Herbicide

# Overview
A herbicide is used to kill unwanted plants. Selective herbicides kill specific 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 all plant material with which they come into contact. Some plants produce natural herbicides, such as the genus Juglans (walnuts).  They are applied in total vegetation control (TVC) programs for maintenance of highways and railroads.  Smaller quantities are used in forestry, pasture systems, and management of areas set aside as wildlife habitat.
Herbicides are widely used in agriculture and in landscape turf management.  In the us, they account for about 70% of all agricultural pesticide use.

# 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 was developed by a British team during World War II and 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 largely unaffected (although high doses of 2,4-D at crucial growth periods can harm grass crops such as maize or cereals).  The low cost of 2,4-D 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 trimethylamine) or one of many esters of the parent 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 many vinous plants, and sedges.  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 does not break down readily (within a few weeks) after being applied.  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 1974 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 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
Certain herbicides cause a variety of health effects ranging from skin rashes to death.  The pathway of attack can arise from improper application resulting in direct contact with field workers, inhalation of aerial sprays, food consumption and from contact with residual soil contamination.  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.  Other alleged health effects can include chest pain, headaches, nausea  and fatigue.  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, there is significant concern regarding health effects.  Some of the herbicides in use are known to be mutagenic, carcinogenic or teratogenic.
However, some herbicides may also have a therapeutic use. Current research aims to use herbicides as an  anti-malaria drug that targets the plant-like apicoplast plastid in the malaria causing parasite Plasmodium falciparum.

# 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.  They are less effective on perennial plants, which are able to regrow from roots or tubers.
- 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.
- Preemergent herbicides are applied to the soil before the crop emerges and prevent germination or early growth of weed seeds.
- 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. The main mechanisms of action are:
- ACCase inhibitors are compounds that kill grasses. Acetyl coenzyme A carboxylase (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 acetolactate synthase (ALS) enzyme (also known as acetohydroxyacid synthase, or AHAS) is the first step in the synthesis of the branched-chain amino acids (valine, leucine, and isoleucine). These herbicides slowly starve affected plants of these amino acids which eventually leads to inhibition of DNA synthesis. They affect grasses and dicots alike. The ALS inhibitor family includes sulfonylureas (SUs), imidazolinones (IMIs), triazolopyrimidines (TPs), pyrimidinyl oxybenzoates (POBs), and sulfonylamino carbonyl triazolinones (SCTs).
- EPSPS inhibitors: The enolpyruvylshikimate 3-phosphate synthase enzyme EPSPS is used in the synthesis of the amino acids tryptophan, phenylalanine and tyrosine. They affect grasses and dicots alike. Glyphosate (Roundup) is a systemic EPSPS inhibitor but inactivated by soil contact.
- Synthetic auxin inaugurated the era of organic herbicides. They were discovered in the 1940s after a long study of the plant growth regulator auxin. Synthetic auxins mimic this plant hormone. 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.
- 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 chlorophyll molecules. As a consequence, oxidation reactions in excess of those normally tolerated by the cell occur, and the plant dies. The triazine herbicides (including atrazine) are PSII inhibitors.

# Organic Herbicides
An organic herbicide is one 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:
- Spices are now effectively used in patented herbicides.
- Vinegar is effective for 5-20% solutions of acetic acid with higher concentrations most effective but mainly destroys surface growth and so respraying to treat regrowth is needed. Resistant plants generally succumb when weakened by respraying.
- Steam has been applied commercially but is now considered uneconomic and inadequate. It kills surface growth but not underground growth and so respraying to treat regrowth of perennials is needed.
- Flame is considered more effective than steam but suffers from the same difficulties.

# Application
Most herbicides are applied as water-based sprays using ground equipment.  Ground equipment varies in design, but large areas can be sprayed using self-propelled sprayers equipped with a long boom, of  60 to 80 feet (20 to 25 m) with flat fan nozzles spaced about every 20 in (500 mm).  Towed, handheld, and even horse-drawn sprayers are also used.
Inorganic herbicides can generally be applied aerially using helicopters or airplanes, and can be applied through irrigation systems (chemigation).

