Water Pollution

Herbicide

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Characteristic Cogengrass circular infestations are often targets of herbicides. Credit: Greg Leach

A herbicide is any of a number of chemical substances intended to kill vegetation. Since the vast majority of herbicides are non-selective in their lethal action, there may be widespread adverse ecological consequences from their use. These outcomes include not only organism death, but may involve mutagenic, developmental and carcinogenic effects to animals and plants.

Herbicides are in broad use for agriculture, golf courses, utility corridors, residential and other land uses. The earliest herbicides were inorganic chemical substances, although modern herbicides are dominated by organic compounds. Presently, there is massive application of chemical herbicides; in the USA alone 480 million kilograms are applied annually.

Widespread herbicide use beginning in the 1940s is responsible for numerous species extinctions, including birds, amphibians, fish and arthropods. In many cases, herbicide use is a contributory cause along with habitat destruction to species endangerment. Many herbicides have persistent effects in the environment, retaining their toxicity as they remain in soils for decades in some cases; furthermore, some herbicides are highly soluble, so that they may enter aquatic systems, where non-selective lethal effects can occur. Often the herbicides undergo chemical change after release into the environment; in some cases, the altered chemicals may have different toxicity effects upon plants and animals. Herbicide use has been linked to certain human diseases and mortality, as well as some types of reproductive and endocrine system impacts.

Environmental accumulation

caption Soil treated with salt herbicide may
may become sterile to all life.
Even though herbicides are intended for certain plant organisms, the net result of their use is accumulation in large amounts in the environment (for example, the soil, aquatic, biotic and atmospheric systems). These effects are amplified by the following phenenoma: (a) high solubility of many classes of herbicides; (b) magnification of concentrations of herbicides upon entry into the food chain and successive concentriation in lipids for higher species in that chain; and (c) persistence of many herbicides and transformation to other harmful metabolites upon residence in soil, water and biota. 

In surface waters of the world atrazine and 2,4 D are ubiquitous, even in western countries that have generally stringent water quality criteria. Atrazine is generally toxic to most fish species at concentrations on the order of one part per billion in water. To understand the magnitude of the ocean accumulation, note that in one month of peak discharge 650 metric tons of herbicide are discharged to the Gulf of Mexico.

caption A-D. Weed growth on agricultural field; E-F. Field application of herbicide Source: Saikat Basu, own work.

Biotransformation

Herbicides are taken up by not only plant, but also animal and bacterial organisms. The process often results in toxicity to the uptake organism, and may engender a chemical transformation in the herbicide itself. These chemical changes are important not only to understand the toxicity within the affected organism, but also to analyze the persistence of each herbicide in the environment and the generation of metabolites that may have different ecological effects from the original herbicide.

Very few herbicides are stable within living organisms. Only the chlorinated aliphatic molecules and glyphosate evince strong stability. For the rest, linkages of alkyl, hydroxyl, carboxyl, amino, amido, nitrile, or halogenated groups are subject to enzymatic or chemical attack via hydrolysis or redox reactions; subsequently, these reactive sites may enable conjugation with sugars, amino acids and thiol-peptides. Generally plants cannot break down aromatic rings, although bacteria are often quite adept at such biotransformation.

Ecological effects

Bioaccumulative effects in plants and higher animals are well known, whereby plants can typically concentrate an herbicide or heavy metal by a factor of 1000 from its environmental concentration in soil or water; correspondingly, higher animals can concentrate a given herbicide by a factor of 2000. Many classes of herbicides act across a wide variety of species. Organo-chlorine compounds, for example, act broadly to inhibit cellular adenosine triphosphatase activity that broadly underlies animal metabolism, and these substances also interfere with fundamental renal and liver function.

Dicamba is toxic to mammals by ingestion or inhalation; it is also toxic to coldwater fishes when present in water bodies. Glyphosate was originally thought to have low ecological impacts, but is now known to have significant adverse impacts upon amphibians, fish, beneficial insects and nitrogen fixing bacteria. Many aliphatics such as acrolein are strongly toxic to fish and wildlife.

