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Glyphosate

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Glyphosate, N-phosphonomethyl glycine, is broad-spectrum herbicide, sold under the brand name RoundUp. It is "the most widely used broad-spectrum herbicide on [a] global scale."[1] Commercial formulations of glyphosate were first sold in 1974.[2] Some genetically engineered crops, called Roundup Ready Crops, have their DNA altered to allow them to withstand glyphosate. These include Roundup Ready soybeans, corn, canola, sugarbeets, and alfalfa.

Contents

History

Glyphosate's usefulness as an herbicide was discovered by Monsanto scientist John E. Franz in 1970.[3] It was first introduced in the herbicide Roundup by Monsanto in 1974.[4] Glyphosate is the active ingredient in Roundup, but the product also includes water and a surfactant, polyoxyethylene-alkylamine (POEA), which allows the herbicide to adhere to a plant's leaves so that the active ingredient can penetrate them. As of 2005, Monsanto's glyphosate products alone were registered in more than 130 countries for use in more than 100 crops.

Much of Roundup's success is due to the perception that it is safe and nontoxic, as well as the fact that it is effective against so many species of plants. However, its safety is the subject of controversy. In 2005, Monsanto wrote:

"Glyphosate binds tightly to most types of soil so it is not available for uptake by roots of nearby plants. It works by disrupting a plant enzyme involved in the production of amino acids that are essential to plant growth. The enzyme, EPSP synthase, is not present in humans or animals, contributing to the low risk to human health from the use of glyphosate according to label directions."[4]

Roundup has won awards due to its perception of effectiveness and safety. In 1987, Franz received the National Medal of Technology for his work on glyphosate. In September 1994, Farm Chemicals magazine named Roundup one of the "Top 10 Products That Changed the Face of Agriculture." In 1996, Monsanto received the Presidential Award for Sustainable Development for "pioneering sustainable technologies" including glyphosate. The same year, Monsanto received the Presidential Green Chemistry Challenge Award for "environmentally responsible systems used in the manufacture of glyphosate herbicides.[4]

Glyphosate Products

Glyphosate is used in a number of herbicide products including:

How It Works

Glyphosate is absorbed through a plant's foliage and then transported throughout the stems, leaves, and roots of the entire plant.

"Glyphosate inhibits plant growth by inhibiting the production of essential aromatic amino acids through competitive inhibition of the enzyme enolpyruvylshikimate phosphate (EPSP) synthase. This is a key enzyme in the shikimic acid pathway for the synthesis of chorismate..., which is a precursor for the essential amino acids phenylalanine, tyrosine, and tryptophan."[5]

In other words, glyphosate prevents plants from making amino acids they need to survive. It does this by inhibiting an enzyme needed to make chorismate, a precursor to those amino acids. One of these amino acids, tryptophan, "is necessary for the synthesis of indolylacetic acid (IAA), the main growth promoter, that can explain the widespread field observation of reduced in depth root growth of plants."[6]

A 1984 study found plants that died following treatment with glyphosate were infected with pathogenic fungi, compared to control plants not treated with glyphosate but planted in the same media that did not yield pathogenic fungi.[7] The study concluded that more research was needed but postulated that glyphosate inhibits the plant's defense mechanisms and/or increases nutrient leakage from treated plants.

When more research was completed, "Rahe and coworkers documented that severe root infection associated with glyphosate-treated plants was due to disruption of synthesis of plant defense compounds, or phytoalexins, through the shikimate pathway there by predisposing plants to attack by soilborne fungal pathogens (Johal and Rahe, 1988; Lévesque et al., 1987).[8][9] Thus, infection by soilborne pathogens caused by the inability of plants to synthesize phytoalexins contributed to the overall herbicidal efficacy of glyphosate and was considered a “secondary mode of action” of glyphosate. These findings were significant because the release of glyphosate into the environment was found to have considerably more and far-reaching effects than the original notion that was limited to only the localized disruption of a specific metabolic pathway within a target plant."[10]

