glyphosate introduction
February 24th, 2010Glyphosate (N-(phosphonomethyl) glycine) is a broad-spectrum systemic herbicide used to kill weeds, especially perennials. It is typically sprayed and absorbed through the leaves, injected into the trunk, or applied to the stump of a tree, or broadcast or used in the cut-stump treatment as a forestry herbicide. Initially patented and sold by Monsanto Company in the 1970s under the tradename Roundup, its U.S. patent expired in 2000. Glyphosate is the most used herbicide in the USA.[3] In the US, 5-8 million pounds are used every year on lawns and yards and 85-90 million pounds are used annually in US agriculture.[3]
Glyphosate’s mode of action is to inhibit an enzyme involved in the synthesis of the amino acids tyrosine, tryptophan and phenylalanine. It is absorbed through foliage and translocated to growing points. Because of this mode of action, it is only effective on actively growing plants; it is not effective as a pre-emergence herbicide.
Some crops have been genetically engineered to be resistant to it (i.e. Roundup Ready). Such crops allow farmers to use glyphosate as a post-emergence herbicide against both broadleaf and cereal weeds. Soy was the first Roundup Ready crop.
Chemistry
Glyphosate is an aminophosphonic analogue of the natural amino acid glycine and the name is a contraction of glycine, phospho-, and -ate. The molecule has several dissociable hydrogens, especially the first hydrogen of the phosphate group. The molecule tends to exist as a zwitterion where a phosphonic hydrogen dissociates and joins the amine group. Glyphosate is soluble in water to 12g/L at room temperature.
Glyphosate was first discovered to have herbicidal activity in 1970 by John E. Franz, while working for Monsanto.[4] Franz received the National Medal of Technology in 1987,[5] and the Perkin Medal for Applied Chemistry[6] in 1990 for his discoveries.
Formulas and tradenames
Although the Roundup trademark is registered with the US Patent Office and still extant, the patent has expired. Glyphosate is marketed in the US and worldwide in different solution strengths under many tradenames[7]: Roundup, Buccaneer, Razor Pro (41%), Genesis Extra II (41% w/ Surfactant), Roundup Pro Concentrate (50.2 %), Rodeo (51.2%), Aquaneat (53.8%), and Aquamaster (53.5%)[8]. These products may contain other ingredients, causing them to have different effects. For example, Roundup was found to have different effects to glyphosate alone.[9] Roundup is a water-based solution containing glyphosate, a surfactant, and other substances. Other formulations contain additional active ingredients to improve the speed of action. In the UK, Weedol Rootkill Plus, which contains glyphosate and pyraflufen-ethyl, and Resolva 24H[10] which contains glyphosate and diquat, are available.
Biochemistry
Glyphosate kills plants by interfering with the synthesis of the amino acids phenylalanine, tyrosine and tryptophan. It does this by inhibiting the enzyme 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS), which catalyzes the reaction of shikimate-3-phosphate (S3P) and phosphoenolpyruvate to form 5-enolpyruvyl-shikimate-3-phosphate (ESP). ESP is subsequently dephosphorylated to chorismate, an essential precursor in plants for the aromatic amino acids: phenylalanine, tyrosine and tryptophan.[11][12] These amino acids are used as building blocks in peptides, and to produce secondary metabolites such as folates, ubiquinones and naphthoquinone. X-ray crystallographic studies of glyphosate and EPSPS show that glyphosate functions by occupying the binding site of the phosphoenol pyruvate, mimicking an intermediate state of the ternary enzyme substrates complex.[13] The shikimate pathway is not present in animals, which instead obtain aromatic amino acids from their diet. Glyphosate has also been shown to inhibit other plant enzymes[14][15], and also has been found to affect animal enzymes.[16]
Use
Roundup used as an alternative to mowing in an apple orchard in Ciardes, Italy
Glyphosate is effective in killing a wide variety of plants, including grasses, broadleaf, and woody plants.[17] It has a relatively small effect on some clover species[18]. By volume, it is one of the most widely used herbicides.[19] It is commonly used for agriculture, horticulture, and silviculture purposes, as well as garden maintenance (including home use).[19]
Glyphosate is supplied in several formulations for different uses:
- Ammonium salt.
- Isopropyl amine salt.
- Glyphosate acid - standalone, as ammonium salt or as isopropyl salt.
- Potassium salt
Products are supplied most commonly in formulations of 120, 240, 360, 480 and 680g active ingredient per litre. The most common formulation in agriculture is 360g, either alone or with added cationic surfactants.
For 360g formulations, European regulations allow applications of up to 12 litres per hectare for control of perennial weeds such as couch grass. More commonly, rates of 3 litres per hectare are practiced for control of annual weeds between crops[20].
