Herbicide
Overview
 
Herbicides, also commonly known as weedkillers, are pesticide
Pesticide
Pesticides are substances or mixture of substances intended for preventing, destroying, repelling or mitigating any pest.A pesticide may be a chemical unicycle, biological agent , antimicrobial, disinfectant or device used against any pest...

s used to kill unwanted plant
Plant
Plants are living organisms belonging to the kingdom Plantae. Precise definitions of the kingdom vary, but as the term is used here, plants include familiar organisms such as trees, flowers, herbs, bushes, grasses, vines, ferns, mosses, and green algae. The group is also called green plants or...

s. Selective herbicides kill specific targets while leaving the desired crop
Crop
Crop may refer to:* Crop, a plant grown and harvested for agricultural use* Crop , part of the alimentary tract of some animals* Crop , a modified whip used in horseback riding or disciplining humans...

 relatively unharmed. Some of these act by interfering with the growth of the weed
Weed
A weed in a general sense is a plant that is considered by the user of the term to be a nuisance, and normally applied to unwanted plants in human-controlled settings, especially farm fields and gardens, but also lawns, parks, woods, and other areas. More specifically, the term is often used to...

 and are often synthetic "imitations" of plant hormones. Herbicides used to clear waste ground, industrial sites, railways and railway embankments are non-selective and kill all plant
Plant
Plants are living organisms belonging to the kingdom Plantae. Precise definitions of the kingdom vary, but as the term is used here, plants include familiar organisms such as trees, flowers, herbs, bushes, grasses, vines, ferns, mosses, and green algae. The group is also called green plants or...

 material with which they come into contact.
Encyclopedia
Herbicides, also commonly known as weedkillers, are pesticide
Pesticide
Pesticides are substances or mixture of substances intended for preventing, destroying, repelling or mitigating any pest.A pesticide may be a chemical unicycle, biological agent , antimicrobial, disinfectant or device used against any pest...

s used to kill unwanted plant
Plant
Plants are living organisms belonging to the kingdom Plantae. Precise definitions of the kingdom vary, but as the term is used here, plants include familiar organisms such as trees, flowers, herbs, bushes, grasses, vines, ferns, mosses, and green algae. The group is also called green plants or...

s. Selective herbicides kill specific targets while leaving the desired crop
Crop
Crop may refer to:* Crop, a plant grown and harvested for agricultural use* Crop , part of the alimentary tract of some animals* Crop , a modified whip used in horseback riding or disciplining humans...

 relatively unharmed. Some of these act by interfering with the growth of the weed
Weed
A weed in a general sense is a plant that is considered by the user of the term to be a nuisance, and normally applied to unwanted plants in human-controlled settings, especially farm fields and gardens, but also lawns, parks, woods, and other areas. More specifically, the term is often used to...

 and are often synthetic "imitations" of plant hormones. Herbicides used to clear waste ground, industrial sites, railways and railway embankments are non-selective and kill all plant
Plant
Plants are living organisms belonging to the kingdom Plantae. Precise definitions of the kingdom vary, but as the term is used here, plants include familiar organisms such as trees, flowers, herbs, bushes, grasses, vines, ferns, mosses, and green algae. The group is also called green plants or...

 material with which they come into contact. Smaller quantities are used in forestry, pasture systems, and management of areas set aside as wildlife habitat
Habitat
* Habitat , a place where a species lives and grows*Human habitat, a place where humans live, work or play** Space habitat, a space station intended as a permanent settlement...

.

Some plants produce natural herbicides, such as the genus Juglans (walnut
Walnut
Juglans is a plant genus of the family Juglandaceae, the seeds of which are known as walnuts. They are deciduous trees, 10–40 meters tall , with pinnate leaves 200–900 millimetres long , with 5–25 leaflets; the shoots have chambered pith, a character shared with the wingnuts , but not the hickories...

s), or the tree of heaven
Tree of heaven
Ailanthus altissima , commonly known as tree of heaven, ailanthus, or in Standard Chinese as chouchun , is a deciduous tree in the Simaroubaceae family. It is native to both northeast and central China and Taiwan. Unlike other members of the genus Ailanthus, it is found in temperate climates rather...

; such action of natural herbicides, and other related chemical interactions, is called allelopathy
Allelopathy
Allelopathy is a biological phenomenon by which an organism produces one or more biochemicals that influence the growth, survival, and reproduction of other organisms. These biochemicals are known as allelochemicals and can have beneficial or detrimental effects on the target organisms...

.

