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Type of chemical used to kill unwanted plants

Herbicides (/us/'/ɜːr//b//ɪ//s//aɪ//d//z/, /uk/'/h//ɜːr// /), also commonly known as weed killers, are substances used to control undesired plants, also known as weeds. Selective herbicides control specific weed species while leaving the desired crop relatively unharmed, while non selective herbicides (sometimes called "total weed killers") kill plants indiscriminately. The combined effects of herbicides, nitrogen fertilizer, and improved cultivars has increased yields (per acre) of major crops by 3x to 6x from 1900 to 2000. In the United States in 2012, about 91% of all herbicide usage, determined by weight applied, was in agriculture. In 2012, world pesticide expenditures totaled nearly $24.7 billion; herbicides were about 44% of those sales and constituted the biggest portion, followed by insecticides, fungicides, and fumigants. where certain formulations have been found to suppress hardwood varieties in favor of conifers after clearcutting, as well as pasture systems.

Prior to the widespread use of herbicides, cultural controls, such as altering soil pH, salinity, or fertility levels, were used to control weeds. Mechanical control including tillage and flooding were also used to control weeds. In the late 19th and early 20th centuries, inorganic chemicals such as sulfuric acid, arsenic, copper salts, kerosene and sodium chlorate were used to control weeds, but these chemicals were either toxic, flammable or corrosive and were expensive and ineffective at controlling weeds.

The major breakthroughs occurred during the Second World War as the result of research conducted independently in the United Kingdom and the United States into the potential use of herbicides in war. The compound 2,4 D was first synthesized by W. G. Templeman at Imperial Chemical Industries. In 1940, his work with indoleacetic acid and naphthaleneacetic acid indicated that "growth substances applied appropriately would kill certain broad leaved weeds in cereals without harming the crops," though these substances were too expensive and too short lived in soil due to degradation by microorganisms to be of practical agricultural use; by 1941, his team succeeded in synthesizing a wide range of chemicals to achieve the same effect at lower cost and better efficacy, including 2,4 D. In the same year, R. Pokorny in the US achieved this as well. Independently, a team under Juda Hirsch Quastel, working at the Rothamsted Experimental Station made the same discovery. Quastel was tasked by the Agricultural Research Council (ARC) to discover methods for improving crop yield. By analyzing soil as a dynamic system, rather than an inert substance, he was able to apply techniques such as perfusion. Quastel was able to quantify the influence of various plant hormones, inhibitors, and other chemicals on the activity of microorganisms in the soil and assess their direct impact on plant growth. While the full work of the unit remained secret, certain discoveries were developed for commercial use after the war, including the 2,4 D compound. When 2,4 D was commercially released in 1946, it became the first successful selective herbicide, triggering a worldwide revolution in agricultural output. It allowed for greatly enhanced weed control in wheat, maize (corn), rice, and similar cereal grass crops, because it kills dicots (broadleaf plants), but not most monocots (grasses). 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 use either an amine salt (often trimethylamine) or one of many esters of the parent compound.

The triazine family of herbicides, which includes atrazine, was introduced in the 1950s; they have the 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. 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 had been first prepared in the 1950s but its herbicidal activity was only recognized in the 1960s. It was marketed as Roundup in 1971. The development of glyphosate resistant crop plants, it is now used very extensively for selective weed control in growing crops. 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 herbicides used in agriculture and gardening are specifically formulated to degrade within a short period after application.

Herbicides can be classified/grouped in various ways; for example, according to their activity, the timing of application, method of application, mechanism of their action, and their chemical structures.

Chemical structure of the herbicide is of primary affecting efficacy. 2,4 D, mecoprop, and dicamba control many broadleaf weeds but remain ineffective against turf grasses. Chemical additives influence selectivity. Surfactants alter the physical properties of the spray solution and the overall phytotoxicity of the herbicide, increasing translocation. Herbicide safeners enhance the selectivity by boosting herbicide resistance by the crop but allowing the herbicide to damage the weed. Selectivity is determined by the circumstances and technique of application. Climatic factors affect absorption including humidity, light, precipitation, and temperature. Foliage applied herbicides will enter the leaf more readily at high humidity by lengthening the drying time of the spray droplet and increasing cuticle hydration. Light of high intensity may break down some herbicides and cause the leaf cuticle to thicken, which can interfere with absorption. Precipitation may wash away or remove some foliage applied herbicides, but it will increase root absorption of soil applied herbicides. Drought stressed plants are less likely to translocate herbicides. As temperature increases, herbicides' performance may decrease. Absorption and translocation may be reduced in very cold weather.