While the preponderance of ecological impacts are strictly deleterious, there are a number of lower organisms whose growth is actually stimulated by herbicide treatment. In particular some bacteria, fungi, mites, nematodes and springtails evince initial growth flourishes upon treatment by a minority of herbicides. It is unclear, however, whether longer term deleterious mutations and fitness reductions are masked by initial growth spurts that are in response to initial stimulation.

Amphibians

caption Malformations in amphibians are common
outcomes of herbicide exposure.
Amphibians are sensitive to many herbicides and their populations have experienced significant decline, and are known to have occasioned mutagenic and reproductive effects from herbicide exposure. Habitat degradation and fragmentation are the greatest sources of amphibian population decline, but herbicides are important contributory factors to such habitat degradation. There are many mechanisms associated with herbicidal action upon amphibians including interference with enzyme activity and mutations, both of which reduce behavioral and reproductive fitness, and hence generally lead to amphibian population declines.

It should be noted that research illustrates herbicide action is likely to be synergistically negative upon amphibian populations with other stressors including parasites, nitrate fertilizer addition to aquatic habitats and climatic oscillations. Atrazine, a very common herbicide, is known to produce de-masculation, birth abnormalities and estrogen imbalance in certain frog species, including the Northern leopard frog.

Fish

caption American minnow is sensitive to low concentrations
of atrazine and toxophene. USGS
Many herbicides are toxic to fish at very low concentrations; for example, toxaphene is lethal to the American minnow (Pimephales promelas) at concentrations on the order of 50 nanograms per liter in natural waters. A number of herbicides are known to induce hepatic and renal lesions in fish; detailed attribution to specific herbicides is often difficult, since it is common for several herbicides to be found in a given aquatic environment. In systematic analyses of mass fish kills, it is characteristic that herbicide and pesticide causes are responsible for the majority of such mass kill incidents. The common herbicide atrazine also induces abnormal tissue growth, reduction in egg production and hormonal change in P. promelas, for concentration levels substantially less than the U.S. EPA chronic exposure limits. Trifluralin is toxic to many fish species, having a 96 hour LC50 of 20 parts per billion for Rainbow trout.

Birds

caption The Peregrine Falcon became endangered due
to herbicide and pesticide use from 1950 to 1980
Source: Kevin Cole
Numerous bird species whose populations are in serious decline have been studied intensively for causation; an extremely high fraction of these cases have demonstrated that herbicide use is implicated in the decline or entrance into an extinction vortex. Due to sucessive concentration of herbicides in the food chain, raptors are exceptionally vulnerable to its persistance in the environment; notable victims of herbicide use include the Peregrine Falcon, Sharp-shinned Hawk, Coopers Hawk, Eurasian Sparrowhawk, Osprey, Bald Eagle, and White-tailed Eagle. Such other species as Brown Pelicans and several Heron species are subject to significant mortality from herbide use.


Besides direct effects, there were notable consistency in secondary impacts, chiefly from herbicide killing of arthropod prey and from seed reduction due to herbicide applications. In terms of acute toxicity dosage of 275 parts per million of propanil is lethal to mallard ducks.

Some herbicides—for example organophosphates—are extremely toxic to avafauna. Diazinon, for example is one hundred times more toxic to birds than mammals on a dosage basis; as an example from one observation set, 900 Canada geese were killed from a flock of 2500 due to diazinon application in one localized wintering area. Diazinon's use has been banned in many locations where bird populations are already rare or endangered, or where notice has been taken of its lethality. 

Mammals

As early as 1966 the National Cancer Insitute conducted research indicating the herbicides 2,4 D and
2.4,5 T induce abnormal fetuses in mice and rats; these syndromes include increased fetus death, as well as eyeless fetuses, occurrences of cleft palates, systic kidneys and enlarged palates. Other studies with reindeer and golden hamsters have shown tetragenic and mutation effects of 2.4 D and 2,4,5 T. Furthermore, these same two herbicides cause eye lesions in most mammals.