Herbicides are widely used in agriculture
Agriculture
Agriculture is the cultivation of animals, plants, fungi and other life forms for food, fiber, and other products used to sustain life. Agriculture was the key implement in the rise of sedentary human civilization, whereby farming of domesticated species created food surpluses that nurtured the...

 and in landscape turf management. In the U.S., they account for about 70% of all agricultural pesticide use.

History

Prior to the widespread use of chemical herbicides, cultural controls
Cultural controls
In agriculture cultural control is the practice of modifying the growing environment to reduce the prevalence of unwanted pests. Examples include changing soil pH or fertility levels, irrigation practices, amount of sunlight, or temperature...

, 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
2,4-Dichlorophenoxyacetic acid
2,4-Dichlorophenoxyacetic acid is a common systemic pesticide/herbicide used in the control of broadleaf weeds. It is the most widely used herbicide in the world, and the third most commonly used in North America...

, often abbreviated 2,4-D. It was first commercialized by the paint company Sherwin-Williams and saw 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 (often trimethylamine
Trimethylamine
Trimethylamine is an organic compound with the formula N3. This colorless, hygroscopic, and flammable tertiary amine has a strong "fishy" odor in low concentrations and an ammonia-like odor at higher concentrations...

) or one of many ester
Ester
Esters are chemical compounds derived by reacting an oxoacid with a hydroxyl compound such as an alcohol or phenol. Esters are usually derived from an inorganic acid or organic acid in which at least one -OH group is replaced by an -O-alkyl group, and most commonly from carboxylic acids and...

s of the parent compound. These are easier to handle than the acid.

2,4-D exhibits relatively good selectivity, meaning, in this case, that it controls a wide number of broadleaf weeds while causing little to no injury to grass crops at normal use rates. An herbicide is termed selective if it affects only certain types of plants, and nonselective if it inhibits a very broad range of plant types. Other herbicides have been more recently developed that achieve higher levels of selectivity than 2,4-D.

The 1950s saw the introduction of the triazine family of herbicides, which includes atrazine
Atrazine
Atrazine, 2-chloro-4--6--s-triazine, an organic compound consisting of an s-triazine-ring is a widely used herbicide. Its use is controversial due to widespread contamination in drinking water and its associations with birth defects and menstrual problems when consumed by humans at concentrations...

, which have current distinction of being the herbicide family of greatest concern regarding groundwater contamination. Atrazine does not break down readily (within a few weeks) after being applied to soils of above neutral pH
PH
In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...

. Under alkaline soil conditions atrazine may be carried into the soil profile as far as the water table by soil water following rainfall causing the aforementioned contamination. Atrazine is thus said to have carryover, a generally 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 (i.e., that decompose quickly) often do not provide season-long weed control.

Health and environmental effects

Herbicides have widely variable toxicity
Toxicity
Toxicity is the degree to which a substance can damage a living or non-living organisms. Toxicity can refer to the effect on a whole organism, such as an animal, bacterium, or plant, as well as the effect on a substructure of the organism, such as a cell or an organ , such as the liver...

. In addition to acute toxicity
Acute toxicity
Acute toxicity describes the adverse effects of a substance that result either from a single exposure or from multiple exposures in a short space of time...

 from high exposures there is concern of possible carcinogenicity as well as other long-term problems such as contributing to Parkinson's disease
Parkinson's disease
Parkinson's disease is a degenerative disorder of the central nervous system...

.

Some herbicides cause a range of health effects ranging from skin rashes to death. The pathway of attack can arise from intentional or unintentional direct consumption, improper application resulting in the herbicide coming into direct contact with people or wildlife, inhalation of aerial sprays, or food consumption prior to the labeled pre-harvest interval. Under extreme conditions, herbicides can also be transported via surface runoff
Surface runoff
Surface runoff is the water flow that occurs when soil is infiltrated to full capacity and excess water from rain, meltwater, or other sources flows over the land. This is a major component of the water cycle. Runoff that occurs on surfaces before reaching a channel is also called a nonpoint source...

 to contaminate distant water sources. Most herbicides decompose rapidly in soils via soil microbial decomposition, hydrolysis, or photolysis.

Phenoxy herbicides are often contaminated with dioxins
Dioxins and dioxin-like compounds
Dioxins and dioxin-like compounds are by-products of various industrial processes, and are commonly regarded as highly toxic compounds that are environmental pollutants and persistent organic pollutants . They include:...

 such as TCDD
2,3,7,8-Tetrachlorodibenzodioxin
2,3,7,8-Tetrachlorodibenzo-p-dioxin is a polychlorinated dibenzo-p-dioxin . It is the most potent compound of the series and became known as a contaminant in Agent Orange, a herbicide used in the Vietnam War, as well as the Seveso disaster...

