Risky Solution? The flawed case against glyphosate.
Written by Ivan Kennedy, AM, FRACI CCHEM
Published in the May/June 2020 edition, Chemistry in Australia, RACI
Contrary to sensational media reports and class actions, global regulators consider glyphosate not to be carcinogenic. Consequences could be dire if market forces dictate its phase-out.
The herbicide glyphosate was released by the Monsanto agrochemical company to the global market, with the trade name of Roundup, in 1974. Because of its unique properties, this new chemical met many of the requirements for an ideal product for controlling weeds, with minimal environmental and human impacts. Yet despite this wonderful beginning, a half century later glyphosate now finds itself accused in courts as harmful, the subject of global class actions.
Glyphosate has a highly specific action in mimicking phosphoenolpyruvate (PEP), substrate to the plant enzyme EPSP synthase. EPSP synthase is essential for shikimate synthesis, a precursor for aromatic amino acids such as phenylalanine and tyrosine and for lignin needed for wood. This enzyme is not found in animals because aromatic amino acids are dietary, explaining this unique specificity of action.
Another advantage is glyphosate’s very low toxicity to animals, with LD50 values greater than those for common salt. (The LD50 value is the acute dose that will cause fatality in 50% of a population of test animals.) As a moderately water-soluble salt ionising from each of its carboxylic acid, amino and phosphate moieties, glyphosate is rapidly excreted in urine if ingested. As a zwitterion salt that dissociates, it has little or no volatility, even under acid conditions.
Glyphosate is quickly inactivated in soil from strong binding to its phosphate group, giving a residue that is degraded mainly by decarboxylation to aminomethyl -phosphonic acid (AMPA), which may be persistent when soil is dry, also bonded to soil by its phosphate group. If concentrated formulations are diluted with water containing soil, glyphosate is rendered inactive by this binding reaction.
Two aspects of its properties led to it becoming the world’s major marketed herbicide (Duke and Powles, doi.org/10.1002/ps.1518), almost one million tonnes annually in many different commercial formulations, since it is now out of patent.
First, it is a broad-spectrum herbicide, which is highly toxic to most annual plant species when applied to leaves but has no residual activity in soil; this inactivation makes it ideal for low tillage and no-till agriculture. This more sustainable agriculture has become very popular because it removes the need for costly ploughing of land in fallow before planting of crops. This has improved crop yields, reducing erosion and the cost of weed control. It also conserves water and nutrients in the soil profile and reduces the use of fossil fuels for ploughing with tillage implements, producing far less CO2 emissions.
Second, Monsanto’s success in producing genetically modified (GM) plants with closed EPSP synthases made it a favoured target for Roundup Ready crops designed to be herbicide resistant. Several crops have been provided with alternative EPSP genes from bacteria resistant to the herbicidal effect of glyphosate, such as soybeans, cotton, canola (rape) and maize.
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Ivan Kennedy AM FRACI CChem is Professor Emeritus in Agricultural & Environmental Chemistry, Institute of Agriculture, University of Sydney. His research with some 50 research students has sought solutions for a range of environmental problems, partly described in several books. He is the RACI’s topic chair and reviewer for symposia in environmental and sustainable chemistry for Paciﬁchem 2020, to be held in Hawaii this December.