Herbicide

Source: Wikipedia, the free encyclopedia.
"Weedkiller" redirects here. For the album by Ashnikko, see Weedkiller (album).
A field after application of a herbicide
Weeds controlled with herbicide

Herbicides (US: /ˈɜːrbɪsaɪdz/, UK: /ˈhɜːr-/), also commonly known as weed killers, are substances used to control undesired plants, also known as weeds.[1] 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.[2] The combined effects of herbicides, nitrogen fertilizer, and improved cultivars has increased yields (per acre) of major crops by three to six times from 1900 to 2000.[3]

In the United States in 2012, about 91% of all herbicide usage, determined by weight applied, was in agriculture.

conifers after clearcutting,[6]
as well as pasture systems.

History

Prior to the widespread use of herbicides,

cultural controls, such as altering soil pH, salinity, or fertility levels, were used to control weeds.[7] 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.[8][9]

First herbicides

Second World War
.

The major breakthroughs occurred during the

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.[15]

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

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.[16] Like other acid herbicides, current formulations use either an amine salt (often trimethylamine) or one of many esters
of the parent compound.

Further discoveries

The triazine family of herbicides, which includes

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.[17] 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.

Terminology

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.

Selectivity

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.[18]

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

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.[18]

Timing of application

Method of application

Persistence

An herbicide is described as having low residual activity if it is neutralized within a short time of application (within a few weeks or months) – typically this is due to rainfall, or reactions in the soil. A herbicide described as having high residual activity will remain potent for the long term in the soil. For some compounds, the residual activity can leave the ground almost permanently barren.[citation needed]

Mechanism of action

Herbicides classified by their mechanism of action

Herbicides interfere with the biochemical machinery that supports plant growth. Herbicides often mimic natural plant hormones, enzyme substrates, and cofactors. They interfere with the metabolism in the target plants. Herbicides are often classified according to their site of action because as a general rule, herbicides within the same site of action class produce similar symptoms on susceptible plants. Classification based on the site of action of the herbicide is preferable as herbicide resistance management can be handled more effectively.[18] Classification by mechanism of action (MOA) indicates the first enzyme, protein, or biochemical step affected in the plant following application:

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 (

2,4,5-T, picloram, dicamba, clopyralid, and triclopyr
.

WSSA and HRAC classification

Using the Weed Science Society of America (WSSA) and herbicide Resistance and World Grains (HRAC) systems, herbicides are classified by mode of action.[28] Eventually the Herbicide Resistance Action Committee (HRAC)[29] and the Weed Science Society of America (WSSA)[30] developed a classification system.[31][32] 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.[33]

Use and application

Further information: Pesticide application
Herbicides being sprayed from the spray arms of a tractor in North Dakota.

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

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.[34]

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.[35]

Misuse and misapplication

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Herbicide volatilisation
or spray drift may result in herbicide affecting neighboring fields or plants, particularly in windy conditions. Sometimes, the wrong field or plants may be sprayed due to error.

Use politically, militarily, and in conflict

Main articles:
Rainbow herbicide and Herbicidal warfare
Handicapped children in Vietnam, most of them victims of Agent Orange, 2004

Although herbicidal warfare uses chemical substances, its main purpose is to disrupt agricultural food production or to destroy plants which provide cover or concealment to the enemy. During the Malayan Emergency, British Commonwealth forces deployed herbicides and defoliants in the Malaysian countryside in order to deprive Malayan National Liberation Army (MNLA) insurgents of cover, potential sources of food and to flush them out of the jungle. Deployment of herbicides and defoliants served the dual purpose of thinning jungle trails to prevent ambushes and destroying crop fields in regions where the MNLA was active to deprive them of potential sources of food. As part of this process, herbicides and defoliants were also sprayed from Royal Air Force aircraft.[36]

The use of herbicides as a chemical weapon by the

Viet Nam Red Cross Society estimates that up to one million people were disabled or have health problems as a result of exposure to Agent Orange.[41] The United States government has described these figures as unreliable.[42]

Health and environmental effects

See also:
Environmental effects of pesticides and Health effects of pesticides

Human health

Many questions exist about herbicides' health and environmental effects, because of the many kinds of herbicide and the myriad potential targets, mostly unintended. For example, a 1995 panel of 13 scientists reviewing studies on the

soft tissue sarcoma and non-Hodgkin lymphoma.[44]

Toxicity

Herbicides have widely variable toxicity. Acute toxicity, short term exposure effects, and chronic toxicity, from long term environmental or occupational exposure. Much public suspicion of herbicides confuses valid statements of acute toxicity with 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.[45] That is, the study shows lack of chronic toxicity, but cannot question the herbicide's acute toxicity.

Health effects

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

adsorb or retain the herbicides. Herbicide properties that increase likelihood of transport include persistence (resistance to degradation) and high water solubility.[46]

Contamination

Cases have been reported where

causal relationship remains unclear.[49]

False claims

Herbicide manufacturers have at times made false or misleading claims about the safety of their products. Chemical manufacturer

Tordon 101 (Dow AgroSciences, owned by the Dow Chemical Company) has claimed Tordon 101 has no effects on animals and insects,[52] in spite of evidence of strong carcinogenic activity of the active ingredient,[53] picloram, in studies on rats.[54]

Ecological effects

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.

