The herbicide 2,4-D was developed to kill broad-leafed plants but not grass and related grain crops, making it an important weed killer and making efficient, targeted weed killing possible on a large scale.
Humans have been at war with weeds since humankind ceased living as hunter-gatherers and became agriculturalists thousands of years ago. Weeds may be defined as plants growing in places where humans do not want them to be. This may be in the middle of an ornamental flower garden or in farm fields with food crops. The first case is an annoyance to individuals; the second may bring about crop failure and lower the chances of human survival, for weeds not only decrease crop yields but also lower crop quality.
Weed killing in agriculture involved laborious tilling by hand until the development of herbicides, substances that kill weeds. As any home gardener knows, tilling is laborious, expensive, and time-consuming. Before 2,4-dichlorophenoxyacetic acid (2,4-D) became widely used in the late 1940’s, it was estimated that weeds diminished agricultural revenues of the United States by more than $3 billion per year. At that time, weed removal by tillage made up 20 to 35 percent of the entire cost of most successful agricultural endeavors.
At the time 2,4-D was introduced in the mid-1940’s, herbicides such as sulfuric acid, sodium chloride, sodium chlorate, and arsenic compounds had been used for about fifty years. The biggest advantage of 2,4-D in comparison with the other known herbicides was that it could be used in tiny amounts and with relative safety. For example, one water spray form of 2,4-D used one part herbicide per one thousand parts of water. Furthermore, 2,4-D in the 1940’s cost only three to eight dollars per acre, compared with fifty dollars or more per acre for any of the other herbicides. Thus, 2,4-D was consequently viewed as being both inexpensive and safe.
Unlike the other types of herbicides, 2,4-D, the most widely used of the weed-killing substances known as the chlorophenoxy herbicides, is a growth regulator that requires only tiny quantities to turn weeds into plants no longer suited to living in their environment. One pound of 2,4-D can replace tens or even hundreds of pounds of most other herbicides. The herbicide is, moreover, simple and inexpensive to produce.
In its pure form, 2,4-D is a white, odorless, and noncorrosive powder that is relatively easy to handle and apply. Because the pure form will not dissolve in water, its application to plants generally requires mixing with oils. Alternatively, 2,4-D can be applied as a water spray, in which case it is first converted into water-soluble forms called esters and amines.
Such water-based forms of 2,4-D are frequently used, although they are usually somewhat more expensive than the pure, unaltered substance. One advantage of the “oil forms” of 2,4-D is that they are not easily washed off the weeds by rain. Rainfall twenty-four hours after the application of the water-soluble forms of 2,4-D very markedly decreases their herbicidal effectiveness. Because there are advantages to 2,4-D preparations both as oils and as water sprays, many different preparations of this herbicide are commercially available.
The discovery of 2,4-D was a product of the extensive search for selective herbicides that arose from discovery of the first such chemicals, the industrially produced nitrophenols and chlorophenols, and the plant hormone indoleacetic acid. The chemical structure of 2,4-D combines features of both of these types of herbicides, making it a widely used weed killer with many applications.
The herbicidal value of 2,4-D was first observed during a systematic study of insecticides carried out by U.S. Department of Agriculture experts from universities across the country. On the basis of their research, the Du Pont Corporation
Low concentrations of the herbicide were shown to cause rapid and distorted plant growth, followed soon thereafter by death, when applied to many of the broad-leafed plants that are costly agricultural pests, including wild mustard, Canada thistle, and black bindweed. In contrast, 2,4-D at low concentrations does not affect grasses and thus does not harm grain crops. This is not true of higher doses, which can harm crop plants, so 2,4-D must be applied carefully to crops.
Among the other useful properties of 2,4-D is its relatively quick disappearance from the soil, within sixty to ninety days after its spray application. Several researchers on the original 2,4-D development panel reported that 2,4-D ingestion by cattle, and by the researchers themselves, caused no ill effects—but, of course, any chemical that kills living things has the potential to be toxic to humans. The Merck Index (1976) points out that human contact with large amounts of 2,4-D can cause eye irritation and gastrointestinal disturbances, and that high concentrations are lethal to rodents. Although toxic doses are much higher than those used herbicidally, weed killers and insecticides should always be used cautiously.
One tremendously important consequence of 2,4-D use was the rapid burgeoning of agricultural crops, which led to huge increases of annual production and billions of dollars of profit, kept U.S. food prices down (contrary to expectation), and increased America’s power as an agricultural nation. At the same time, U.S. citizens were ensured adequate nutritional intake, and the probability of a U.S. famine was greatly lowered. Similar consequences have been seen in other industrialized nations that use 2,4-D.
