Herbicides

Introduction

Weeds can be considered as plants growing in the wrong places and often having few, if any, desirable properties. They compete with crops and garden plants for light and nutrients from the soil. Traditionally, weeds are removed by hand, but they can also be dealt with by applying chemicals called herbicides that will kill them. Herbicides are widely used today in agriculture, horticulture, and in recreational gardening.

A successful herbicide is specific, killing only the weed and not the crop. It should also not be toxic to humans or other animals, and should break down rapidly so that it does not contaminate soil or groundwater. The modern trend is toward using genetic modification to create crops that are resistant to herbicides. Such crops are protected against the impact of herbicides used on nearby weeds. However, the general public has been reluctant to accept such developments.

Historical Background and Scientific Foundations

Pesticides are chemicals that kill off any animal or plant that threatens a crop. Herbicides are a type of pesticide that kills weeds, which are plants that compete with crops for resources. Although some organisms produce their own pesticide molecules, modern pesticides tend to be synthetic organic chemicals produced by the chemical industry, usually from raw materials derived from petroleum. The first herbicides were produced in the 1940s and formed part of the so-called Green Revolution, where pesticides and fertilizers began to be widely used in agriculture in an attempt to boost crop productivity.

Today, chemists put as much effort into creating a new herbicide as they would a new drug. There are several different classes of herbicide and hundreds of different products. Some inhibit photosynthesis, so the weed turns yellow and dies, others are so-called root mitotic inhibitors, which stop root cells from dividing so the root withers away. One of the oldest herbicides is 2,4-dichlorophenoxyacetic acid (2,4-D), which is a hormonal agent that makes weeds grow extremely rapidly and then disintegrate. One of the most widely used herbicides today is glyphosphate, known as Roundup, which inhibits a specific plant enzyme. It is a broad-spectrum herbicide, which kills most weeds and is often used to clear ground.

The most important property of a herbicide is selectivity, which is the ability to kill the weed, but not the crop or any other plant or animal. Modern herbicides must not harm the environment by persisting in the soil or in groundwater. They are potentially toxic to humans if deliberately or accidentally ingested, so people using them on a large scale need to be properly trained and wear protective clothing. Most herbicides are meant for either foliar application, in which case the leaves of the weeds are sprayed, or application to the ground. They are usually applied in agriculture from a low-flying plane or using a tractor.

Impacts and Issues

In the past, there have been problems with a herbicide called 2,4,5-trichlorophenoxyacetic acid (2,4,5-T), a popular herbicide for broad-leaved weeds. The manufacturing process contaminated the product with dioxins, which are very toxic to humans. Accordingly, 2,4,5-T was withdrawn from use in the United States in 1983. The herbicide 2,4,5-T was also a component of Agent Orange, used as a defoliant by the U.S. military during the Vietnam war, so-called because it was stored in orange barrels. Its use has been linked to ill health in both Vietnam veterans and the Vietnamese people.

Today, the science of herbicides has advanced to include plants genetically modified to be resistant to glyphosphate or glufosinate. These plants are not affected if they are sprayed with the relevant herbicide because they carry a gene for resistance to it. This means that crop residue can be left on land after spraying, which has the benefit of reducing soil erosion. However, there is concern that the resistance genes could be transferred in the wild to weeds, creating a breed of superweed, which would not be sensitive to the herbicide. There is

WORDS TO KNOW

FOLIAR: Pertaining to the leaf.

HERBICIDE SELECTIVITY: The ability to kill weeds while leaving a crop unharmed.

also public concern over genetically modified food crops, so herbicide resistance has tended to be confined to crops like alfalfa or cotton that are not meant for human consumption.

See Also Agricultural Practice Impacts; Groundwater Quality

BIBLIOGRAPHY

Books

Hanson, J. R. Chemistry in the Garden. CambridgeRoyal Society of Chemistry, 2007.

Web Sites

Division of Crop Sciences, University of Illinois. “How Herbicides Work.” http://web.aces.uiuc.edu/vista/pdf_pubs/HERBWORK.PDF (accessed March 25, 2008).

Mount Shasta Herald. “Growers Hear About New Roundup Ready Alfalfa.” February 16, 2005. http://www.monsanto.co.uk/news/ukshowlib.phtml?uid=8599 (accessed March 25, 2008).

U.S. Department of Agriculture. “Agricultural Chemical Usage.” http://www.nass.usda.gov/Statistics_by_State/Pennsylvania/Publications/Annual_Statistical_Bulletin/2006_2007/nur_chemuse.pdf (accessed March 25, 2008).