Paraquat and diquat are two herbicides known to be toxic to mammals. Sethyoxydim in as low a dosage of 40 parts per million has been shown to decrease litter size, reduce fetal weights, cause severe maternal weight loss, increased fetal resorptions and spontaneous abortions in rabbits; this same herbicide causes liver and bone marrow toxicity in dogs.

Arthropods

A fundamental outcome of herbicide use is dramatic reduction of numbers of insects, in many cases due simply to large scale reduction in host plant organisms; a more basic truism is the reduction in biodiversity as a consequence of the success of vast expanses of "weed free" crops. It is precisely the presence of diverse flora in untreated areas that gives rise to the greatest diversity and abundance of insects. Pollinating insects, including bees, have shown particular vulnerability to herbicide applications. Colony collapse disorder, or the widespread reduction in honeybee populations, are suggested as linked to intensive herbicide use as a causative factor, likely due to the large reduction of forage habitat to honeybees; monocultures produced by effective herbicide use are not suitable for multi-season pollen forage required by honeybees. The LD50 dose of propanil for bee toxicity is 240 micrograms per bee. Even glyphosate is known to be toxic to bees at the level of 0.1 milligram per bee. 2,4-D is highly toxic to bees, with lethality occurring at a dose of eleven micrograms per bee.

Herbicides have toxic effects on aquatic arthropods; for example, trifluorin is toxic to daphnia, a group of small aquatic crustaceans at dosage of one half part per million for the 96 hour LC50. Paraquat has a similar level of toxicity to daphnia, evincing a 96 hour LC50 of 1.2 parts per million.

Plants

caption Endangered Pimelea spicata: extinction
threatened by glyphosate. Source: Peter Halasz
Since most herbicides are notably non-selective, there are massive adverse impacts to native vegetation from the broad scale worldwide use of herbicides. Many plant communities are significantly damaged and fragmented, with particular impacts on rare and endangered flora, which have a limited range and limited number of metapopulations. Roadside spraying and aerial application of crop herbicides are particularly responsible for major losses of individual species, as well as habitat area.

Unintended consequences of herbicidal control of invasive plants can cause endangerment or extirpation of entire populations of endangered plants. For example, the endangered shrub, Pimelea spicata, has had entire metapopulations endangered by glyphosate applications intended to eradicate bitou bush.

Human health effects

One of the most widely used herbicides is 2,4-D, which has been classified as a human carcinogen by the International Agency for Research on Cancer. One Southeast Asian study with over 3000 subjects showed the rated of birth defects quadrupled with exposure to one parent of the herbicide 2,4,5-T . Paraquat is a well established human toxin, whose lethal ingestion dose may be as low as 1.5 grams; death usually occurs from respiratory or renal failure.

Studies in 2008 found atrazine to increase gene activity linked to abnormal human birth weight when over-expressed in the placenta. Atrazine, the second most common herbicide applied in the USA,  also targeted a gene that has been amplified in the uterus of women with unexplained infertility. 2,4-D a common herbicide and 2,4,5-T are implicated in human reproductive failure and miscarriage according to reports in the New York Times. In a Minnesota study of 1537 children, parental exposure to glyphosate was correlated with increased birth defects. Pre-natal exposure to the herbicide nitrofen has produced increased mortality of babies, with noted malformations of cardiac tissue.

A number of herbicides exhibit pronounced carcinogenic effects that pose risks for consumers and farmworkers. For example, epidemiological research shows increased risk of cancer, notably soft-tissue sarcomas and non-Hodgkin's lymphomas, in people occupationally exposed to chlorophenoxy herbicides. Triazine herbicide exposure has evinced strong correlation with increases in breast cancer incidence.

The herbicide glyphosate is most often applied under the formulation and trade name of Roundup, a substance which produces symtoms of sore throat, abdominal pain and vomiting; however, it is thought that some of these effects are due to the presence of the surfactant polyoxyethyleneamine.

For the herbicide alachlor the EPA has recognized skin and eye irritation; renal, spleen  and liver damage risk; and cancer of the lining of nasal cavity and eyelids. The use of alachlor as a herbicide has been banned by the European Union.

