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A herbicide is a pesticide used to kill unwanted plants. Selective herbicides kill certain targets while leaving the desired crop relatively unharmed. Some of these act by interfering with the growth of the weed and are often based on plant hormones. Herbicides used to clear waste ground are nonselective and kill every plant with which they come into contact.1 History
Prior to the widespread use of chemical herbicides, cultural controls, such as altering soil pH, salinity, or fertility levels, were used to control weeds.
The first widely used herbicide was 2,4 dichlorophenoxyacetic acid, often abbreviated 2,4-D. It first saw widespread production and use in the late 1940s. It is easy and inexpensive to manufacture, and kills many broadleaf plants while leaving grasses unaffected. Its low cost has led to continued usage today. Like other acid herbicides, current formulations utilize either an amine salt (usually trimethyl amine) or one of many esters ( ester) of the base compound. These are easier to handle than the acid.
2,4-D exhibits relatively poor selectivity, meaning that it causes stress to non-target plants. It is also less effective against some broadleaf weeds, including sedges and many vinous plants. A herbicide is termed selective if it affects only certain types of plants, and nonselective if it inhibits most any type of plant. Other herbicides have been more recently developed to achieve the desired selectivity.
The 1970s saw the introduction of atrazine , which has the dubious distinction of being the herbicide of greatest concern for groundwater contamination. Atrazine does not break down readily (within a few weeks) after being appplied. Instead it is carried deep into the soil by rainfall causing the aforementioned contamination. Atrazine is said to have high carryover, a very undesirable property for herbicides.
Glyphosate, frequently sold under the brand name Roundup, was introduced in the late 1980s for non-selective weed control. It is now a major herbicide in selective weed control in growing crop plants due to the development of crop plants that are resistant to it. The pairing of the herbicide with the resistant seed led to the consolidation of the seed and chemistry industry in the late 1990s.
Modern chemical herbicides for agriculture are specifically formulated to decompose within a short period after application. This is desirable as it allows crops which may be affected by the herbicide to be grown on the land in future seasons.
2 Uses
Herbicides are widely used in management of landscape turf and in agriculture. They are used in total vegetation control (TVC) programs for maintenance of highways and railroads. Relatively smaller quantities are used in forestry, pasture systems, and management of areas set aside as wildlife habitat.
3 Classification of herbicides
Herbicides can be grouped by chemical family, mode of action, and type of vegetation controlled.
By activity:
- Contact herbicides destroy only the plant tissue in contact with the chemical spray. Generally, these are the fastest acting herbicides. They are ineffective on perennial plants, which are able to regrow from roots or tubers.
- Systemic herbicides are foliar-applied and are translocated through the plant. They destroy a greater amount of plant tissue than contact herbicides.
- Soil-borne herbicides are applied to the soil and are taken up by the roots of the target plant.
- Pre-emergent herbicides are applied to the soil and prevent germination or early growth of weed seeds.
Their classification by mechanism of action (MOA) indicates the first enzyme, protein, or biochemical step affected in the plant following application. The main mechanisms of action are:
- ACCase inhibitors are compounds that kill grasses. ACCase is part of the first step of lipid synthesis. Thus, ACCase inhibitors affect cell membrane production in the meristems of the grass plant. The ACCases of grasses are sensitive to these herbicides, whereas the ACCases of dicot plants are not.
- ALS inhibitors: the ALS enzyme is the first step in the synthesis of the amino acids valine, leucine, and isoleucine. These herbicides also affect the cell membrane at the meristem. They affect grasses and dicots alike.
- EPSPs inhibitors: The enzyme EPSPs is used in the synthesis of the amino acids tryptophan, phenylalanine and tyrosine. The effect of this group of compounds is noted on the entire plant. They affect grasses and dicots alike.
- Synthetic auxin inaugurated the era of organic herbicides. They were discovered in the 1940s after a long study of the plant growth regulator auxinAuxins are a group of plant growth substances (often called phytohormones or plant hormones), the most common example being indoleacetic acid (IAA), responsible for raising the pH around cells, making the cell wall less rigid and allowing elongation.. They have several points of action on the cell membrane, and are effective in the control of dicot plants.
- Photosystem II inhibitors reduce electron flow from water to NADPH2+ at the photochemical step in photosynthesis. They bind to the Qb site on the D2 protein, and prevent quinone from binding to this site. Therefore, this group of compounds cause electrons to accumulate on chlorophyllChlorophyll is the green photosynthetic pigment present in chloroplasts, which provides the energy necessary for photosynthesis. The intense green color of chlorophyll is due to its strong absorbence in the red and blue regions of the electromagnetic spec molecules. As a consequence, oxidation reactions in excess of those normally tolerated by the cell occur, and the plant dies.
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