; research suggested that such contamination results in a small rise in cancer risk after exposure to these herbicides. Triazine
Triazine
A triazine is one of three organic chemicals, isomeric with each other, whose molecular formula is 333 and whose empirical formula is CHN.- Structure :...

 exposure has been implicated in a likely relationship to increased risk of breast cancer
Breast cancer
Breast cancer is cancer originating from breast tissue, most commonly from the inner lining of milk ducts or the lobules that supply the ducts with milk. Cancers originating from ducts are known as ductal carcinomas; those originating from lobules are known as lobular carcinomas...

, although a causal relationship remains unclear.

Herbicide manufacturers have at times made false or misleading claims about the safety of their products. Chemical manufacturer Monsanto Company agreed to change its advertising after pressure from New York
New York
New York is a state in the Northeastern region of the United States. It is the nation's third most populous state. New York is bordered by New Jersey and Pennsylvania to the south, and by Connecticut, Massachusetts and Vermont to the east...

 attorney general
Attorney General
In most common law jurisdictions, the attorney general, or attorney-general, is the main legal advisor to the government, and in some jurisdictions he or she may also have executive responsibility for law enforcement or responsibility for public prosecutions.The term is used to refer to any person...

 Dennis Vacco
Dennis Vacco
Dennis C. Vacco is an American lawyer and politician. He graduated a B.A. from Colgate University in 1974, a J.D...

; Vacco complained about misleading claims that its spray-on glyphosate based herbicides, including Roundup, were safer than table salt and "practically non-toxic" to mammals, birds, and fish. Roundup is toxic and has resulted in death
Death
Death is the permanent termination of the biological functions that sustain a living organism. Phenomena which commonly bring about death include old age, predation, malnutrition, disease, and accidents or trauma resulting in terminal injury....

 after being ingested in quantities ranging from 85 to 200 mL, although it has also been ingested in quantities as large as 500 mL with only mild or moderate symptoms. The manfucturer of Tordon 101 (Dow AgroSciences
Dow AgroSciences
Dow AgroSciences LLC is a wholly owned subsidiary of the Dow Chemical Company specializing in not only agricultural chemicals such as pesticides, but also seeds and biotechnology solutions. The company is based in Indianapolis, Indiana, in the United States...

, owned by the Dow Chemical Company
Dow Chemical Company
The Dow Chemical Company is a multinational corporation headquartered in Midland, Michigan, United States. As of 2007, it is the second largest chemical manufacturer in the world by revenue and as of February 2009, the third-largest chemical company in the world by market capitalization .Dow...

) has claimed that Tordon 101 has no effects on animals and insects, in spite of evidence of strong carcinogenic activity of the active ingredient Picloram
Picloram
Picloram is a systemic herbicide used for general woody plant control, sold under the trade names Tordon and Grazon. It also controls a wide range of broad-leaved weeds, but most grasses are resistant...

 in studies on rats.

The risk of Parkinson's disease has been shown to increase with occupational exposure to herbicides and pesticides. The herbicide paraquat
Paraquat
Paraquat is the trade name for N,N′-dimethyl-4,4′-bipyridinium dichloride, one of the most widely used herbicides in the world. Paraquat, a viologen, is quick-acting and non-selective, killing green plant tissue on contact. It is also toxic to human beings and animals...

 is suspected to be one environmental factor causing Parkinson's disease.

All commercially-sold, organic and non-organic herbicides must be extensively tested prior to approval for sale and labeling by the Environmental Protection Agency. However, because of the large number of herbicides in use, there is significant concern regarding health effects.
In addition to health effects caused by herbicides themselves, commercial herbicide mixtures often contain other chemicals, including inactive ingredients, which have negative impacts on human health. For example, Roundup contains adjuvants which, even in low concentrations, were found to kill human embryonic, placental, and umbilical cells in vitro
In vitro
In vitro refers to studies in experimental biology that are conducted using components of an organism that have been isolated from their usual biological context in order to permit a more detailed or more convenient analysis than can be done with whole organisms. Colloquially, these experiments...

. One study also found that Roundup caused genetic damage, but found that the damage was not caused by the active ingredient.