Worldwide view

Herbicides are widely used worldwide, with the earliest uses arising in developed countries; application methods typically involve aerial spraying or truck-borne spray rigs. The earliest uses involved many chemicals that are persistent in the environment. In western countries the use of many persistent herbicides toxic to animals have been banned, but many of these substances such as DDT remain in widespread use in China, Brazil, India and many underdeveloped countries.

Herbicides were a cornerstone feature of the so-called green revolution which promised the increase of crop production in the 1970s. Unfortunately the green revolution resulted in unintended consequences of massive ecological damage, extensive herbicide and pesticide soil residues and prdigious loss of topsoils concomitant with the tillage practices associated with these intensive chemical practices. In many world locations, such as the North China Plain, the green revolution actually produced unsustainable peaks in production, such that crop yields have been systematically declining for the last two decades.

In Europe, over 64,000 tonnes of herbicide per annum are applied to agricultural uses, with France and Germany  the principal users of herbicides; however, on a land intensity basis, both Belgium and the Netherlands exceed the French and German per hectare application. Triazines are the most common type of herbicides used in Europe, although some european countries began a ban on certain triazines starting in the 1990s.

In North America, triazines are the most commonly used class of herbicide for the USA. However, glyphosate is extremely widely used in the USA as well, with an estimated application of over 100 million pounds per annum.

In Asia, triazines are highly widespread, being the most applied herbicide in China.

Classes of herbicides

There are a large number of chemical compounds in use as herbicides including organic and inorganic molecules.

Organic chemicals

caption Molecular structure of the herbicide 2,4-D Phenoxycarboxylic acids

This class includes phenoxyacetic acids as well as phenoxypropionic and phenoxybutyric acid herbicides. The most well known chemical of this group is 2,4 D, a rather water soluble phenoxyacetic acid. Also in this group is 2,4,5-T, the main ingredient in Agent Orange, a well known defoliant used in the Vietnam War. 

Upon degradation, all phenoxycarboxylic acids undergo hydrolysis and the respective substituted phenols being the chief metabolites, which are deemed persistent in the environment because of the number of chlorine atoms. Phenoxyacetic acids are important plant growth regulators, which disrupt plant protein synthesis and hormone balance.

Benzoates

caption Salvia reflexa, a species targeted by benzoates.
Source: G.A.Cooper, USDA

This is a relatively recent class of herbicides, only becoming recognized in the 1990s. The most notable element of the class is pyrithiobac, which can control plant species within the genus Amaranthus and other broadleaf herbs such as Salvia reflexa, Senna occidentalis and Sida spinosa.

Benzonitriles

Dichlobenil and some of the hydroxybenzonnitriles are the most widely used benzonitriles.  The killing effects of substituted iodine are especially pronounced. Benzonitriles act by photosynthetic inhibition and by interruption of oxidative phosphorylation. The benzonitriles are used chiefly as selective post-emergent chemicals, primarily in grain farming

Benzothiadiazoles

Bentazon, first introduced to commercial use in 1968, is the prime example of this herbicide class. This molecule was advanced for application for cereal crop purposes to combat weed species resistant to 2,4-D. It is specifically utilized for soybeans, maize, wheat, barley and rice farming. Bentazon is a particularly lethal molecule with respect to post-emergent application to compositae, Convolvulaceae, Brassicaceae, Cyperaceae, Amaranthaceae, Solanaceae, Polygonaceae and Ambrosiaceae families.

Triazines

caption Non-Hodgkins lymphoma is linked to triazine. The chief compounds within this class are chlortriazines and methylthiotriazines.  The killing efficacy of triazines increase with the introduction of methoxy, chloro or alkylthiogroups into the molecule. As a class the triazines are diverse with respect to their persistence and selectivity. The chief mechanism of action is inhibition of photosynthesis. Major uses of triazines are in the tropics for corn cultivation. Even though triazines are subject to photodegradation, soil persistence is notable, since all residues do not remain on the surface. Metabolites of the triazines are frequently found in groundwater where the triazines have been applied. 