Non selective herbicides, generally known as defoliants, are used to clear industrial sites, waste grounds, railways, and railway embankments. Paraquat, glufosinate, and glyphosate are non selective herbicides. 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: Acetolactate synthase (ALS; also known as acetohydroxyacid synthase, or AHAS) is part of 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 the inhibition of DNA synthesis. They affect grasses and dicots alike. The ALS inhibitor family includes various sulfonylureas (SUs) (such as flazasulfuron and metsulfuron methyl), imidazolinones (IMIs), triazolopyrimidines (TPs), pyrimidinyl oxybenzoates (POBs), and sulfonylamino carbonyl triazolinones (SCTs). The ALS biological pathway exists only in plants and microorganisms (but not animals), thus making the ALS inhibitors among the safest herbicides. EPSPS inhibitors: 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 inactivated by soil contact. Other members of this class are chlorbromuron, pyrazon, isoproturon, bromacil, and terbacil. Photosystem I inhibitors steal electrons from ferredoxins, specifically the normal pathway through FeS to Fdx to NADP+, leading to direct discharge of electrons on oxygen. As a result, reactive oxygen species are produced and oxidation reactions in excess of those normally tolerated by the cell occur, leading to plant death. Bipyridinium herbicides (such as diquat and paraquat) inhibit the FeS to Fdx step of that chain, while diphenyl ether herbicides (such as nitrofen, nitrofluorfen, and acifluorfen) inhibit the Fdx to NADP+ step. Tyrosine breakdown products are used by plants to make carotenoids, which protect chlorophyll in plants from being destroyed by sunlight. If this happens, the plants turn white due to complete loss of chlorophyll, and the plants die. Mesotrione and sulcotrione are herbicides in this class; a drug, nitisinone, was discovered in the course of developing this class of herbicides. Complementary to mechanism based classifications, herbicides are often classified according to their chemical structures or motifs. Similar structural types work in similar ways. For example, aryloxphenoxypropionates herbicides (diclofop chlorazifop, fluazifop) appear to all act as ACCase inhibitors. Knowing about herbicide chemical family grouping serves as a short term strategy for managing resistance to site of action. The phenoxyacetic acid mimic the natural auxin indoleacetic acid (IAA). This family includes MCPA, 2,4 D, and 2,4,5 T, picloram, dicamba, clopyralid, and triclopyr.

Using the Weed Science Society of America (WSSA) and herbicide Resistance and World Grains (HRAC) systems, herbicides are classified by mode of action. Eventually the Herbicide Resistance Action Committee (HRAC) and the Weed Science Society of America (WSSA) developed a classification system. Groups in the WSSA and the HRAC systems are designated by numbers and letters, inform users awareness of herbicide mode of action and provide more accurate recommendations for resistance management.

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 long booms, of with spray nozzles spaced every apart. Towed, handheld, and even horse drawn sprayers are also used. On large areas, herbicides may also at times be applied aerially using helicopters or airplanes, or through irrigation systems (known as chemigation). 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 weed plants. This allows treatment of taller grassland weeds by direct contact without affecting related but desirable shorter plants in the grassland sward beneath. The method has the benefit of avoiding spray drift. In Wales, a scheme offering free weed wiper hire was launched in 2015 in an effort to reduce the levels of MCPA in water courses. There is little difference in forestry in the early growth stages, when the height similarities between growing trees and growing annual crops yields a similar problem with weed competition. Unlike with annuals however, application is mostly unnecessary thereafter and is thus mostly used to decrease the delay between productive economic cycles of lumber crops. The use of herbicides as a chemical weapon by the U. S. military during the Vietnam War has left tangible, long term impacts upon the Vietnamese people and U. S soldiers that handled the chemicals. More than 20% of South Vietnam's forests and 3.2% of its cultivated land were sprayed at least once between during the war. The government of Vietnam says that up to four million people in Vietnam were exposed to the defoliant, and as many as three million people have suffered illness because of Agent Orange, while the Red Cross of Vietnam estimates that up to one million people were disabled or have health problems as a result of exposure to Agent Orange. The United States government has described these figures as unreliable.