Genetically modified crops
In 1996, genetically modified soybeans were made commercially available.[21] Current Roundup Ready crops include soy, maize (corn), sorghum, canola, alfalfa, and cotton, with wheat still under development. These cultivars greatly improved conventional farmers’ ability to control weeds, since glyphosate could be sprayed on fields without damaging the crop. As of 2005, 87% of U.S. soybean fields were planted with glyphosate-resistant varieties.[22][23]
While the use of Roundup Ready crops may have increased the usage of herbicides measured in pounds applied per acre[24], its use has changed the herbicide use profile away from atrazine, metribuzin, and alachlor. This has the benefit of reducing the dangers of herbicide runoff into drinking water.[25][26]
In 1999, a review of Roundup Ready soybean crops found that, compared to the top conventional varieties, they had a 6.7% lower yield [24]. This so called “yield drag” follows the same pattern observed when other traits are introduced into soybeans by conventional breeding [27] and can not be attributed to the Roundup Ready trait or the GM nature of the crop since Monsanto have recently released Roundup Ready 2 soybeans, which are claimed to yield 7-11% higher than RR version 1 [28]. There have been no reports of “yield drag” with the other Roundup Ready crops maize, sorghum or canola.
Development
Some micro-organisms have a version of 5-enolpyruvoyl-shikimate-3-phosphate synthetase (EPSPS) that is resistant to glyphosate inhibition. The version used in genetically modified crops was isolated from Agrobacterium strain CP4 (CP4 EPSPS) that was resistant to glyphosate.[29][30] This CP4 EPSPS gene was cloned and transfected into soybeans. The CP4 EPSPS gene was engineered for plant expression by fusing the 5′ end of the gene to a chloroplast transit peptide derived from the petunia EPSPS. This transit peptide was used because it had shown previously an ability to deliver bacterial EPSPS to the chloroplasts of other plants. The plasmid used to move the gene into soybeans was PV-GMGTO4. It contained three bacterial genes, two PC4 EPSPS genes, and a gene encoding beta-glucuronidase (GUS) from Escherichia coli as a marker. The DNA was injected into the soybeans using the particle acceleration method. Soybean cultivar A54O3 was used for the transformation. The expression of the GUS gene was used as the initial evidence of transformation. GUS expression was detected by a staining method in which the GUS enzyme converts a substrate into a blue precipitate. Those plants that showed GUS expression were then taken and sprayed with glyphosate, and their tolerance was tested over many generations.
Other uses
Herbicides sprayed over a coca field in Colombia
Glyphosate is one of a number of herbicides used by the United States government to spray Colombian coca fields through Plan Colombia. Its health effects, effects on legal crops, and effectiveness in fighting the war on drugs have been widely disputed.[31] There are reports that widespread application of glyphosate in attempts to destroy coca crops in South America have resulted in the development of glyphosate-resistant strains of coca known as Boliviana negra, which have been selectively bred to be both “Roundup Ready” and larger and higher yielding than the original strains of the plant.[32] However, there are no reports of glyphosate-resistant coca in the peer-reviewed literature. In addition, since spraying of herbicides is not permitted in Colombian national parks, this has encouraged coca growers to move into park areas, cutting down the natural vegetation, and establishing coca plantations within park lands.
Toxicity
Glyphosate is rated least dangerous in comparison to other herbicides and pesticides, such as those from the organochlorine family.[33] Roundup has a United States Environmental Protection Agency (EPA) Toxicity Class of III for oral and inhalation exposure.[34] It has been rated as class I (Severe) for eye irritation.
A recent study, on the other hand, has shown that Roundup formulations and metabolic products cause the death of human embryonic, placental, and umbilical cells in vitro even at low concentrations. The effects are not proportional to Glyphosate concentrations but dependent on the nature of the adjuvants used in the formulation.[35]
Humans
Glyphosate has a United States Environmental Protection Agency Toxicity Class of III in 1993,[34] . It has been rated as class I (Severe) for eye irritation. Glyphosate is being evaluated for effects to unborn fetuses and their development. It is currently on the USEPA Endocrine Disrupter Screening list, published in 2007.[36][37]
Outside its intended use, glyphosate can be lethal. For example, with intentional poisonings there is approximately a 10% mortality for those ingesting glyphosate, compared to 70% for those ingesting paraquat.[38]
Laboratory toxicology studies suggest that other ingredients combined with glyphosate may have greater toxicity than glyphosate alone. For example, a study comparing glyphosate and Roundup found that Roundup had a greater effect on aromatase than glyphosate alone.[9]
Statistics from the California Environmental Protection Agency’s Pesticide Illness Surveillance Program indicate that glyphosate-related incidents are one of the highest reported of all pesticides.[39][40] However, incident count does not take into account the number of people exposed and the severity of symptoms associated with each incident.[40] For example, if hospitalization were used as a measure of the severity of pesticide related incidents, then glyphosate would be considered relatively safe, since, over a 13 year period in California, none of the 515 pesticide-related hospitalizations recorded were attributed to glyphosate.[40]
Greenpeace states that the acute human toxicity of glyphosate is very low, but note that, as mentioned above, other added chemicals (particularly surfactants, e.g. polyoxy-ethyleneamine, POEA), can be more toxic than glyphosate itself.[17] Over-application, or application directly to the soil may impact earthworms.