Acetamides

Acetochlor is a prominent example of this herbicide class; it may be used in pre- or post-emergent treatments of most annual grasses and an assortment of broadleaf vegetation. The first chlorinated acetamide to be used was alachlor, which is employed in a range of agricultural applications including soybeans, groundnuts, sorghum, maize and cotton crops. Butachlor is invoked for growing rice as well as sorghum, groundnuts and potatoes.

Aliphatics

Aliphatics such as acrolein have been used since the 1930s, but chiefly in control of aquatic vegetation. Dalapon was developed for use in control of certain perennial grasses, especially for agricultural applications such as sugarcane, coffee, tea and orchard crops.

Ureas and phenylureas

caption Molecular structure of the herbicide Diuron. Phenylureas such as diuron and monouron were the earliest widely used ureas. Subsequently thiadiazolylureas have been added to the arsenal of urea herbicides; the majority of these chemicals are phenyl substituted. Most of these molecules are absorbed via leaves and roots of the affected plants. 

Sulphonylureas are a special class of phenylureas; these chemicals act by inhibition of acetolactate synthesis. This set of herbicides hydrolyzes most swiftly in a low pH environment. These molecules are also absorbed by roots or leaves and are transported via xylem and phloem. The first sulphonylurea introduced to the market wa sulphuron in the year 1981; its use rapidly penetrated into the cereal market.

Aromatic acids

The aromatic acids behave as growth regulators after they are taken up through roots or leaves.  Benzoic  acid derivatives, and particularly dicamba, are notable herbicides in widespread use. Dicamba is applied in forestry, viticulture and grassland settings. Dicamba is non-selective and kills most broadleaf plants and legumes. Picloram is another widely used aromatic acid, manifesting much higher persistence than dicamba. Some of the aromatic acids have been banned for use as early as 1979 due to acute toxicity. 

Carbamates and thiocarbamates

Carbamates are esters of carbanilic acid and are represented by compunds such as asulam and desmedipham. Asulam, first studied in 1965, is applied in post emergent circumstances to sugarcane and orchard crops. It achieves growth stunting and necrosis in targeted plant species. Desmedipham is used in post-emergent applications for sugarbeet growing, and is targeted against annual broadleaf species such as Solanum nigrum. Sonchus arvensis and Ambrosia artemisiifolia. The thiocarbamates are extremely volatile and may evaporate to become air pollutants if not formulated to bond with soil. They are also highly toxic to a broad range of plants; for example, they may deform maize seedlings, unless there is a safener incorporated into the formulation. Furthermore carbamates are generally toxic to key animal species; for example, carbaryl is acutely toxic to earthworms and bees.

Cineoles

An example of this class is cinmethylin, whose efficacy appears to depend upon inhibition of mitosis in meristemic plant parts. This herbicide is targeted on certain annual grasses, broadleaf plants and sedges. Its degradation is inhibited in some rice crops where anaerobic conditions reduce microbial activity.

Cyclohexanediones

This class of herbicides is most effective in targeting annual and perennial grasses. Cycloxidim, an example of this group, kills annual and perennial grasses and does not act on broadleaf species. A surfactant or oil adjuvant is required in formulation for maximum efficacy. Sethoxydim is another member of the cyclohexanedione class, which chemical is used to control annual and perennial grasses for crops such as alfalfa, cotton and sunflowers.

Dinitroanilines

This class of molecules has low water solubility and high volatility; moreover, these chemicals have strong adsorption characteristics to soil particles. Although seed germination is not inhibited, root growth impacts are the major route of action. Benefin is an example compound used in lettuce and tobacco crops to kill such plants as genera Setaria, Digitaria. Brachiaria and Cenchrus.

Diphenyl ethers

This class of herbicides is regarded as chiefly contact application, since they are not effectively translocated by plants. Acifluorfen, an example of this class, is used to kill Brassica kaber, Datura stramonium, Sesbania exaltata and Sorghum halepense. Bifenox is used to control certain broadleaf weeds such as Kochia scoparia and Datura carota.