Impact on Non-Target Plants

Once glyphosate travels to a plant's roots, it is "released into the rhizosphere," (the area immediately around the roots), "where it is immobilized at the soil matrix or microbially degraded.[11] However, some of the glyphosate remains in dead plant tissues. A 2009 study found that non-target plants continue to be impacted by glyphosate toxicity up to three weeks after glyphosate application.[12]

A 2009 study examined the relationship between previous glyphosate use, tillage method (conventional, minimal, or no-till), and plant diseases caused by fungal pathogens of the genus Fusarium.[13] The study found "a relationship between previous glyphosate use and increased Fusarium infection of spikes and subcrown internodes of wheat and barley, or Fusarium colonization of crop residues." However, the study adds, "because of the close association between noncereal crops, reduced tillage, and glyphosate use, it was not possible to completely separate the effects of these factors on Fusarium infections." (No-till or minimum till practices are often combined with herbicide use, specifically, glyphosate use. Thus, the scientists were not able to determine whether any increase or decrease in plant disease was related to tillage strategy, glyphosate use, or both.) The study recommended more research in this area.

Impacts of Glyphosate Drift

At sub-lethal doses of glyphosate, such as the amounts a plant might be exposed to from spray drift, plants are still impacted. A study examined sunflowers treated with small amounts of glyphosate (to simulate spray drift) found:[14]

"In conclusion, the results presented in this study showing that glyphosate is especially inhibitory to ferric reductase complement the recently published report (Eker etal., 2006) that glyphosate exerts a strong inhibitory influence on ferric reductase activity of Fe-deficient roots and impairs the uptake and translocation of Fe in plants. These impairments could be a major reason for the increasingly observed Fe deficiency chlorosis in cropping systems associated with widespread glyphosate usage as reported for different parts of the USA (Franzen etal., 2003; Jolley etal., 2004). Such strong interference of glyphosate with root uptake and root-to-shoot transport of Fe in crop plants may represent a potential threat to human and animal nutrition because of possible reduction of Fe in edible plants parts (e.g. seed/grain)."

In other words, low doses of glyphosate equal to the amount plants are exposed to in spray drift, can result in iron deficiencies in the plants. For crops destined as animal feed or as human food, this could result in decreased dietary iron.

Impact on Microorganisms

Several sources have found that glyphosate impacts soil microorganisms. For example:

"Subsequent research on glyphosate interactions with soil microorganisms demonstrated that although glyphosate was metabolized by a segment of the microbial population, it was also toxic to several bacteria and fungi; the net effect glyphosate appeared to be a disruption of soil and root microbial community composition because selected components of the microbial community were stimulated while others were suppressed."[15][16][17]
"Herbicides, including glyphosate, can inhibit or stimulate the growth of fungal pathogens, and can either increase or decrease disease development through direct or indirect means (Altman, 1993; Levesque and Rahe, 1992).[18][19] Levesque and Rahe (1992) showed evidence that herbicides can have a direct effect on various components of the soil microflora, such as plant pathogens, antagonists, or mycorrhizae, which can potentially increase or decrease the incidence of plant disease. Pathogens able to infect weeds can also increase their inoculum potential after weeds have been sprayed with herbicides, which could subsequently affect host crops."[20]

Glyphosate in the Environment

Although glyphosate is highly soluble in water, its tendency to bond to soils makes it unlikely to leach into groundwater or runoff "significantly." (Studies have found about 1%-2% of glyphosate may runoff in rainfall after glyphosate is applied.[21])