Still unresolved, however, is the question of whether 2,4-D and its breakdown products will cause environmental problems and health problems to those who eat foods that contain traces of these substances. It is clear, however, that unlike the persistent insecticide dichloro-diphenyl-dichloroethane (DDT), 2,4-D lingers in the environment only briefly, which should make negative consequences from the herbicide’s use much less extensive.
Another important effect of 2,4-D has been its great ability to kill poison ivy, ragweed, and many of the other plants that cause hay fever and associated respiratory allergies. The herbicide has been sprayed widely on roadsides and in empty lots, which has reputedly had beneficial results. It has yet to be shown that 2,4-D causes health problems when sprayed for this purpose or when sprayed on golf courses and lawns.
A major negative effect of 2,4-D was caused by its very close relative, trichlorinated phenoxyacetic acid (2,4,5-T), which is a very potent defoliant and killer of woody plants such as underbrush and trees. When it was found that a mixture of 2,4-D and 2,4,5-T produced various compound herbicides useful for clearing land, the U.S. military purchased huge quantities of one combination that was shipped in bright orange drums and very soon named Agent Orange. This chemical was used during the Vietnam War to defoliate the Vietnamese jungle and thus to help locate Viet Cong troops. The defoliant worked, but Agent Orange was found to contain an extremely toxic chemical called dioxin, a product of the manufacture of 2,4,5-T. Dioxin was subsequently identified as causing many severe health problems to U.S. service personnel who delivered Agent Orange or otherwise came in contact with it during the war. Many studies link dioxin to cancer.
Because 2,4-D is an organic chlorine compound, many of which are carcinogens, and because human cancers may take twenty or thirty years to develop, the herbicide must continue to be handled sparingly and with the utmost caution.
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Ashton, Floyd M., and Alden S. Crafts. Mode of Action of Herbicides. New York: Wiley-Interscience, 1973. A solid scientific treatise with information on the chemistry, use, and problems associated with herbicides. Also includes a section on 2,4-D and other chlorophenoxy insecticides. -
Bender, David L., and Bruno Leone. The Environmental Crisis: Opposing Viewpoints. San Diego, Calif.: Greenhaven Press, 1991. A thoughtful, provocative book with essays on environmental topics from differing viewpoints. Includes discussions of pesticides and thoughtful commentaries on the roles of government, the public, and activists in seeking solutions for environmental problems. -
Hildebrand, E. M. “War on Weeds.” Science 103 (April 19, 1946): 465-468, 492. An effective article that gives a brief but solid overview of herbicides, touching on their significance, history, types, and uses. Also contains a valuable section on 2,4-D. -
Inderjit, ed. Weed Biology and Management. Boston: Kluwer Academic, 2004. Extensive text on the biology of weeds and the science behind their control and elimination. Bibliographic references and index. -
Muzik, Thomas J. Weed Biology and Control. New York: McGraw-Hill, 1970. A solid approach to weed control that covers many alternatives. Includes a clear explanation of 2,4-D chemistry and biology. -
Singh, Harminder Pal, Daizy Rani Batish, and Ravinder Kumar Kohli, eds. Handbook of Sustainable Weed Management. New York: Food Products Press, 2006. Handbook devoted to methods for controlling weeds without disturbing environmental ecosystems or otherwise harming the planet. Bibliographic references and index. -
Tukey, Harold B. “2,4-D, a Potent Growth Regulator of Plants.” Scientific Monthly 64 (February, 1947): 93-97. Provides detailed information on 2,4-D, including aspects of 2,4-D manufacture and a list of researchers involved in its development and testing. Also discusses methods of application for specific uses and its relative safety under different conditions. -
“2,4-D and Weed Control.” Consumer’s Research Bulletin 18 (August, 1946): 22-24. Presents the early history of 2,4-D and mentions many of the forms in which it was sold in 1946, the methods of 2,4-D action, and some of its viable uses. -
Winholz, Martha, Susan Budavari, Lorraine Y. Stroumtsos, and Margaret N. Fertig. The Merck Index. 9th ed. Rahway, N.J.: Merck, 1970. Gives physical and chemical data, synthetic references, toxicities, and uses for thousands of industrial chemicals, including 2,4-D. Very useful source.
Portable Aerosol Containers Are Introduced
Congress Amends the Water Pollution Control Act
Dioxin Causes Chloracne in West German Chemical Workers
United States Sprays Agent Orange in Vietnam
Carson Publishes Silent Spring
Green Revolution
Jensen Finds PCBs in Animal Tissues
Environmental Defense Fund Is Founded
Pesticide Poisons the Rhine River
DDT Ban Signals New Environmental Awareness