Imidazolidinones and imidazolinones

The imidazolidinone buthidazole acts against a plethora of broadleaf and grass species including Elytrigia repens and Teraxacum offinale. Imazaquin is an example of the imidazolinone family, which acts on a wide range of broadleaf plants including member of the genera Ipomoea and Euphorbia.

Imines

A chief example of this class is CGA-248757, which is used at above ground control of certain broadleaf species; it has no effect in the root zone. Species killed by this herbicide include Arbutilon theophrasti, and the compound is sufficiently potent that three grams per hectare may be utilized for effective control.

N-Phenylphthallmides

Flumiclorac is a premier example of this herbicide class. Target plants evince severe damage symptoms within 24 hours of application in conditions of intense sunlight. Example weeds that are killed by this chemical are Amaranthus species and Euphorbia maculata.

Nitriles

Bromoxynil is a good example of a nitrile herbicide, with efficacy against a gamut of broad leaved species including Chorispora tenella, Chenopodium album, and Capsella bursa-pastoris. Besides use for rye, triticale, oats, sorghum, maize and wheat, this compound is also used in a wide variety of industrial, rail right-of-way and roadside applications. Dichlobenil is used against certain plants with rhizomal propagation such as Cyperus rotunda, but also for more conventionally rooted species such as Poa annua.

Organic arsenicals

These chemicals are applied in foliar exposures and are considered among the most persistent herbicides; furthermore, they are strongly adsorbed by soils and have a low mobility in all soils except very sandy varieties, where they may offer moderate mobility. Example compounds include cacodylic acid and monosodium methanearsonate.

Organo-chlorines

There are a large variety of organo-chlorines such as DDT. These substances tend to be low in water solubility and accordingly high in biomagnification, making them toxic to many animal species.  While these substances are banned for commercial use in the USA and some other western countries, they remain in widespread use in developing countries.

Oxadiazoles and oxydiazolidines

The oxadiazole class is typified by oxadiazon, a chemical used in agricultural applications for tea, banana, rubber, onion, garlic, potato and rice. This herbicide is most often used against bulb, rhizome and other deep rooted species, including Cynodon dactylon and Lollium perenne; it is also applied for killing of a variety of genus Urtica taxa. When used a post-emergent, oxadiazon is only effective in the very early growth stage of the target species. Methazole is a representative of the oxydiazolidines, and is used for a range of annual and perennial weeds for diverse crops such as grapes, oats, maize, wheat and soybeans.

Phenols

The first organic herbicide used commercially was a phenolic molecule, introduced in 1932. While that particular cresol is no longer in broad usage, modern cousins are quite widely applied. For example, dinoseb, the most toxic of the substituted dintrophenols, is employed in bean, pea and potato crops.

Phenylpyridazines

Pyridate, an example of the phenylpyridazines, is used in agricultural settings which produce onions, rapeseed, alfalfa and grapes. Usage of this compound started in farms in the year 1974. Initial damage to plants is noted by withering at leaf edges within one day, and the death is complete after four to nine more days.

Phenyl Triazinones

Sulfentrazone is one of the newer herbicides, appearing only in the year 1991. Sida spinosa, Cucumis melo and Cassia occidentalis are target species for killing. By adding a surfactant to the formulation, sulfentrazone can also be applied for control of cyperus esculentus.

Phthalamates

Naptalam can be applied either in pre- or post-emergent methods attacking broadleaf species such as Xanthium strumarium and certain annual grasses such as Setaria faberi. This herbicide is specifically used for cucumber, watermelon, and other melon crops. Targeted weeds experience a pronounced antigeotrophic syndrome, where roots fail to achieve the customary downward curvature.

Phosphonic acid derivatives

By far the most widely used chemical in this class is glyphosate, whose mode of killing is inhibition of an enzyme involved in the synthesis of the amino acids tyrosine, tryptophan and phenylalanine. It is absorbed through leaves and transported to growing points. Because of this route of action, flyphosate is effective only on actively growing plants, and not useful as a preemergent. gluphosinate is widely lethal to a wide range of plants and is suitable only for use for certain crops which have been genetically engineered.

