Malcolm D., Stephen O. Duke, and Carl Fedtke. Physiology of Herbicide Action. Englewood Cliffs, NJ: Prentice-Hall, 1993.

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