In general, many questions exist about the health and environmental effects of many herbicides because of the large number of herbicides and the myriad potential targets, mostly unintended. For example, a 1995 panel of 13 scientists reviewing studies on the carcinogenicity of 2,4 D had divided opinions on the likelihood 2,4 D causes cancer in humans. as of 1992, studies on phenoxy herbicides were too few to accurately assess the risk of many types of cancer from these herbicides, even although evidence was stronger that exposure to these herbicides is associated with increased risk of soft tissue sarcoma and non Hodgkin lymphoma. Herbicides have widely variable toxicity in addition to acute toxicity arising from ingestion of a significant quantity rapidly, and chronic toxicity arising from environmental and occupational exposure over long periods. Much public suspicion of herbicides revolves around a confusion between valid statements of acute toxicity as opposed to equally valid statements of lack of chronic toxicity at the recommended levels of usage. For instance, while glyphosate formulations with tallowamine adjuvants are acutely toxic, their use was found to be uncorrelated with any health issues like cancer in a massive US Department of Health study on 90,000 members of farmer families for over a period of 23 years. That is, the study shows lack of chronic toxicity, but cannot question the herbicide's acute toxicity. 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 labelled preharvest interval. Under some conditions, certain herbicides can be transported via leaching or surface runoff to contaminate groundwater or distant surface water sources. Generally, the conditions that promote herbicide transport include intense storm events (particularly shortly after application) and soils with limited capacity to adsorb or retain the herbicides. Herbicide properties that increase likelihood of transport include persistence (resistance to degradation) and high water solubility. Cases have been reported where Phenoxy herbicides are contaminated with dioxins such as TCDD; research has suggested such contamination results in a small rise in cancer risk after occupational exposure to these herbicides. Triazine exposure has been implicated in a likely relationship to increased risk of breast cancer, 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 attorney general Dennis Vacco; 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 (though proof that this was ever said is hard to find). Roundup is toxic and has resulted in death 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 manufacturer of Tordon 101 (Dow AgroSciences, owned by the Dow Chemical Company) has claimed Tordon 101 has no effects on animals and insects, in spite of evidence of strong carcinogenic activity of the active ingredient, picloram, in studies on rats.

Herbicide use generally has negative impacts on many aspects of the environment. Insects, non targeted plants, animals, and aquatic systems subject to serious damage from herbicides. Impacts are highly variable.

Atrazine has often been blamed for affecting reproductive behavior of aquatic life, but the data do not support this assertion.

Bird populations are one of many indicators of herbicide damage. Most observed effects are due not to toxicity, but to habitat changes and the decreases in abundance of species on which birds rely for food or shelter. Herbicide use in silviculture, used to favor certain types of growth following clearcutting, can cause significant drops in bird populations. Even when herbicides which have low toxicity to birds are used, they decrease the abundance of many types of vegetation on which the birds rely. Herbicide use in agriculture in the UK 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.