Ecological effects

Commercial herbicide use generally has negative impacts on bird
Bird
Birds are feathered, winged, bipedal, endothermic , egg-laying, vertebrate animals. Around 10,000 living species and 188 families makes them the most speciose class of tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Extant birds range in size from...

 populations, although the impacts are highly variable and often require field studies to predict accurately. Laboratory studies have at times overestimated negative impacts on birds due to toxicity, predicting serious problems that were not observed in the field. Most observed effects are due not to toxicity but to habitat changes and the decrease in abundance of species birds rely on for food or shelter. Herbicide use in silviculture
Silviculture
Silviculture is the practice of controlling the establishment, growth, composition, health, and quality of forests to meet diverse needs and values. The name comes from the Latin silvi- + culture...

, used to favor certain types of growth following clearcutting
Clearcutting
Clearcutting, or clearfelling, is a controversial forestry/logging practice in which most or all trees in an area are uniformly cut down. Clearcutting, along with shelterwood and seed tree harvests, is used by foresters to create certain types of forest ecosystems and to promote select species that...

, can cause significant drops in bird populations. Even when herbicides are used which have low toxicity to birds, the herbicides decrease the abundance of many types of vegetation which the birds rely on. Herbicide use in agriculture in Britain
United Kingdom
The United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...

 has been linked to a decline in seed-eating bird species which rely on the weeds killed by the herbicides. Heavy use of herbicides in neotropical agricultural areas has been one of many factors implicated in limiting the usefulness of such agricultural land for wintering migratory birds.

Scientific uncertainty

The health and environmental effects of many herbicides is unknown, and even within the scientific community there is often disagreement on the risk. For example, a 1995 panel of 13 scientists reviewing studies on the carcinogenicity of 2,4-D had divided opinions on the likelihood that 2,4-D causes cancer in humans. , there were too few studies on phenoxy herbicides to accurately assess the risk of many types of cancer from these herbicides, even though evidence was stronger that exposure to these herbicides is associated with increased risk of soft tissue sarcoma
Soft tissue sarcoma
A soft-tissue sarcoma is a form of sarcoma that develops in connective tissue, though the term is sometimes applied to elements of the soft tissue that are not currently considered connective tissue.-Risk factors:...

 and non-Hodgkin lymphoma
Non-Hodgkin lymphoma
The non-Hodgkin lymphomas are a diverse group of blood cancers that include any kind of lymphoma except Hodgkin's lymphomas. Types of NHL vary significantly in their severity, from indolent to very aggressive....

.

Resistance

Scientists generally agree that selection pressure applied to weed populations for a long enough period of time eventually leads to resistance. Plants have developed resistance to Atrazine
Atrazine
Atrazine, 2-chloro-4--6--s-triazine, an organic compound consisting of an s-triazine-ring is a widely used herbicide. Its use is controversial due to widespread contamination in drinking water and its associations with birth defects and menstrual problems when consumed by humans at concentrations...

 and to ALS-inhibitors, and more recently, to glyphosate herbicides. Marestail
Conyza canadensis
Conyza canadensis is an annual plant native throughout most of North America and Central America. Common names include Horseweed, Canadian Horseweed, Canadian Fleabane, Coltstail, Marestail and Butterweed.It is an annual plant growing to 1.5 m tall, with sparsely hairy stems...

 is one weed that has developed glyphosate resistance.

Classification

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 rhizomes, 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 are capable of controlling perennial plants and may be slower acting but ultimately more effective than contact herbicides.


By use:
  • Soil-applied herbicides are applied to the soil and are taken up by the roots and/or hypocotyl of the target plant. There are three main types of soil-applied herbicides:
  1. Pre-plant incorporated herbicides are soil applied prior to planting and mechanically incorporated into the soil. The objective for incorporation is to prevent dissipation through photodecomposition
    Photodissociation
    Photodissociation, photolysis, or photodecomposition is a chemical reaction in which a chemical compound is broken down by photons. It is defined as the interaction of one or more photons with one target molecule....

     and/or volatility.
  2. Preemergent herbicides
    Preemergent herbicides
    Preemergent herbicides prevent the germination of seeds by inhibiting a key enzyme. In some areas of the world, preemergent herbicides are used to prevent crabgrass from appearing in summer lawns.-Brands:-Crabgrass:...

    are applied to the soil before the crop emerges and prevent germination or early growth of weed seeds.
  3. Post-emergent herbicides are applied after the crop has emerged.


Their classification by mechanism of action
Mechanism of action
In pharmacology, the term mechanism of action refers to the specific biochemical interaction through which a drug substance produces its pharmacological effect...

 (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
    Acetolactate synthase
    The acetolactate synthase enzyme is a protein found in plants and micro-organisms. ALS catalyzes the first step in the synthesis of the branched-chain amino acids ....