A review of the toxicological data on Roundup shows that there are at least 58 studies of the effects of Roundup itself on a range of organisms.[41] This review concluded that “for terrestrial uses of Roundup minimal acute and chronic risk was predicted for potentially exposed non-target organisms”. It also concluded that there were some risks to aquatic organisms exposed to Roundup in shallow water. More recent research suggests glyphosate induces a variety of functional abnormalities in fetuses and pregnant rats.[42] Also in recent mammalian research, glyphosate has been found to interfere with an enzyme involved testosterone production in mouse cell culture[43] and to interfere with an estrogen biosynthesis enzyme in cultures of human placental cells.[44]
There is a reasonable correlation between the amount of Roundup ingested and the likelihood of serious systemic sequelae or death. Ingestion of >85 mL of the concentrated formulation is likely to cause significant toxicity in adults. Gastrointestinal corrosive effects, with mouth, throat and epigastric pain and dysphagia are common. Renal and hepatic impairment are also frequent, and usually reflect reduced organ perfusion. Respiratory distress, impaired consciousness, pulmonary oedema, infiltration on chest x-ray, shock, arrythmias, renal failure requiring haemodialysis, metabolic acidosis and hyperkalaemia may supervene in severe cases. Bradycardia and ventricular arrhythmias are often present pre-terminally. Dermal exposure to ready-to-use glyphosate formulations can cause irritation, and photo-contact dermatitis has been reported occasionally; these effects are probably due to the preservative Proxel (benzisothiazolin-3-one). Severe skin burns are very rare. Inhalation is a minor route of exposure, but spray mist may cause oral or nasal discomfort, an unpleasant taste in the mouth, tingling and throat irritation. Eye exposure may lead to mild conjunctivitis, and superficial corneal injury is possible if irrigation is delayed or inadequate.[45]
Other species
The direct toxicity of pure glyphosate to mammals and birds is low.[46][unreliable source?] The acute oral toxicity of Roundup is > 5,000 mg/kg in the rat.[47] It showed no toxic effects when fed to animals for 2 years, and only produced rare cases of reproductive effects when fed in extremely large doses to rodents and dogs. It has not demonstrated any increase in cancer rates in animal studies and is poorly absorbed in the digestive tract. Glyphosate has no significant potential to accumulate in animal tissue.[48][49]
An in vitro study indicates that glyphosate formulations could harm earthworms[50] and beneficial insects.[51] However, the reported effect of glyphosate on earthworms has been criticized.[41] The results conflict with results from field studies where no effects were noted for the number of nematodes, mites, or springtails after treatment with Roundup at 2 kilograms active ingredient per hectare.[52] Glyphosate can negatively affect nitrogen-fixing bacteria,[53] and increase the susceptibility of plants to disease.[54] A 2005 study concluded that certain amphibians may be at risk from glyphosate use.[55]
Certain surfactants used in some glyphosate formulations have higher toxicity to fish and invertebrates, resulting in some formulations of glyphosate not being registered for use in aquatic applications.[56] Monsanto produces glyphosate products with alternative surfactants that are specifically formulated for aquatic use, for example “Biactive” and “AquaMaster”.[57] According to Monsanto, “Conservation groups have chosen glyphosate formulations because of their effectiveness against most weeds as glyphosate has very low toxicity to wildlife”.[58] Glyphosate is used with five different salts, but commercial formulations of it contain surfactants, which vary in nature and concentration. As a result, human poisoning with this herbicide is not with the active ingredient alone, but with complex and variable mixtures.[45]
Glyphosate’s effect on soil life may be limited, because when glyphosate comes into contact with the soil, it rapidly binds to soil particles and is inactivated.[59][60] Unbound glyphosate is degraded by bacteria. Low activity because of binding to soil particles suggests that glyphosate’s effects on soil flora are limited. Low glyphosate concentrations can be found in many creeks and rivers in U.S. and Europe.[citation needed]
The United States Environmental Protection Agency,[59] the EC Health and Consumer Protection Directorate, and the UN World Health Organization have all concluded that pure glyphosate is not carcinogenic. Opponents of glyphosate claim that Roundup has been found to cause genetic damage, citing Peluso et al.[61] The authors concluded that the damage was “not related to the active ingredient, but to another component of the herbicide mixture“.