Glyphosate in the Soil

Glyphosate can reach the soil by washing off the foliage of plants, via spray drift, by exudation from the roots of treated plants, or by the decomposition of treated plants. However, "risks of glyphosate toxicity to non-target organisms in soils are generally considered as marginal,since glyphosate is almost instantaneously inactivated by adsorption to clay minerals and cationic binding sites of the soil matrix (Piccoloetal.,1992;Dong-Meietal.,2004), while glyphosate in the soil solution is prone to rapid microbial degradation (Giesy et al., 2000)."[22][23] In other words, glyphosate residues in the soil are not considered hazardous as it either breaks down quickly or binds to minerals that make it no longer a threat to plants. Glyphosate that biodegrades usually breaks down into carbon dioxide and ammonium (NH4+).[24] In an analysis of 47 studies, 50% of glyphosate broke down in the soil in time periods ranging from 1.2 days to 197.3 days. The arithmetic mean amount of time was 32 days and the geometric mean was 17 days.[25]

Although glyphosate is mostly broken down by microbes or bound to the soil, a 2009 study found that "the root tissue of glyphosate-treated weeds represents a storage pool for glyphosate."[26]

Glyphosate in Water

Although most glyphosate applied to soil does not run off into waterways, sometimes glyphosate is applied to aquatic environments directly. In flowing water, it is dissipated via "tributary dilution, dispersion, and loss through processes such as absorption to suspended particulate matter or sediments and microbial degradation."[27] The half-life of glyphosate in water has been estimated to be from 7 to 14 days.[28]