     (ALS) enzyme (also known as acetohydroxyacid synthase, or AHAS) is the first step in the synthesis of the branched-chain amino acids (valine
    Valine
    Valine is an α-amino acid with the chemical formula HO2CCHCH2. L-Valine is one of 20 proteinogenic amino acids. Its codons are GUU, GUC, GUA, and GUG. This essential amino acid is classified as nonpolar...

    , leucine
    Leucine
    Leucine is a branched-chain α-amino acid with the chemical formula HO2CCHCH2CH2. Leucine is classified as a hydrophobic amino acid due to its aliphatic isobutyl side chain. It is encoded by six codons and is a major component of the subunits in ferritin, astacin and other 'buffer' proteins...

    , and isoleucine
    Isoleucine
    Isoleucine is an α-amino acid with the chemical formula HO2CCHCHCH2CH3. It is an essential amino acid, which means that humans cannot synthesize it, so it must be ingested. Its codons are AUU, AUC and AUA....

    ). These herbicides slowly starve affected plants of these amino acid
    Amino acid
    Amino acids are molecules containing an amine group, a carboxylic acid group and a side-chain that varies between different amino acids. The key elements of an amino acid are carbon, hydrogen, oxygen, and nitrogen...

    s 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). ALS is a biological pathway that exists only in plants and not in animals thus making the ALS-inhibitors among the safest herbicides.

  • EPSPS inhibitors: The enolpyruvylshikimate 3-phosphate synthase enzyme EPSPS is used in the synthesis of the amino acids tryptophan
    Tryptophan
    Tryptophan is one of the 20 standard amino acids, as well as an essential amino acid in the human diet. It is encoded in the standard genetic code as the codon UGG...

    , phenylalanine
    Phenylalanine
    Phenylalanine is an α-amino acid with the formula C6H5CH2CHCOOH. This essential amino acid is classified as nonpolar because of the hydrophobic nature of the benzyl side chain. L-Phenylalanine is an electrically neutral amino acid, one of the twenty common amino acids used to biochemically form...

     and tyrosine
    Tyrosine
    Tyrosine or 4-hydroxyphenylalanine, is one of the 22 amino acids that are used by cells to synthesize proteins. Its codons are UAC and UAU. It is a non-essential amino acid with a polar side group...

    . 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
    Auxin
    Auxins are a class of plant hormones with some morphogen-like characteristics. Auxins have a cardinal role in coordination of many growth and behavioral processes in the plant's life cycle and are essential for plant body development. Auxins and their role in plant growth were first described by...

    . 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 D1 protein, and prevent quinone from binding to this site. Therefore, this group of compounds cause electrons to accumulate on chlorophyll
    Chlorophyll
    Chlorophyll is a green pigment found in almost all plants, algae, and cyanobacteria. Its name is derived from the Greek words χλωρος, chloros and φύλλον, phyllon . Chlorophyll is an extremely important biomolecule, critical in photosynthesis, which allows plants to obtain energy from light...

     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) and urea derivatives (diuron) are photosystem II inhibitors.

  • Photosystem I inhibitors steal electrons from the normal pathway through FeS - Fdx - NADP leading to direct discharge of electrons on Oxygen. As result ROS (reactive oxygen species) are produced and oxidation reactions in excess of those normally tolerated by the cell occur leading to plant death.

Bipirydiums herbicides (like Diquat and Paraquat) hit "Fe-S - Fdx step".
Diphenilethers herbicides (like Nitrofen , Nitrofluorfen, Acifluoren) hit "Fdx - NADP step".

Organic herbicides

Recently the term organic has come to imply products used in organic farming
Organic farming
Organic farming is the form of agriculture that relies on techniques such as crop rotation, green manure, compost and biological pest control to maintain soil productivity and control pests on a farm...

. Under this definition an organic herbicide is one that can be used in a farming enterprise that has been classified as organic. Commercially-sold organic herbicides are expensive and may not be affordable for commercial farming. Depending on the application, they may be less effective than synthetic herbicides and are generally used along with cultural and mechanical weed control practices.

Homemade organic herbicides include:
  • Corn gluten meal (CGM) is a natural preemergence weed control used in turfgrass, which reduces germination of many broadleaf and grass weeds.

  • Spice
    Spice
    A spice is a dried seed, fruit, root, bark, or vegetative substance used in nutritionally insignificant quantities as a food additive for flavor, color, or as a preservative that kills harmful bacteria or prevents their growth. It may be used to flavour a dish or to hide other flavours...

    s are now effectively used in herbicides.

