Getting to grips with glyphosate

Glyphosate has its supporters and detractors. Dr Charlie Reinhardt examines the science behind the world’s most widely used herbicide.

The raging debate on the pros and cons of biotech crops and their supporting technologies is muddled by absolutist views that are both supportive and dismissive of this technology.  Biotech crop technology has been hailed by some as “one-in-a-hundred years’ technology”, and vilified by others in both the scientific fraternity and general public.

Supporters of biotech crops claim that the technology can overcome otherwise insurmountable challenges faced by resource-poor farmers struggling with drought and high weed and pest pressure. Detractors raise concerns about risks to human health, threats to biodiversity, and users of biotech crops being exploited by multinational seed companies.

Adoption of biotech crops is hindered by four main factors: Fear of losing produce market share in countries where the technology is banned; consumer suspicion of scientific knowledge and market share being held by only a few multinational companies; scepticism about the benefits of biotech crops for farmers; and food safety concerns.

As scientists in the agriculture sector, we cannot claim objectivity on a topic so fraught with divergent views, but we can attempt to discover where the preponderance of scientifically sound support lies – for or against biotech crops. In comparing positive and negative environmental effects of biotech crops, both the proven and potential environmental impact should be compared with the impact of the practices and technologies replaced by them.

Virtually all findings published in reputable scientific journals and analyses of these by independent authorities such as the Food and Agriculture Organisation, the UN and the World Health Organisation have found that the environmental benefits of substituting conventional crops with biotech crops are usually substantial. Moreover, they pose neither serious nor persistent toxicity risks.

Of course, this win-win scenario can only be possible if the biotechnologies are used correctly, wisely and conscientiously, but unfortunately this is not always the case. Consider the case of glyphosate herbicide, the world’s most popular and widely used herbicide, which suffers from much unfair, unsubstantiated criticism.

Glyphosate is one of the least toxic pesticides used in agriculture, with a lower acute toxicity than aspirin and many other commonly ingested compounds.

Toxicity of chemical compounds is expressed as LD50 values. LD50 is the amount of the product lethal by ingestion to 50% of a population of test animals or organisms under laboratory conditions. LD50 values are expressed in milligrams of pesticide per kilogram (mg/kg) of body weight, so the larger the LD50 value, the less toxic the chemical, and conversely, the lower the value, the more toxic it is. The LD50 value of glyphosate is >5,000mg/ kg, table salt is 3 000mg/kg, caffeine is 192mg/kg and nicotine is 50mg/kg.

Rapid degrading
Glyphosate degrades more rapidly through microbial action in soil than most herbicides and pesticides, and the molecule’s rapid, strong binding to soil mineral fractions further reduces its potential impact on soil health and the risk of contaminating water resources.

These benign properties of glyphosate are accentuated by the fact that the glyphosate molecule is made up of two components:

  • Glycine (a natural amino acid in all plants);
  • A phosphate group, which means that glyphosate on degradation returns only pure carbon, hydrogen, oxygen, nitrogen and phosphorus to the environment.

Some claim that glyphosate has negative effects on the nutritional status of biotech crops that are glyphosate-resistant. Micronutrient cation concentrations in soil solutions are much greater than would be found for glyphosate in soil solutions. Consider a glyphosate application rate of 1kg/ha (soil concentration = 0,75μg/g soil, assuming incorporation to a depth of 10cm and a soil bulk density of 1,33g/cm3), and an average soil adsorption coefficient of Kd = 100.

The amount of glyphosate in soil solution would be 0,044μmol/l, which is much smaller than typical Mn+2 (manganese), Zn+2 (zinc), Cu+2 (copper), and Fe+3 (iron) concentrations found in soil solutions from productive agricultural soils. In typical agricultural soils in the US, concentrations of Mn+2, Zn+2, Cu+2, and Fe+3 in soil solutions have been found to be on average 480X, 220X, 80X, and 310X greater than the glyphosate in solution, respectively.

Therefore, free cation concentrations, for example Mn+2, would not be reduced appreciably by glyphosate present in soil, even at the highest recommended application rates and assuming that all the glyphosate in solution reacted with Mn.
Moreover, glyphosate degrades rapidly in soil, whereas micronutrients do not (although micronutrient concentrations in soil can fluctuate). The same principles apply to theoretical glyphosate interactions with micronutrients occurring in the plant.

Revolutionary
Glyphosate has revolutionised crop production not only through its connection with glyphosate-resistant crops, better known as Roundup Ready crops, but also because this latter type of biotech cropping system provides support for reduced tillage (minimum- and no-till), a key driver in conservation agriculture.

Reduced tillage and fewer trips across the land to apply herbicide and pesticide help to reduce fossil fuel consumption and the loss of soil through wind and water erosion. According to a 2005 estimate, the use of glyphosate-resistant crops was comparable to a worldwide carbon emission reduction equivalent to removing four million family cars from the road.

New technology
We should not reject biotechnology innovations that have been subjected to intense, extended scientific investigation, as in the case of glyphosate-resistant crops and related biotech crops. Rather, we should remain alert and objective in our assessments of them.

In this respect, glyphosate has been implicated in weed resistance having evolved in certain weeds (32 species globally), but we should consider this in the light of there being more than 200 weed species that have developed resistance to several other herbicides.

Finally, underpinning all these truths is an imperative: best agricultural practices must be in place so that the proven values of biotech crops can be expressed in optimal crop yield and quality.

Dr Charlie Reinhardt is extraordinary professor of weed science in the Department of Plant Production and Soil Science, University of Pretoria, and project leader in the SA Herbicide Resistance Initiative. Phone him on 083 442 3427 or email
[email protected].