Bioaccumulation is a concern, both in terrestrial[55] and aquatic environments,[56] and is heavily dependent on both the kind of herbicide and the conditions. For example, fish in dark aquariums bioaccumulated 14 times more trifluralin than fish kept in well lit aquariums in a 1977 study.[57]

Aquatic life

microbial communities in soil.[56]

Bird populations

Bird populations are one of many indicators of herbicide damage. Most observed effects are due not to toxicity,

neotropical agricultural areas has been one of many factors implicated in limiting the usefulness of such agricultural land for wintering migratory birds.[61]

Resistance

One major complication 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.[62] Herbicide resistance was first observed in 1957, and since has evolved repeatedly in weed species from 30 families across the globe.[63] Weed resistance to herbicides has become a major concern in crop production worldwide.[64]

Resistance to herbicides is often attributed to overuse as well as the strong evolutionary pressure on the affected weeds.

glyphosate resistance evolved rapidly in part because when glyphosate use first began, it was continuously and heavily relied upon for weed control.[67] This caused incredibly strong selective pressure upon weeds, encouraging mutations conferring glyphosate resistance to persist and spread.[68]

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.[69][70][71]

Plants developed resistance to

Marestail is one weed that has developed glyphosate resistance.[72]
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 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.[67]

As of 2023, Palmer amaranth with resistance to six different herbicide modes of action has emerged.[74] 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.[75] 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.[68][76]

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.[77]

Evolutionary insights

When mutations occur in the genes responsible for the biological mechanisms that herbicides interfere with, these mutations may cause the herbicide mode of action to work less effectively. This is called target-site resistance. Specific mutations that have the most helpful effect for the plant have been shown to occur in separate instances and dominate throughout resistant weed populations. This is an example of convergent evolution.[63] Some mutations conferring herbicide resistance may have fitness costs, reducing the plant's ability to survive in other ways, but over time, the least costly mutations tend to dominate in weed populations.[63]

Recently, incidences of non-target site resistance have increasingly emerged, such as examples where plants are capable of producing enzymes that neutralize herbicides before they can enter the plant's cells –

metabolic resistance. This form of resistance is particularly challenging, since plants can develop non-target-site resistance to herbicides their ancestors were never directly exposed to.[77]

Biochemistry of resistance

Resistance to herbicides can be based on one of the following biochemical mechanisms:[78][79][80]

The following terms are also used to describe cases where plants are resistant to multiple herbicides at once:

  • Cross-resistance: In this case, a single resistance mechanism causes resistance to several herbicides. The term target-site cross-resistance is used when the herbicides bind to the same target site, whereas non-target-site cross-resistance is due to a single non-target-site mechanism (e.g., enhanced metabolic detoxification) that entails resistance across herbicides with different sites of action.
  • Multiple resistance: In this situation, two or more resistance mechanisms are present within individual plants, or within a plant population.

Resistance management

See also: Integrated pest management

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.[82] By relying on diverse weed control methods, including non-herbicide methods of weed control, the selection pressure on weeds to evolve resistance can be lowered. Researchers warn that if herbicide resistance is combatted only with more herbicides, "evolution will most likely win."[66] In 2017, the USEPA issued a revised Pesticide Registration Notice (PRN 2017-1), which provides guidance to pesticide registrants on required pesticide resistance management labeling. This requirement applies to all conventional pesticides and is meant to provide end-users with guidance on managing pesticide resistance.[83] 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.[84]

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.[82]

Agronomic factors influencing the risk of herbicide resistance development[original research?]
Factor Low risk High risk
Cropping system Good rotation Crop monoculture
Cultivation system Annual ploughing Continuous minimum tillage
Weed control Cultural only Herbicide only
Herbicide use Many modes of action Single modes of action
Control in previous years Excellent Poor
Weed infestation Low High
Resistance in vicinity Unknown Common

Approaches to treating resistant weeds

Alternative herbicides

When resistance is first suspected or confirmed, the efficacy of alternatives is likely to be the first consideration. If there is resistance to a single group of herbicides, then the use of herbicides from other groups may provide a simple and effective solution, at least in the short term. For example, many triazine-resistant weeds have been readily controlled by the use of alternative herbicides such as dicamba or glyphosate.[82]

Mixtures and sequences

The use of two or more herbicides which have differing modes of action can reduce the selection for resistant genotypes. Ideally, each component in a mixture should:

  • Be active at different target sites
  • Have a high level of efficacy
  • Be detoxified by different biochemical pathways
  • Have similar persistence in the soil (if it is a residual herbicide)
  • Exert negative cross-resistance
  • Synergise the activity of the other component

No mixture is likely to have all these attributes, but the first two listed are the most important. There is a risk that mixtures will select for resistance to both components in the longer term. One practical advantage of sequences of two herbicides compared with mixtures is that a better appraisal of the efficacy of each herbicide component is possible, provided that sufficient time elapses between each application. A disadvantage with sequences is that two separate applications have to be made and it is possible that the later application will be less effective on weeds surviving the first application. If these are resistant, then the second herbicide in the sequence may increase selection for resistant individuals by killing the susceptible plants which were damaged but not killed by the first application, but allowing the larger, less affected, resistant plants to survive. This has been cited as one reason why ALS-resistant Stellaria media has evolved in Scotland recently (2000), despite the regular use of a sequence incorporating mecoprop, a herbicide with a different mode of action.[82]

Natural herbicide

The term organic herbicide has come to mean herbicides intended for

tree of heaven; such actions of natural herbicides, and other related chemical interactions, is called allelopathy
. The applicability of these agents is unclear.

Farming practices and resistance: a case study

Herbicide resistance became a critical problem in

acetyl coenzyme A carboxylase.[86][87]

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

long-chain fatty acid inhibitors, were effective against ryegrass.[86]

See also

Further information: List of herbicides

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Further reading

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