# Terminology
- Control is the destruction of unwanted weeds, or the damage of them to the point where they are no longer competitive with the crop.
- Suppression is incomplete control still providing some economic benefit, such as reduced competition with the crop.
- 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.

# Major herbicides in use today
- 2,4-D, a broadleaf herbicide in the phenoxy group used in turf and in no-till field crop production.  Now mainly used in a blend with other herbicides that act as synergists, it is the most widely used herbicide in the world, third most commonly used in the United States. It is an example of synthetic auxin(plant hormone).
- atrazine, a triazine herbicide used in corn and sorghum for control of broadleaf weeds and grasses.  Still used because of its low cost and because it works as a synergist when used with other herbicides, it is a photosystem II inhibitor.
- clopyralid is a broadleaf herbicide in the pyridine group, used mainly in turf, rangeland, and for control of noxious thistles.  Notorious for its ability to persist in compost. It is another example of synthetic auxin.
- dicamba, a persistent broadleaf herbicide active in the soil, used on turf and field corn. It is another example of synthetic auxin.
- Glyphosate, a systemic nonselective (it kills any type of plant) herbicide used in no-till burndown and for weed control in crops that are genetically modified to resist its effects. It is an example of an EPSPs inhibitor.
- Imazapyr, is a non-selective herbicide used for the control of a broad range of weeds including terrestrial annual and perennial grasses and broadleaved herbs, woody species, and riparian and emergent aquatic species.
- Imazapic, is a selective herbicide for both the pre- and post-emergent control of some annual and perennial grasses and some broadleaf weeds.  Imazapic kills plants by inhibiting the production of branched chain amino acids (valine, leucine, and isoleucine), which are necessary for protein synthesis and cell growth.
- Linuron, is a non-selective herbicide used in the control of grasses and broadleafed weeds. It works by inhibiting photosynthesis.
- metoalachlor, a pre-emergent herbicide widely used for control of annual grasses in corn and sorghum; it has largely replaced atrazine for these uses.
- Paraquat, a nonselective contact herbicide used for no-till burndown and in aerial destruction of marijuana and coca plantings.  More acutely toxic to people than any other herbicide in widespread commercial use.
- picloram, a pyridine herbicide mainly used to control unwanted trees in pastures and edges of fields. It is another synthetic auxin.
- Triclopyr

# Herbicides of historical interest
- 2,4,5-Trichlorophenoxyacetic acid (2,4,5-T) was a widely used broadleaf herbicide until being phased out starting in the late 1970s.  While 2,4,5-T itself is of only moderate toxicity, the manufacturing process for 2,4,5-T contaminates this chemical with trace amounts of  2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD).  TCDD is extremely toxic to humans.  With proper temperature control during production of 2,4,5-T, TCDD levels can be held to about .005 ppm.  Before the TCDD risk was well understood, early production facilities lacked proper temperature controls.  Individual batches tested later were found to have as much as 60 ppm of TCDD.
- 2,4,5-T was withdrawn from use in the USA in 1983, at a time of heightened public sensitivity about chemical hazards in the environment.  Public concern about dioxins was high, and production and use of other (non-herbicide) chemicals potentially containing TCDD contamination was also withdrawn.  These included pentachlorophenol (a wood preservative) and PCBs (mainly used as stabilizing agents in transformer oil).  Some feel that the 2,4,5-T withdrawal was not based on sound science.  2,4,5-T has since largely been replaced by dicamba and triclopyr.
- Agent Orange was a herbicide blend used by the U.S. military in Vietnam between January 1965 and April 1970 as a defoliant.  It was a mixture of 2,4,5-T, 2,4-D, and picloram.  Because of TCDD contamination in the 2,4,5-T component, it has been blamed for serious illnesses in many veterans who were exposed to it.  However, research on populations exposed to its dioxin contaminant have been inconsistent and inconclusive.  Agent Orange often had much higher levels of TCDD than 2,4,5-T used in the US.  The name Agent Orange is derived from the orange color-coded stripe used by the Army on barrels containing the product. It is worth noting that there were other blends of synthetic auxins at the time of the Vietnam War whose containers were recognized by their colors, such as Agent Purple and Agent Pink.