Pyrazollums

Difenzoquat can be viewed as an example of the pyrazollum class. This herbicide is employed in post-emergent applications to control wild oats; its efficacy is enhanced by formulation with a non-ionic surfactant. The pathology of treated plants follows a course of chlorotic symtoms to meristems, followed by necrosis.

Pyridazinones and pyridinones

The pyridazinones typically attack the photsynthetic function, with a noticable whitening of leaf vein tissue. Norflurizon is an example compound in this class of fluorinated organics. This pre-emergent chemical is used to kill a number of plant species including the grasses Eleusine indica and Panicum dichotomiflorum, as well as broadleaf taxa such as Cirsium arvense and species within the genus Portulaca. Fluridone was developed as an aquatic herbicide for application on the water surface or benthic environment; the kill is effective for members of the genera Najas, Elodea, Potamageton and Utricularia.

Pyridines and related nitrogenous compounds

Pyrichlor is a notable pyridine herbicide whose uses include roadside and industrial land uses. Thiazophyr is another widely used compound in this class, whose uses include a number of agricultural applications such as soybeans, cotton, vineyards, peanuts and certain fruit crops. Paraquat and diquat are other widely applied herbicides in this class; both are effective applied on above ground plant parts and are fast acting. 

Quinollinecaroxylic acids

Quinclorac is an auxin type herbicide that is chiefly deployed in both transplanted rice seedlings and direct seeded rice farming. It is effective against species such as convolvulus arvensis and Echinocloa crus-galli. Quinclorac is a persistent chemical in the soil environment, containing potentially damaging levels of phytotoxins to any future crops rotated into the treated zone at least a year from application.

Tetrahydropyrimidinones

These compounds are expected to be most useful in controlling a broad range of monocots, dicots and sedges in protecting rice crops. Their bleaching action is similar to fluoride compounds in the pyridazinone class. Specific species killed by include Scirpus juncoides, Cyperus difformis, Monochoria vaginalis, Cyperus serotinus and Eleocharis acicularis.

Triazinones

Metribuzin, as an example of the triazinone class, is highly toxic to many broadleaf annuals including Datura stramonium and members of the genus Brassica.

Triazoles

The triazoles exert plant killing by bleaching action exerted on leaves and shoots. Amitrole is a prominent representative of this class, and causes particular damage to meristems.

Triazolopyrimidine sulfonanilides

Flumetsulam is a prime representative of this herbicide class, and it exhibits damage mechanisms of chlorosis similar to other fluorine bearing herbicides. Most of the weed species that are acted upon by flumetsam are broadleaved plants and not grasses.

Uracils

Substituted uracils have been known to demonstrate herbicidal action for almost five decades. Prominent examples of this class are terbicil and bromacil. The lithium salts of bromacil are utilized in citrus and pineapple farming applications; this formulation is also used in brush control, and in control of individual broadleaf species such as Tribulus terrestris.

Inorganic chemicals

Sodium compounds

The earliest known use of herbicides was in 146 BC, when the Romans sterilized agricultural lands of Carthage using sodium chloride. Between 1896 and the 1930s a variety of sodium salts came into usage including sodium chlorate, sodium arsenite, sodium nitrite, sodium metaborate and sodium tetraborate; none of these remain in systematic use in modern times.

Heavy metal sulphates

Arriving in the marketplace at a similar timeframe of the early 1900s copper and ferrous sulfates were used until the dominance of organic compounds arose in the 1940s.

Low biological impact methods

In the past several decades there is emerging interest in reducing the widespread ecological impacts of chemicals of historic herbicide usage. Thus some research has focussed upon formulations that avoid acute toxicity to organisms, especially non-target species. An example of such a herbicide is corn meal gluten, which has virtually no toxicity, but acts by inhibiting seed germination in certain plant species.

References

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Citation

Hogan, C. (2014). Herbicide. Retrieved from http://www.eoearth.org/view/article/51cbefe97896bb431f69f723

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