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Resistance to glyphosate

Glyphosate is already the world's most used herbicide. Its use is anticipated to increase even further with the introduction of genetically engineered, glyphosate resistant crops. Topsy Jewell reports on the environmental implications of introducing this new technology.

Glyphosate was developed by the US multinational Monsanto. World-wide sales are worth over US$1,500 million annually and account for 50% of the company's agricultural turn-over. Growth in sales is expected to increase to $2,000 million over the next 5 years, equivalent to over 40,000 tonnes of active ingredient. Use of glyphosate in the European Union increased by 129% between 1991 and 1995. Its popularity has been encouraged by the claims of Monsanto, that glyphosate is not harmful to humans and is environmentally safe. 
    However, since its introduction over 25 years ago, scientists have developed a much greater understanding and appreciation of the complexity of the effects chemical contaminants have in the human body and ecological systems. Correspondingly, researchers have employed more elaborate and sensitive tests to evaluate the impacts of glyphosate. The result is that there is now a strong body of independent research that demonstrates that glyphosate is a harmful chemical. These results are summarised in the table below. In addition, the authors of the research studies reviewed in this report consistently remarked that there is an enormous lack of information on the effects of glyphosate in the environment and difficulty in measuring subtle, long term impacts.
    Monsanto claims that the use of glyphosate-tolerant crops will reduce the quantity of  older, more toxic herbicides used by farmers. In the short term, the use of herbicides may appear to decline. In the long term, however, there are other considerations which will affect the levels of herbicides and possibly other pesticides used. These include:

  • The development of glyphosate resistance in weeds or the use of practices to minimise the risks of weed resistance will perpetuate the practice of applying mixtures of herbicides that have a range of adverse environmental impacts. 

  • The presence of glyphosate-resistant  volunteers (unintentionally grown crops) will require the use of additional  applications of herbicides other than glyphosate. The Danish government has expressed concern that the greater volumes and varieties of herbicides required in removing volunteer and feral plants will have impacts on soil and groundwater.

  • The impact on pest insect populations and diseases after long term use of glyphosate may mean that farmers are increasing the use of insecticides and fungicides.

Rather than making significant reductions in pesticide use, in the long term, herbicide resistant crops will intensify chemical dependence in agriculture. This happens at a time when the adverse impacts of herbicides has stimulated investment in research and development of weed control practices that dramatically reduce reliance on herbicides. 
    Moreover, it is not even clear the glyphosate tolerant crops are going to produce higher yields. Herbicide resistant Roundup Ready cotton was introduced commercially in the US in 1997. In the first growing season, 30,000 acres (12,000 ha) failed. A quarter of the 200 farmers licensed to grow the cotton found that cotton bolls were deformed and bolls were dropping off early.
    The risks and benefits of glyphosate herbicides and the introduction of glyphosate-resistant crops therefore can no longer be evaluated in isolation, but must be seen in the light of other approaches that significantly reduce herbicide use. The perceived benefits of glyphosate-resistant crops may prove non-essential where other, more environmentally beneficial, methods can be applied.

Monsanto's Claims

Independent Research Findings

Roundup has a low irritational potential for eye and skin and otherwise is not a risk to human health.

Roundup is amongst the top most reported pesticides causing poisoning incidents (mainly skin irritation) in several countries. It also causes a range of acute symptoms including, recurrent eczema, respiratory problems, elevated blood pressure, allergic reactions.

Roundup does not cause any adverse reproductive effects

In laboratory tests on rabbits glyphosate caused long lasting, harmful effects on semen quality and sperm counts.

Roundup is not mutagenic in mammals.

DNA damage has been observed in laboratory experiments in mice organs and tissue.

Roundup is environmentally safe.

  • In the agricultural environment, glyphosate is toxic to some beneficial soil organisms, beneficial arthropod predators, and increases crops' susceptibility to diseases.

  • Sub-lethal doses of glyphosate from spray drift damages wildflower communities and can affect some species up to 20 metres away from the sprayer.

  • The use of glyphosate in arable areas may cause dieback in hedgerow trees.

Roundup is rapidly inactivated in soil and water.

  • Glyphosate is very persistent in soils and sediments.

  • Glyphosate inhibited the formation of nitrogen fixing nodules on clover for 120 days after treatment.

  • Glyphosate residues were found in lettuce, carrot, and barley when planted a year after glyphosate was applied.

Roundup is immobile and does not leach from soils.

  • Glyphosate can readily desorb from soil particles in a range of soil types. It can be extensively mobile and leach to lower soil layers.

  • Glyphosate can be carried by soil particles suspended in run off.

Roundup does not contaminate drinking water when used by local authorities on hard surfaces.

In the UK, levels of glyphosate above the EU limit have been detected by the Welsh Water Company every year  since 1993. The Drinking Water Inspectorate recommends that glyphosate be monitored, particularly, in areas where it is used by local authorities on hard surfaces.

It is nearly impossible for glyphosate resistance to evolve in weeds.

In 1996, glyphosate resistant ryegrass was discovered in Australia. 

Outcrossing in oilseed rape crops (and the transfer of genes from transgenic crops) occurs over a short distance and can be easily managed.

The densities of oil seed rape pollen are much higher and their dispersal patterns differ from around large fields compared to those found in experimental plots. Wind dispersal of  pollen occurs over much greater distances and at higher concentrations than predicted by experimental plots. Significant levels of gene flow from transgenic oil seed crops is inevitable.

Roundup Ready crops will  reduce levels of herbicide use.

Herbicide resistant crops will intensify and increase dependency on herbicide use in agriculture rather than lead to any significant reductions. A variety of herbicides will have to be reintroduced to control glyphosate resistant volunteers, feral populations of crops and resistant weeds.

Source: References cited in Health and Environmental Impacts of Glyphosate, 
(Details available from the Pesticides Trust [now PAN UK]).

Health and Environmental Impacts of Glyphosate is a report by the Pesticides Trust [now PAN UK]  produced for Friends of the Earth.

Topsy Jewell is an independent environmental researcher and a member of the Pesticides Trust Board.

 [This article first appeared in Pesticides News No. 41, September 1998, page 5]
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