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Herb icides are che micals or biological agents used as pesticides to kill a weed, which is any plant growing where it is not wanted, usually those abundant enough to take over flowerbeds, gardens, or reduce yield in agricultural fields. Reasons for using herbicides include improving the aesthetic appeal of a landscape or ornamental garden, increasing crop yields, or to kill intoxicants or narcotic plants grown as illegal substances. Herbicides have also been used to expose enemy combatants in jungle areas.
Until the time of World War II, weeds were controlled mechanically. This included plowing, weeding by hand, or using a hoe. Cultural controls are methods used to cultivate the land so it is less hospitable to weed development. Altering the pH of the soil, its salinity, and its fertility are other ways to hamper weed development.
Some plants are natural herbicides. Sassafras trees (Sassafras albidum) act as herbicides with allelopathic toxins produced in their roots. Allelopathy occurs when one plant interferes chemically with the development of another. Other allelopathic plants include black walnuts, sagebrush, and sunflowers.
Most herbicides are synthetic chemicals manu factured for specific purposes. There are a number of ways to classify herbicides, such as by the kind of vegetation they control, the activity, use, chemical family name, or mode of action. Contact and systemic herbicides are “activity” herbicides. Contact herbicides kill plant tissue on contact. They are fastacting and are generally most effective on annuals. Systemic herbicides work by being taken into the plant, which it then poisons. These herbicides are usually applied by spraying on the leaves or by applying it to the soil.
Preemergent herbicides are applied to the soil before the crop emerges to prevent weeds from germinating. Postemergent herbicides are applied after the crop has emerged. Some herbicides inhibit the biology, enzymes, or proteins of a plant, interfering with the mechanism of action of a weed. The ACCase (Acetyl coenzyme A carboxylase) inhibitors are used to kill grasses by interfering with the lipid synthesis in their cell membranes. Acetohydroxyacid synthase (AHAS) acetolactate synthase (ALS), inhibitors interfere with enzymes or with amino acid production in plants, causing them to slowly starve to death. Other inhibitors interfere with the production of amino acids in plants. Glyphosate, sold commercially as Roundup, is an enolpyruvuylshikimate 3-phosphate synthase enzyme (EPSPS) inhibitor. The first organic herbicide was synthetic auxin, which mimics plant growth hormones to interfere with plant development. Photosystem II inhibitors interfere with electron flow in photosynthesis.
Herbicides are used in enormous quantities in landscaping, landscape turf management, and in agriculture. They are also used along highways in maintenance programs to control vegetation, which are called total vegetation control (TVC) programs. Herbicides are also used extensively in the management of wildlife areas, in forestry, and in pasture management systems. They are also used to eliminate or reduce the growth of weeds in lakes. Most widely used herbicides are mixed with water and sprayed with various equipment, such as a container with a pumping sprayer for small garden spray units. Herbicides may also be sprayed in industrial volume. Railroad and powerline right of ways are often cleansed of weeds in this manner. Airplanes or helicopters can also be used to aerially spray a large volume over wide areas. Chemigation is the method for spraying herbicides through irrigation equipment.
Many of the large number of herbicides in use pose potential health effects on humans, some of them serious. Damaging effects can range from rashes to cancer to immediate death. If applicators are not used properly, gardeners or field workers can inhale aerial sprays. Traces of herbicides can also remain on foods and be consumed. Triazine herbicides have been listed as a human carcinogen, linked especially with breast cancer. Low levels of exposure are believed to disrupt endocrine production. The Environmental Protection Agency has issued a warning concerning the danger of high levels of exposure. Atrazine is an herbicide that has been linked to heart attacks, strokes, eye damage, and birth defects.
Agent Orange was used extensively in the Vietnam War by the United States as a defoliant. Between 1962-71, about 20 million gallons of herbicides were sprayed on the jungles in order to expose enemy troop movements on the Ho Chi Mien Trail and other areas. In the process, American troops were exposed in sufficient doses to cause a number of illnesses.
Herbicides have also caused serious ecological damage. Surface runoff of herbicides has contributed to the pollution of lakes, streams, and rivers used as sources of potable water. Fish kills and other downstream negative health impacts have occurred because herbicides leached or were washed into watersheds. Negative effects from herbicides can also come from soil contamination. Many herbicides deteriorate once sprayed onto an area, but others remain for a sufficiently long period of time that they pose a long-term health danger.
- De Padro and J. Jorrin, eds., Weed and Crop Resistance to Herbicides (Springer-Verlag, 1997);
- D. Dodge, ed., Herbicides and Plant Management (Cambridge University Press, 1990);
- T. Meyer and E.M. Thurman, eds., Herbicide Metabolites in Surface Water and Groundwater (Oxford University Press, 1996);
- National Academies Press Staff, Veterans and Agent Orange: Committee to Review the Health Effects in Vietnam Veterans of Exposure to Herbicides (National Academies Press, 2003);
- B. Powles and D.L. Shaner, Herbicide Resistant Weed Management in World Grain Crops (CRC Press, 2001);
- M. Roe, R.J. Kuhr, and J.D. Burton, eds., Herbicide Activity: Toxicology, Biochemistry and Molecular Biology (IOS Press, Inc., 1997).