One major complications to the use of herbicides for weed control is the ability of plants to evolve herbicide resistance, rendering the herbicides ineffective against target plants. Out of 31 known herbicide modes of action, weeds have evolved resistance to 21. 268 plant species are known to have evolved herbicide resistance at least once. Herbicide resistance was first observed in 1957, and since has evolved repeatedly in weed species from 30 families across the globe. Weed resistance to herbicides has become a major concern in crop production worldwide. Resistance to herbicides is often attributed to overuse as well as the strong evolutionary pressure on the affected weeds. To minimize resistance, rotational programs of herbicide application, where herbicides with multiple modes of action are used, have been widely promoted. This caused incredibly strong selective pressure upon weeds, encouraging mutations conferring glyphosate resistance to persist and spread. However, in 2015, an expansive study showed an increase in herbicide resistance as a result of rotation, and instead recommended mixing multiple herbicides for simultaneous application. As of 2023, the effectiveness of combining herbicides is also questioned, particularly in light of the rise of non target site resistance. Plants developed resistance to atrazine and to ALS inhibitors relatively early, but more recently, glyphosate resistance has dramatically risen. Marestail is one weed that has developed glyphosate resistance. Glyphosate resistant weeds are present in the vast majority of soybean, cotton and corn farms in some U. S. states. Weeds that can resist multiple other herbicides are spreading. Few new herbicides are near commercialization, and none with a molecular mode of action for which there is no resistance. Because most herbicides could not kill all weeds, farmers rotate crops and herbicides to stop the development of resistant weeds. A 2008–2009 survey of 144 populations of waterhemp in 41 Missouri counties revealed glyphosate resistance in 69%. Weeds from some 500 sites throughout Iowa in 2011 and 2012 revealed glyphosate resistance in approximately 64% of waterhemp samples. As of 2023, 58 weed species have developed glyphosate resistance. Weeds resistant to multiple herbicides with completely different biological action modes are on the rise. In Missouri, 43% of waterhemp samples were resistant to two different herbicides; 6% resisted three; and 0.5% resisted four. In Iowa 89% of waterhemp samples resist two or more herbicides, 25% resist three, and 10% resist five. Annual bluegrass collected from a golf course in the U. S. state of Tennessee was found in 2020 to be resistant to seven herbicides at once. Rigid ryegrass and annual bluegrass share the distinction of the species with confirmed resistance to the largest number of herbicide modes of action, both with confirmed resistance to 12 different modes of action; however, this number references how many forms of herbicide resistance are known to have emerged in the species at some point, not how many have been found simultaneously in a single plant. In 2015, Monsanto released crop seed varieties resistant to both dicamba and glyphosate, allowing for use of a greater variety of herbicides on fields without harming the crops. By 2020, five years after the release of dicamba resistant seed, the first example of dicamba resistant Palmer amaranth was found in one location.

Due to herbicide resistance – a major concern in agriculture – a number of products combine herbicides with different means of action. Integrated pest management may use herbicides alongside other pest control methods. Integrated weed management (IWM) approach utilizes several tactics to combat weeds and forestall resistance. This approach relies less on herbicides and so selection pressure should be reduced. An example of a fully executed label compliant with the USEPA resistance management labeling guidance can be seen on the specimen label for the herbicide, cloransulam methyl, updated in 2022. Optimising herbicide input to the economic threshold level should avoid the unnecessary use of herbicides and reduce selection pressure. Herbicides should be used to their greatest potential by ensuring that the timing, dose, application method, soil and climatic conditions are optimal for good activity. In the UK, partially resistant grass weeds such as Alopecurus myosuroides (blackgrass) and Avena genus (wild oat) can often be controlled adequately when herbicides are applied at the 2 3 leaf stage, whereas later applications at the 2 3 tiller stage can fail badly. Patch spraying, or applying herbicide to only the badly infested areas of fields, is another means of reducing total herbicide use.

Herbicide resistance became a critical problem in Australian agriculture, after many Australian sheep farmers began to exclusively grow wheat in their pastures in the 1970s. Introduced varieties of ryegrass, while good for grazing sheep, compete intensely with wheat. Ryegrasses produce so many seeds that, if left unchecked, they can completely choke a field. Herbicides provided excellent control, while reducing soil disrupting because of less need to plough. Within little more than a decade, ryegrass and other weeds began to develop resistance. In response Australian farmers changed methods. By 1983, patches of ryegrass had become immune to Hoegrass (diclofop methyl), a family of herbicides that inhibit an enzyme called acetyl coenzyme A carboxylase. Ryegrass populations were large, and had substantial genetic diversity, because farmers had planted many varieties. Ryegrass is cross pollinated by wind, so genes shuffle frequently. To control its distribution farmers sprayed inexpensive Hoegrass, creating selection pressure. In addition, farmers sometimes diluted the herbicide in order to save money, which allowed some plants to survive application. When resistance appeared farmers turned to a group of herbicides that block acetolactate synthase. Once again, ryegrass in Australia evolved a kind of "cross resistance" that allowed it to rapidly break down a variety of herbicides. Four classes of herbicides become ineffective within a few years. In 2013 only two herbicide classes, called Photosystem II and long chain fatty acid inhibitors, were effective against ryegrass.