Articles and resources

Related SourceWatch articles

References

  1. ↑ Tsehaye Tesfamariam, S. Bott, I. Cakmak, V. Römheld, G. Neumann, "Glyphosate in the rhizosphere – role of waiting times and different glyphosate binding forms in soils for phytoxicity to non-target plants," European Journal of Agronomy (2009), 31:126-132.
  2. ↑ John P. Giesy, Stuard Dobson, and Keith R. Solomon, 2000, "Ecotoxicological Risk Assessment for Roundup Herbicide," Rev Environ Contam Toxicol 167:35-120.
  3. ↑ Inventor of the Week: Roundup, Accessed July 12, 2012.
  4. ↑ 4.0 4.1 4.2 History of Monsanto's Glyphosate Herbicides, Monsanto, 2005.
  5. ↑ John P. Giesy, Stuard Dobson, and Keith R. Solomon, 2000, "Ecotoxicological Risk Assessment for Roundup Herbicide," Rev Environ Contam Toxicol 167:35-120.
  6. ↑ Preface "Glyphosate interactions with physiology, nutrition, and diseases of plants: Threat to agricultural sustainability?", European Journal of Agronomy 31 (2009) 111-113.
  7. ↑ Gurmukh S. Johal and James E. Rahe, "Effect of soilborne plant-pathogenic fungi on the herbicidal action of glyphosate on bean seedlings," Phytopathology (1984), 74:950-955.
  8. ↑ Johal, G.S., Rahe, J.E., 1988. Glyphosate, hypersensitivity and phytoalexins accumulation in the incompatible bean anthracnose host-parasite interaction. Physiol. Mol. Plant Pathol. 32, 267-281
  9. ↑ Lévesque, C.A., Rahe, J.E., Eaves, D.M., 1987. Effects of glyphosate on Fusarium spp.: its influence on root colonization of weeds, propagule density in the soil, and crop emergence. Can. J. Microbiol. 33, 354-360.
  10. ↑ Preface "Glyphosate interactions with physiology, nutrition, and diseases of plants: Threat to agricultural sustainability?", European Journal of Agronomy 31 (2009) 111-113.
  11. ↑ Tsehaye Tesfamariam, S. Bott, I. Cakmak, V. Römheld, G. Neumann, "Glyphosate in the rhizosphere – role of waiting times and different glyphosate binding forms in soils for phytoxicity to non-target plants," European Journal of Agronomy (2009), 31:126-132.
  12. ↑ Tsehaye Tesfamariam, S. Bott, I. Cakmak, V. Römheld, G. Neumann, "Glyphosate in the rhizosphere – role o If waiting times and different glyphosate binding forms in soils for phytoxicity to non-target plants," European Journal of Agronomy (2009), 31:126-132.
  13. ↑ M.R. Fernandez, R.P. Zentner, P. Basnyat, D. Gehl, F. Selles, and Don Huber, "Glyphosate associations with cereal diseases caused by Fusarium spp. in the Canadian Prairies," European Journal of Agronomy (2009), 31:133-143.
  14. ↑ Levent Ozturk, Atilla Yazici, Selim Eker, Ozgur Gokmen, Volker Römheld, and Ismail Cakmak, "Glyphosate inhibition of ferric reductase activity in iron deficient sunflower roots," New Phytologist (2008), 177:899-906.
  15. ↑ Preface "Glyphosate interactions with physiology, nutrition, and diseases of plants: Threat to agricultural sustainability?", European Journal of Agronomy 31 (2009) 111-113.
  16. ↑ Wardle, D.A., Parkinson, D.A., 1992. Influence of the herbicides 2,4-D and glyphosate on soil microbial biomass and activity: a field experiment. Soil Biol. Biochem. 24, 185-186.
  17. ↑ Busse, M.D., Ratcliffe, A.W., Shestak, C.J., Powers, R.F., 2001. Glyphosate toxicity and the effects of long-term vegetation control on soil microbial community. Soil Biol. Biochem. 33, 1777-1789.
  18. ↑ Altman, J., 1993. Pesticide-pathogen interactions in plant disease. In: Altman, J.(Ed.), Pesticide Interactions in Crop Production, Beneficial and Deleterious Effects. CRC Press, Inc., Boca Raton, FL, pp. 315–332.
  19. ↑ C. André Lévesque and James E. Rahe, Herbicide Interactions with Fungal Root Pathogens, with Special Reference to Glyphosate, Annual Review of Phytopathology (1992), Vol. 30: 579-602.
  20. ↑ M.R. Fernandez, R.P. Zentner, P. Basnyat, D. Gehl, F. Selles, and Don Huber, "Glyphosate associations with cereal diseases caused by Fusarium spp. in the Canadian Prairies," European Journal of Agronomy (2009), 31:133-143.
  21. ↑ John P. Giesy, Stuard Dobson, and Keith R. Solomon, 2000, "Ecotoxicological Risk Assessment for Roundup Herbicide," Rev Environ Contam Toxicol 167:35-120.
  22. ↑ Tsehaye Tesfamariam, S. Bott, I. Cakmak, V. Römheld, G. Neumann, "Glyphosate in the rhizosphere – role of waiting times and different glyphosate binding forms in soils for phytoxicity to non-target plants," European Journal of Agronomy (2009), 31:126-132.
  23. ↑ John P. Giesy, Stuard Dobson, and Keith R. Solomon, 2000, "Ecotoxicological Risk Assessment for Roundup Herbicide," Rev Environ Contam Toxicol 167:35-120.
  24. ↑ John P. Giesy, Stuard Dobson, and Keith R. Solomon, 2000, "Ecotoxicological Risk Assessment for Roundup Herbicide," Rev Environ Contam Toxicol 167:35-120.
  25. ↑ John P. Giesy, Stuard Dobson, and Keith R. Solomon, 2000, "Ecotoxicological Risk Assessment for Roundup Herbicide," Rev Environ Contam Toxicol 167:35-120.
  26. ↑ Tsehaye Tesfamariam, S. Bott, I. Cakmak, V. Römheld, G. Neumann, "Glyphosate in the rhizosphere – role of waiting times and different glyphosate binding forms in soils for phytoxicity to non-target plants," European Journal of Agronomy (2009), 31:126-132.
  27. ↑ John P. Giesy, Stuard Dobson, and Keith R. Solomon, 2000, "Ecotoxicological Risk Assessment for Roundup Herbicide," Rev Environ Contam Toxicol 167:35-120.
  28. ↑ John P. Giesy, Stuard Dobson, and Keith R. Solomon, 2000, "Ecotoxicological Risk Assessment for Roundup Herbicide," Rev Environ Contam Toxicol 167:35-120.

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