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Herbicides - Types Of Herbicides, Use Of Herbicides, Environmental Effects Of Herbicide Use

Science EncyclopediaScience & Philosophy: Habit memory: to Heterodont

A herbicide is a chemical used to kill or otherwise manage certain species of plants considered to be pests. Plant pests, or weeds, compete with desired crop plants for light, water, nutrients, and space. This ecological interaction may decrease the productivity and yield of crop plants, thereby resulting in economic damage. Plants may also be judged to be weeds if they interfere with some desired aesthetic effect, as is the case of weeds in lawns.

Clearly, the designation of plants as weeds involves a human judgment. However, in other times and places weeds may be judged to have positive values. For example, in large parts of North America, the red raspberry (Rubus strigosus) is widely considered to be one of the most important weeds in forestry. However, this species also has positive attributes. Its fruits are gathered and eaten by people and wildlife. This vigorously growing plant also provides useful ecological services. For example, it binds soil and helps prevent erosion, and takes up nutrients from the soil, which might otherwise be leached away by rainwater because there are so few plants after disturbance of the site by clear-cutting or wildfire. These ecological services help to maintain site fertility.

Still, it is undeniable that in certain situations weeds exert a significant interference with human purposes. To reduce the intensity of the negative effects of weeds on the productivity of desired agricultural or forestry crops, fields may be sprayed with a herbicide that is toxic to the weeds, but not to the crop species. The commonly used herbicide 2,4-D, for example, is toxic to many broad-leaved (that is, dicotyledonous) weeds, but not to wheat, maize or corn, barley, or rice, all of which are members of the grass family (Poaceae), and therefore monocotyledonous. Consequently, the pest plants are selectively eliminated, while maintaining the growth of the desired plant species.

Modern, intensively managed agricultural systems have an intrinsic reliance on the use of herbicides and other pesticides. Some high-yield varieties of crop species are not very tolerant of competition from weeds. Therefore, if those crops are to be successfully grown, herbicides must be used. Many studies have indicated the shorter-term benefits of herbicide use. For example, studies of the cultivation of maize in Illinois have demonstrated that the average reduction of yield was 81% in unweeded plots, while a 51% reduction was reported in Minnesota. Yields of wheat and barley can be reduced by 25-50% as a result of competition from weeds. To reduce these important, negative influences of weeds on agricultural productivity, herbicides are commonly applied to agricultural fields. As noted above, the herbicide must be toxic to the weeds, but not to the crop species.

Chlorophenoxy acid herbicides

Chlorophenoxy acid herbicides cause toxicity to plants by mimicking their natural hormone-like auxins, and thereby causing lethal growth abnormalities. These herbicides are selective for broad-leaved or angiosperm plants, and are tolerated by monocots and conifers at the spray rates normally used. These chemicals are moderately persistent in the environment, with a half-life in soil typically measured in weeks, and a persistence of a year or so. The most commonly used compounds are 2,4-D (2,4-Dichlorophenoxyacetic acid); 2,4,5-T (2,4,5-Trichlorophenoxyacetic acid); MCPA (2-Methyl-4-chlorophenoxyacetic acid); and silvex [2-(2,4,5-Trichlorophenoxy)-propionic acid].

Triazine herbicides

Triazine herbicides are mostly used in corn agriculture, and sometimes as soil sterilants. These chemicals are not very persistent in surface soils, but they are mobile and can cause a contamination of groundwater. Important examples of this class of chemicals are: atrazine [2-Chloro-4-(ethylamino)-6-(isopropylamino)s-triazine]; cynazine [2-(4-Chloro-6-ethylamino -5-triazin-2-ylamino)-2-methylpropionitrile]; hexazinone [3-Cyclohexyl-6-(dimethyl-amino)-1-methyl-1,3,5-triazine-2,4(1H,3H)-dione]; metribuzin [4-Amino-6-tert-butyl-3-(methylthio)-as-triazin-5(4H)-one]; and simazine [2-chloro-4,6-bis-(ethyl-amino)-s-triazine].

Organic phosphorus herbicides

Organic phosphorus herbicides are few, but they include the commonly used chemical, glyphosate (N-phosphonomethyl-glycine). Glyphosate has a wide range of agricultural uses, and it is also an important herbicide in forestry. To kill plants, glyphosate must be taken up and transported to perennating tissues, such as roots and rhizomes, where it interferes with the synthesis of certain amino acids. Because glyphosate can potentially damage many crop species, its effective use requires an understanding of seasonal changes in the vulnerability of both weeds and crop species to the herbicide. Glyphosate is not mobile in soils, has a moderate persistence, and is not very toxic to animals. Recently, varieties of certain crops, notably the oilseed canola, have been modified through genetic engineering (transgenics) to be tolerant of glyphosate herbicide. Previously, there were no effective herbicides that could be applied to canola crops to reduce weed populations, but now glyphosate can be used for this purpose. However, this has become controversial because many consumers do not want to eat foods made from transgenic crops.

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