A chemical compound which kills weeds. Nonselective herbicides may be used to kill all vegetation before cultivation and planting begin. Once the crop has emerged, selective herbicides are used. These target the troublesome weeds and leave other weeds and the growing crop unharmed. Some have a residual action in the soil - allowing, for example, weed seedlings to be exposed to the chemicals as they emerge, which then kills them. There are no ‘organic’ herbicides. Organic systems rely on planned rotations, competitive crops, mulching, and mechanical means of weed control.
A herbicide is a pesticide used to kill unwanted plants. Selective herbicides kill specific targets while leaving the desired crop relatively unharmed. Herbicides used to clear waste ground are nonselective and kill all plant material with which they come into contact. Some plants produce natural herbicides, such as the genus Juglans (walnuts). Herbicides are widely used in agriculture and in landscape turf management.
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. Mechanical control (including tillage) was also (and still is) used to control weeds.
The first widely used herbicide was 2,4-dichlorophenoxyacetic acid, often abbreviated 2,4-D. It is easy and inexpensive to manufacture, and kills many broadleaf plants while leaving grasses largely unaffected (although high doses of 2,4-D at crucial growth periods can harm grass crops such as maize or cereals). 2,4-D's low cost has led to continued usage today and it remains one of the most commonly used herbicides in the world. Like other acid herbicides, current formulations utilize either an amine salt (usually trimethyl amine) or one of many esters (ester) of the base compound.
2,4-D exhibits relatively poor selectivity, meaning that it causes stress to non-target 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 desired selectivities.
The 1970s saw the introduction of atrazine, which has the dubious distinction of being the herbicide of greatest concern for groundwater contamination. Atrazine is said to have high carryover, a very undesirable property for herbicides. 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 contributed to the consolidation of the seed and chemistry industry in the late 1990s.
Many 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. However, herbicides with low residual activity (ie decompose quickly) often do not provide season-long weed control.
Health effects
Herbicides have been alleged to cause a variety of health effects ranging from skin rashes to death. Herbicides can also be transported via surface runoff to contaminate distant surface waters and hence another pathway of ingestion through extraction of those surface waters for drinking. Some herbicides decompose rapidly in soils and other types have more persistent characteristics with longer environmental half-lives. Most herbicides (primarily the non-organic) must be extensively tested prior to labeling by the Environmental Protection Agency. However, because of the large number of herbicides in use, many are concerned about potential health effects.
Classification of herbicides
Herbicides can be grouped by activity, use, chemical family, mode of action, or type of vegetation controlled.
By activity:
Contact herbicides destroy only the plant tissue in contact with the chemical. Generally, these are the fastest acting herbicides. Systemic herbicides are translocated through the plant, either from foliar application down to the roots, or from soil application up to the leaves. They can destroy a greater amount of plant tissue than contact herbicides.By use:
Soil-applied herbicides are applied to the soil and are taken up by the roots of the target plant. Post-emergent herbicides are applied after the crop has emerged.Their classification by mechanism of action (MOA) indicates the first enzyme, protein, or biochemical step affected in the plant following application. 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. These herbicides slowly starve affected plants of these amino acids which eventually leads to inhibition of DNA synthesis. EPSPS inhibitors: The enolpyruvylshikimate 3-phosphate synthase enzyme EPSPS is used in the synthesis of the amino acids tryptophan, phenylalanine and tyrosine. Synthetic auxin inaugurated the era of organic herbicides. They have several points of action on the cell membrane, and are effective in the control of dicot plants. 2,4-D is a synthetic auxin herbicide. The triazine herbicides (including atrazine) are PSII inhibitors.
Organic Herbicides
By organic is meant a herbicide that can be used in a farming enterprise that has been classified as organic. Organic herbicides are expensive and may not be affordable for commercial production. They are much less effective than synthetic herbicides but of course do not inject unnatural chemicals into the environment.
Organic herbicides include:
Spice spices are now effectively used in patented herbicides.Application
Most herbicides are applied as water-based sprays using ground equipment.
Inorganic herbicides can generally be applied aerially using helicopters or airplanes, and can be applied through irrigation systems (chemigation). Crop Safety, for selective herbicides, is the relative absence of damage or stress to the crop. Most selective herbicides cause some visible stress to crop plants.
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