Herbicides are chemical pesticides that are used to manage vegetation. Usually, herbicides are used to reduce the abundance of weedy plants, so as to release desired crop plants from competition. This is the context of most herbicide use in agriculture, forestry, and for lawn management. Sometimes herbicides are not used to protect crops, but to reduce the quantity or height of vegetation, for example along highways and transmission corridors. The reliance on herbicides to achieve these ends has increased greatly in recent decades, and the practice of chemical weed control appears to have become an entrenched component of the modern technological culture of humans, especially in agroecosystems.
The total use of pesticides in the United States in the mid-1980s was 957 million lb per year (434 million kg/year), used over 592,000 square miles (148 million hectares). Herbicides were most widely used, accounting for 68 % of the total quantity [646 million lb per year (293 million kg/year)], and applied to 82 % of the treated land [484,000 square miles per year (121 million hectares/year)]. Note that especially in agriculture, the same land area can be treated numerous times each year with various pesticides.
A wide range of chemicals is used as herbicides, including: (1) chlorophenoxy acids, especially 2,4-D and 2,4,5-T, which have an auxin-like growth-regulating property and are selective against broadleaved angiosperm plants; (2) triazines such as atrazine, simazine, and hexazinone; (3) chloroaliphatics such as dalapon and trichloroacetate; (4) the phosphonoalkyl chemical, glyphosate, and (5) inorganics such as various arsenicals, cyanates, and chlorates.
The intended ecological effect of any pesticide application is to control a pest species, usually by reducing its abundance to below some economically acceptable threshold. In a few situations, this objective can be attained without important nontarget damage. For example, a judicious spot-application of a herbicide can allow a selective kill of large lawn weeds in a way that minimizes exposure to nontarget plants and animals.
Of course, most situations where herbicides are used are more complex and less well-controlled than this. Whenever a herbicide is broadcast-sprayed over a field or forest, a wide variety of on-site, nontarget organisms is affected, and sprayed herbicide also drifts from the target area. These cause ecotoxicological effects directly, through toxicity to nontarget organisms and ecosystems, and indirectly, by changing habitat or the abundance of food species of wildlife.
An important controversy related to herbicides focused on the military use of herbicides during the Vietnam war. During this conflict, the United States Air Force broadcast-sprayed herbicides to deprive their enemy of food production and forest cover. More than 5,600 square miles (1.4 million hectares) were sprayed at least once, about 1/7 the area of South Vietnam. More than 55 million lb (25 million kg) of 2,4-D, 43 million lb (21 million kg) of 2,4,5-T, and 3.3 million lb (1.5 million kg) of picloram were used in this military program. The most frequently used herbicide was a 50:50 formulation of 2,4,5-T and 2,4-D known as Agent Orange. The rate of application was relatively large, averaging about 10 times the application rate for silvicultural purposes. About 86% of spray missions were targeted against forests, and the remainder against cropland.
In addition, the Agent Orange used in Vietnam was contaminated by the dioxin isomer known as TCDD, an incidental by-product of the manufacturing process of 2,4,5-T. Using post-Vietnam manufacturing technology, the contamination by TCDD in 2,4,5-T solutions can be kept to a concentration well below the maximum of 0.1 parts per million (ppm) set by the United States Environmental Protection Agency (EPA). However, the 2,4,5-T used in Vietnam was grossly contaminated with TCDD, with a concentration as large as 45 ppm occurring in Agent Orange, and an average of about 2.0 ppm. Perhaps 243-375 lb (110-170 kg) of TCDD was sprayed with herbicides onto Vietnam. TCDD is well known as being extremely toxic, and it can cause birth defects and miscarriage in laboratory mammals, although as is often the case, toxicity to humans is less well understood. There has been great controversy about the effects on soldiers and civilians exposed to TCDD in Vietnam, but epidemiological studies have been equivocal about the damages. It seems likely that the effects of TCDD added little to human mortality or to the direct ecological effects of the herbicides that were sprayed in Vietnam.
This is the complete article, containing 710 words
(approx. 2 pages at 300 words per page).