  • Vinegar
    Vinegar
    Vinegar is a liquid substance consisting mainly of acetic acid and water, the acetic acid being produced through the fermentation of ethanol by acetic acid bacteria. Commercial vinegar is produced either by fast or slow fermentation processes. Slow methods generally are used with traditional...

     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
    Steam
    Steam is the technical term for water vapor, the gaseous phase of water, which is formed when water boils. In common language it is often used to refer to the visible mist of water droplets formed as this water vapor condenses in the presence of cooler air...

     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
    Flame
    A flame is the visible , gaseous part of a fire. It is caused by a highly exothermic reaction taking place in a thin zone...

     is considered more effective than steam but suffers from the same difficulties.

  • D-limonene
    Limonene
    Limonene is a colourless liquid hydrocarbon classified as a cyclic terpene. The more common D isomer possesses a strong smell of oranges. It is used in chemical synthesis as a precursor to carvone and as a renewably-based solvent in cleaning products....

     (citrus
    Citrus
    Citrus is a common term and genus of flowering plants in the rue family, Rutaceae. Citrus is believed to have originated in the part of Southeast Asia bordered by Northeastern India, Myanmar and the Yunnan province of China...

     oil). D-limonene (citrus oil) is a natural degreasing agent that strips the waxy skin or cuticle from weeds, causing dehydration and ultimately death .

  • Saltwater or salt applied in appropriate strengths to the rootzone will kill most plants .

  • Monocerin
    Monocerin
    Monocerin is a polyketide metabolite that originates from various fungal species. It has been shown to display antifungal, plant pathogenic, and insecticidal characteristics. Monocerin has been isolated from Dreschlera monoceras,D. ravenelii, Exserohilum turcicum, and Fusarium...

     produced by certain fungi will kill certain weeds such as Johnson Grass
    Johnson grass
    Sorghum halepense, commonly called Johnsongrass, is a plant in the grass family, Poaceae, native to the Mediterranean region, but growing throughout Europe and the Middle East. The plant has been introduced to all continents except Antarctica and most larger islands and archipelagos...

     .

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
Sprayer
A sprayer is a device used to spray a liquid.In agriculture, a sprayer is a piece of equipment that spray nozzles to apply herbicides, pesticides, and fertilizers to agricultural crops...

 equipped with a long boom, of 60 to 80 ft (18.3 to 24.4 m) with flat fan nozzles spaced about every 20 inches (508 mm). Towed, handheld, and even horse-drawn sprayers are also used.

Synthetic organic herbicides can generally be applied aerially using helicopters or airplanes, and can be applied through irrigation systems (chemigation).

A new method of herbicide application involves ridding the soil of its active weed seed bank rather than just killing the weed. Researchers at the Agricultural Research Service
Agricultural Research Service
The Agricultural Research Service is the principal in-house research agency of the United States Department of Agriculture . ARS is one of four agencies in USDA's Research, Education and Economics mission area...

 have found that applying herbicides to fields late in the weed’s growing season greatly reduces its seed production, and therefore fewer weeds will return the following season. If herbicides are applied at the correct stage in the weed’s growing season, then the weed’s presence in the soil seed bank will greatly be reduced. Because most weeds are annual grasses, their seeds will only survive in soil for a year or two, so this method will be able to “weed out” the weed with only a few years of herbicide application.

Weed-wiping may also be used, where a wick wetted with herbicide is suspended from a boom and dragged or rolled across the tops of the taller plants. This allows treatment of taller grassland weeds by direct contact without affecting related but desirable plants in the grassland sward beneath.

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.

In current use

  • 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 allow lower rates of herbicides to be used, 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
    Auxin
    Auxins are a class of plant hormones with some morphogen-like characteristics. Auxins have a cardinal role in coordination of many growth and behavioral processes in the plant's life cycle and are essential for plant body development. Auxins and their role in plant growth were first described by...

     (plant hormone) .
  • aminopyralid
    Aminopyralid
    Aminopyralid is a selective hormone-based herbicide manufactured by Dow AgroSciences for control of broadleaf weeds on grassland, such as docks, thistles, and nettles...

     is a broadleaf herbicide in the pyridine group, used to control broadleaf weeds on grassland, such as docks, thistles and nettles. Notorious for its ability to persist in compost .
  • atrazine
    Atrazine
    Atrazine, 2-chloro-4--6--s-triazine, an organic compound consisting of an s-triazine-ring is a widely used herbicide. Its use is controversial due to widespread contamination in drinking water and its associations with birth defects and menstrual problems when consumed by humans at concentrations...

