defoliant, any one of several chemical compounds that, when applied to plants, can alter their metabolism, causing the leaves to drop off. In agriculture defoliants are used to eliminate the leaves of a crop plant so they will not interfere with the harvesting machinery. Their main military objective is to deprive the enemy of cover. In addition they have been used on food crops that have been considered potential sustenance for the enemy. Unfortunately, defoliants are not discriminating chemical weapons. Normally applied from the air, they are difficult to confine to a desired area and may thus contaminate watercourses, with disastrous effects on fish and other aquatic life. Human beings are also known to suffer toxic effects from them. Further, indigenous populations have suffered severe malnutrition when their food crops have been defoliated to prevent their use by the enemy. Possibly the severest and most long-lasting side effect of military defoliation is the disruption of the fragile jungle ecosystem. Contrary to appearances, the soils that underlie rain forests are not abundant in plant nutrients. Therefore, the ecosystem generally conserves and recycles the nutrients that do exist. Any massive disruption, such as killing or interfering with the metabolism of large numbers of plants, inevitably causes large amounts of nutrients to be washed away by rainfall. Also, when sunlight, which would normally be blocked by the leaves, falls on the claylike soil of the jungle, the soil bakes and becomes very hard. These conditions operate against reestablishment of normal vegetation. During the Vietnam War, U.S. forces used the defoliant Agent Orange, which is widely thought to have led to cancer and other side effects among military personnel and civilians.

A defoliant is any chemical sprayed or dusted on plants to cause its leaves to fall off. A classic example of a highly toxic defoliant used for tactical purposes is Agent Orange, which was used widely by the U.S. military during the Vietnam War from 1961 to 1970.

Defoliants differ from herbicides in that the former seeks mainly to strip leaves from plants, and the latter is used to destroy or inhibit the growth of plants. Defoliation is not always caused by defoliants. It can be many different environmental factors that cause a plant's leaves to fall off.

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