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Glyphosate

[glahy-fos-eyt]

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Glyphosate (N-(phosphonomethyl) glycine) is a non-selective systemic herbicide, absorbed through the leaves, injected into the bole, or applied to the stump of a tree, used to kill weeds, especially perennials and broadcast or used in the cut-stump treatment as a forestry herbicide. Some crops have been genetically engineered to be resistant to it. Glyphosate was first sold by Monsanto under the tradename Roundup, but is no longer under patent. It is now also available in other formulations, e.g. Resolva 24H, which contains glyphosate and diquat.

Chemistry

Glyphosate is an aminophosphonic analogue of the natural amino acid glycine and the name is a contraction of glycine, phospho-, and -ate. Glyphosate was first discovered to have herbicidal activity in 1970 by John E. Franz, while working for Monsanto. Franz received the National Medal of Technology in 1987 , and the Perkin Medal for Applied Chemistry. in 1990 for his discoveries.

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. These amino acids are used as building blocks in peptides, and to produce secondary metabolites such as folates, ubiquinones and naphthoquinone. 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 and also has been found to affect animal enzymes.

Use

Glyphosate is effective in killing a wide variety of plants, including grasses, broadleaf, and woody plants. It has a relatively small effect on some clover species. By volume, it is one of the most widely used herbicides. It is commonly used for agriculture, horticulture, and silviculture purposes, as well as garden maintenance (including home use).

Glyphosate is supplied in several formulations for different uses:

Ammonium salt.
Isopropyl amine salt.
Glyphosate acid - standalone, as ammonium salt or as isopropyl 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.

Genetically modified crops

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. This CP4 EPSPS gene was cloned and transfected into soybeans, and in 1996, genetically modified soybeans were made commercially available. This greatly improved the ability to control weeds in soybean fields since glyphosate could be sprayed on fields without hurting the crop. As of 2005, 87% of U.S. soybean fields were planted with glyphosate resistant varieties. The use of such Roundup Ready crops has changed the herbicide use profile away from atrazine, metribuzin and alachlor. This has the benefit of reducing the dangers of herbicide run off into drinking water.

Other uses

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.

Toxicity

Glyphosate is less toxic than a number of other herbicides and pesticides, such as those from the organochlorine family.

Humans

A review of the literature in 2000 concluded that "under present and expected conditions of new use, there is no potential for Roundup herbicide to pose a health risk to humans". This review considered the likely effects experienced by the two groups most likely to have high exposures, herbicide applicators and children aged 1-6, noting the exposure in those subpopulations was not a health concern. Glyphosate has an EPA Toxicity Class of III in 1993, but more recent studies suggest that IV is appropriate for oral, dermal, and inhalation exposure. It has been rated as class I (Severe) for eye irritation, however.

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.

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. Statistics from the Californian Environmental Protection Agencies Pesticide Illness Surveillance Program indicate that glyphosate related incidents are one of the highest reported of all pesticides. However, incident count does not take into account the number of people exposed and the severity of symptoms associated with each incident. For example if hospitalization is 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.

Greenpeace states that "the acute 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.

Other species

The direct toxicity of pure glyphosate to mammals and birds is low. In vitro studies indicate glyphosate formulations could harm earthworms and beneficial insects. However, these 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.

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. Monsanto produces glyphosate products with alternative surfactants that are specifically formulated for aquatic use, for example "Biactive" and "AquaMaster". According to Monsanto, "Conservation groups have chosen glyphosate formulations because of their effectiveness against most weeds as glyphosate has very low toxicity to wildlife".

When glyphosate comes into contact with the soil, it rapidly binds to soil particles and is inactivated. 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.

Mammal research indicates oral intake of 1% glyphosate induces changes in liver enzyme activities in pregnant rats and their fetuses.

Endocrine disruptor debate

In vitro studies have shown glyphosate affects progesterone production in mammalian cells and can increase the mortality of placental cells. Whether these studies classify glyphosate as an endocrine disruptor is a matter of debate.

Some 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 not occur in an entire organism. Additionally, current in vitro studies expose cell lines to concentrations orders of magnitude greater than would be found in real conditions, and through pathways that would not be experienced in real organism.

Others 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 topic has been planned, and should shed more light on the debate.

References

EU (2002). Review report for the active substance glyphosate Retrieved October 28, 2005.
JP Giesy, KR Solomon, S Dobson (2000). "Ecotoxicological Risk Assessment for Roundup Herbicide". Reviews of Environmental Contamination and Toxicology 167 35–120.
KR Solomon, DG Thompson (2003). "Ecological risk assessment for aquatic organisms from over-water uses of glyphosate". Journal of Toxicology and Environmental Health 6 289–324.
Environmental Health Criteria 159: Glyphosate. World Health Organization, (1994).
History of Glyphosate. Monsanto.