    , 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 extrodinarily well on a broad spectrum of weeds common in the U.S. corn belt, Atrazine is commonly used with other herbicides to reduce the over-all rate of atrazine and to lower the potential for groundwater contamination, it is a photosystem II inhibitor .
  • clopyralid
    Clopyralid
    Clopyralid is a selective herbicide used for control of broadleaf weeds, especially thistles and clovers....

     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
    Dicamba
    Dicamba is an herbicide. Brand names for formulations of this herbicide include Banvel, Oracle and Vanquish...

    , a post-emergent broadleaf herbicide with some soil activity, used on turf and field corn. It is another example of a synthetic auxin.
  • Glufosinate ammonium, a broad-spectrum contact herbicide and is used to control weeds after the crop emerges or for total vegetation control on land not used for cultivation.
  • Fluroxypyr, a systemic, selective herbicide used for the control of broad-leaved weeds in small grain cereals, maize, pastures, range land and turf. It is a synthetic auxin. In cereal growing, fluroxypyr's key importance is control of cleavers, Galium aparine. Other key broad-leaved weeds are also controlled.
  • Glyphosate, a systemic non-selective (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
    Imazapyr
    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....

     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
    Imazapic
    Imazapic is a chemical used as an herbicide. It attacks weeds, and perennial grass. It has a half-live of around 120 days in soil....

    , 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
    Valine
    Valine is an α-amino acid with the chemical formula HO2CCHCH2. L-Valine is one of 20 proteinogenic amino acids. Its codons are GUU, GUC, GUA, and GUG. This essential amino acid is classified as nonpolar...

    , leucine
    Leucine
    Leucine is a branched-chain α-amino acid with the chemical formula HO2CCHCH2CH2. Leucine is classified as a hydrophobic amino acid due to its aliphatic isobutyl side chain. It is encoded by six codons and is a major component of the subunits in ferritin, astacin and other 'buffer' proteins...

    , and isoleucine
    Isoleucine
    Isoleucine is an α-amino acid with the chemical formula HO2CCHCHCH2CH3. It is an essential amino acid, which means that humans cannot synthesize it, so it must be ingested. Its codons are AUU, AUC and AUA....

    ), which are necessary for protein synthesis and cell growth.
  • Linuron is a non-selective herbicide used in the control of grasses and broadleaf weeds. It works by inhibiting photosynthesis.
  • Metolachlor
    Metolachlor
    Metolachlor is an organic compound that is widely used as an herbicide. It is a derivative of aniline and is a member of the chloroacetanilide herbicides. It is highly effective toward grasses but its application is also controversial.-Agricultural use:...

     is a pre-emergent herbicide widely used for control of annual grasses in corn and sorghum; it has displaced some of the atrazine
    Atrazine
    Atrazine, 2-chloro-4--6--s-triazine, an organic compound consisting of an s-triazine-ring is a widely used herbicide. Its use is controversial due to widespread contamination in drinking water and its associations with birth defects and menstrual problems when consumed by humans at concentrations...

     in these uses.
  • Paraquat
    Paraquat
    Paraquat is the trade name for N,N′-dimethyl-4,4′-bipyridinium dichloride, one of the most widely used herbicides in the world. Paraquat, a viologen, is quick-acting and non-selective, killing green plant tissue on contact. It is also toxic to human beings and animals...

    , a non-selective 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.
  • Pendimethalin
    Pendimethalin
    Pendimethalin is an herbicide used in premergence and postemergence applications to control annual grasses and certain broadleaf weeds. It inhibits cell division and cell elongation...

    , a pre-emergent herbicide widely used to control annual grasses and some broadleaf weeds in a very wide range of crops, including corn, soybeans, wheat, cotton, many tree and vine crops, and many turfgrass species.
  • Picloram
    Picloram
    Picloram is a systemic herbicide used for general woody plant control, sold under the trade names Tordon and Grazon. It also controls a wide range of broad-leaved weeds, but most grasses are resistant...

    , a pyridine herbicide mainly used to control unwanted trees in pastures and edges of fields. It is another synthetic auxin.
  • Sodium chlorate
    Sodium chlorate
    Sodium chlorate is a chemical compound with the chemical formula . When pure, it is a white crystalline powder that is readily soluble in water. It is hygroscopic. It decomposes above 250 °C to release oxygen and leave sodium chloride...

    , a non-selective herbicide, considered phytotoxic to all green plant parts. It can also kill through root absorption.
  • Triclopyr
    Triclopyr
    Triclopyr is a systemic, foliar herbicide in the pyridine group. It is used to control broadleaf weeds while leaving grasses and conifers unaffected....