Mammal research indicates oral intake of 1% glyphosate induces changes in liver enzyme activities in pregnant rats and their fetuses.[62]
Aquatic effects
Fish and aquatic invertebrates are more sensitive to Roundup than terrestrial organisms.[41] Glyphosate is generally less persistent in water than in soil, with 12 to 60 day persistence observed in Canadian pond water, yet persistence of over a year have been observed in the sediments of ponds in Michigan and Oregon.[34]
The EU classifies Roundup as R51/53 Toxic to aquatic organisms, may cause long-term adverse effects in the aquatic environment.[63]
Roundup is not registered for aquatic uses[64] and studies of its effects on amphibians indicate it is toxic to them.[65] Other glyphosate formulations that are registered for aquatic use have been found to have negligible adverse effects on sensitive amphibians.[66]
Endocrine disruptor debate
In vitro studies have shown glyphosate affects progesterone production in mammalian cells[67] and can increase the mortality of placental cells.[9] Whether these studies classify glyphosate as an endocrine disruptor is debated.
Some[who?] feel that in vitro studies are insufficient, and are waiting to see if animal studies show a change in endocrine activity, since a change in a single cell line may or may not impact an entire organism. Additionally, current in vitro studies expose cell lines to concentrations orders of magnitude greater than would be found in expected exposures, and through pathways that would not be typically experienced in real organisms.
Others[who?] feel that in vitro studies, particularly ones identifying not only an effect, but a chemical pathway, are sufficient evidence to classify glyphosate as an endocrine disruptor, on the basis that even small changes in endocrine activity can have lasting effects on an entire organism that may be difficult to detect through whole organism studies alone. Further research on the endocrine effects of glyphosate is ongoing, including through the EPA endocrine screening program on 73 chemicals, published in 2007.
Environmental degradation
When glyphosate comes into contact with the soil, it can be rapidly bound to soil particles and be inactivated.[34] Unbound glyphosate can be degraded by bacteria.[68] However, glyphosate has been shown to increase the infection rate of wheat by fusarium head blight in fields that have been treated with glyphosate.[69]
In soils, half-lives vary from as little as 3 days at a site in Texas to 141 days at a site in Iowa.[70] In addition, the glyphosate metabolite aminomethylphosphonic acid has been shown to persist up to 2 years in Swedish forest soils.[71] Glyphosate adsorption varies depending on the kind of soil.[72]
Resistance in weeds and microorganisms
The first documented cases of weed resistance to glyphosate were found in Australia, involving rigid ryegrass near Orange, New South Wales.[73] Some farmers in the United States have expressed concern that weeds are now developing with glyphosate resistance, with 13 states now reporting resistance, and this poses a problem to many farmers, including cotton farmers, that are now heavily dependent on glyphosate to control weeds.[74][75] Farmers associations are now reporting 103 biotypes of weeds within 63 weed species with herbicide resistance[74][75]. This problem is likely to be exacerbated by the use of roundup-ready crops [76].
Legal cases
False advertising
In 1996 Monsanto was accused of false and misleading advertising of glyphosate products, prompting a law suit by the New York State attorney general.[77]
On Fri Jan 20, 2007, Monsanto was convicted of false advertising of Roundup for presenting Roundup as biodegradable and claiming that it left the soil clean after use. Environmental and consumer rights campaigners brought the case in 2001 on the basis that glyphosate, Roundup’s main ingredient, is classed as “dangerous for the environment” and “toxic for aquatic organisms” by the European Union. Monsanto France planned to appeal the verdict at the time.[78]
Scientific fraud
On two occasions the United States Environmental Protection Agency has caught scientists deliberately falsifying test results at research laboratories hired by Monsanto to study glyphosate.[79][80][81] In the first incident involving Industrial Biotest Laboratories, an EPA reviewer stated after finding “routine falsification of data” that it was “hard to believe the scientific integrity of the studies when they said they took specimens of the uterus from male rabbits”.[82][83][84] In the second incident of falsifying test results in 1991, the owner of the lab (Craven Labs), and three employees were indicted on 20 felony counts, the owner was sentenced to 5 years in prison and fined 50,000 dollars, the lab was fined 15.5 million dollars and ordered to pay 3.7 million in restitution.[85][86] Craven laboratories performed studies for 262 pesticide companies, including Monsanto.
Monsanto has stated that the studies have been repeated, and Roundup’s EPA certification does not now use any studies from Craven Labs or IBT. Monsanto also claims that the Craven Labs investigation was started by the EPA after a pesticide industry task force discovered irregularities.[87]