    , a systemic, foliar herbicide in the pyridine group. It is used to control broadleaf weeds while leaving grasses and conifers unaffected.

Historical interest: 2,4,5-T

  • 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
    Pentachlorophenol
    Pentachlorophenol is an organochlorine compound used as a pesticide and a disinfectant. First produced in the 1930s, it is marketed under many trade names...

     (a wood preservative) and PCBs
    Polychlorinated biphenyl
    Polychlorinated biphenyls are a class of organic compounds with 2 to 10 chlorine atoms attached to biphenyl, which is a molecule composed of two benzene rings. The chemical formula for PCBs is C12H10-xClx...

     (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
    Dicamba
    Dicamba is an herbicide. Brand names for formulations of this herbicide include Banvel, Oracle and Vanquish...

     and triclopyr
    Triclopyr
    Triclopyr is a systemic, foliar herbicide in the pyridine group. It is used to control broadleaf weeds while leaving grasses and conifers unaffected....

    .

  • Agent Orange
    Agent Orange
    Agent Orange is the code name for one of the herbicides and defoliants used by the U.S. military as part of its herbicidal warfare program, Operation Ranch Hand, during the Vietnam War from 1961 to 1971. Vietnam estimates 400,000 people were killed or maimed, and 500,000 children born with birth...

     was an herbicide blend used by the U.S. military in Vietnam between January 1965 and April 1970 as a defoliant. It was a 50/50 mixture of the n-butyl esters of 2,4,5-T and 2,4-D. Because of TCDD contamination in the 2,4,5-T component, it has been blamed for serious illnesses in many veterans and Vietnamese people 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
    Vietnam War
    The Vietnam War was a Cold War-era military conflict that occurred in Vietnam, Laos, and Cambodia from 1 November 1955 to the fall of Saigon on 30 April 1975. This war followed the First Indochina War and was fought between North Vietnam, supported by its communist allies, and the government of...

     whose containers were recognized by their colors, such as Agent Purple
    Agent Purple
    Agent Purple is the code name for a powerful herbicide and defoliant used by the U.S. military and the Canadian Government in their Herbicidal Warfare program during the Vietnam War. The name comes from the purple stripe painted on the barrels to identify the contents...

     and Agent Pink
    Agent Pink
    Agent Pink is the code name for a powerful herbicide and defoliant used by the U.S. military in its Herbicidal Warfare program during the Vietnam War. The name comes from the pink stripe painted on the barrels to identify the contents. It was one of the so-called "rainbow herbicides" that included...

     .

See also

  • Index of pesticide articles
    Index of pesticide articles
    This is an index of articles relating to pesticides.-A:Abamectin- Acephate- Acetochlor- Acibenzolar- Acibenzolar-S-methyl- Acrylonitrile- Action level- Agent Blue- Agent Green- Agent Orange- Agent Pink- Agent Purple- Agent White- Agrochemical...



  • Bioherbicide
    Bioherbicide
    A bioherbicide is a biologically based control agent for weeds. Among the three major types of pesticides herbicides are used to control weeds, or undesirable plants. A bioherbicide is a biologically based control agent for weeds. Among the three major types of pesticides (agricultural...

  • List of environmental health hazards
  • Rainbow Herbicides
    Rainbow Herbicides
    The Rainbow Herbicides are a group of chemicals used by the United States military in Southeast Asia during the Vietnam War. Success with Project AGILE field tests with herbicides in South Vietnam in 1961 led to the formal herbicidal program Trail Dust...

  • Soil contamination
    Soil contamination
    Soil contamination or soil pollution is caused by the presence of xenobiotic chemicals or other alteration in the natural soil environment....

  • Surface runoff
    Surface runoff
    Surface runoff is the water flow that occurs when soil is infiltrated to full capacity and excess water from rain, meltwater, or other sources flows over the land. This is a major component of the water cycle. Runoff that occurs on surfaces before reaching a channel is also called a nonpoint source...

  • Weed
    Weed
    A weed in a general sense is a plant that is considered by the user of the term to be a nuisance, and normally applied to unwanted plants in human-controlled settings, especially farm fields and gardens, but also lawns, parks, woods, and other areas. More specifically, the term is often used to...



Further reading

  • A Brief History of On-track Weed Control in the N.S.W. SRA during the Steam Era Longworth, Jim Australian Railway Historical Society Bulletin
    Australian Railway History
    Australian Railway History , is the premier magazine covering railway history in Australia...

    , April, 1996 pp99–116

External links

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