Does glyphosate affect the availability of micronutrients in plants?
Glyphosate acid dissociates at functional soil pH’s and is therefore present in the form of ionic species that form complexes, or chelate, with counterions such as metal ions. Some metal ions are important micronutrients in plants. Manganese (Mn) or iron (Fe) ions for instance are important factors in photosynthesis, nitrogen metabolism and many vital enzymatic pathways in plants. Both manganese and iron deficiency appears in plants as interveinal chlorosis, or yellowing of leaves between the veins.
The fact that glyphosate has the potential to form complexes with these micronutrients has led to the speculation that glyphosate could reduce the availability of micronutrients to the plants, either through reduced uptake due to chelation with glyphosate in the soil, or through reduced availability within plants that have been genetically modified (GM) to be glyphosate resistant (GR). This theory was initially put forward in the United States by Professor Don Huber who claimed that extensive use of glyphosate causes a micronutrient deficiency in GR-plants, making them more susceptible to diseases. This theory is based largely on the claim that glyphosate is a strong chelator and that it therefore significantly alters the availability of micronutrients by formation of complexes, but no data has been provided to support the claim. However, a recent publication describes a detailed model of metal ion complexation within the plant incorporating the effects of many different naturally-occurring compounds such as citric and glutamic acid, in addition to glyphosate. The model clearly demonstrates that micronutrients exist in plants as naturally occurring complexes, and that glyphosate has a minimal impact on micronutrient complexation 9.
Nevertheless, several research groups have reported that glyphosate adversely affects the micronutrient levels in glyphosate-sensitive plants. However, in these experiments glyphosate was applied directly onto glyphosate-sensitive plants which subsequently showed root damage and other signs of herbicidal effects, suggesting that plant growth rates were impaired by the herbicidal activity of glyphosate and not as a result of a lack of nutrients. Also, studies on glyphosate-tolerant soybeans have reported that glyphosate applications adversely affect micronutrient levels. However, these studies were primarily conducted in greenhouses, while field trials conducted under normal agronomic conditions by numerous researchers have shown no effect of glyphosate on micronutrients in glyphosate-tolerant crops 1,2,4,5,6,10,12, 13, 14, 15, 16. For European agriculture all these results or findings are, however, of minor importance, since glyphosate-tolerant crops are not grown for food or feed production in the European member states.
- How likely is it that crops take up glyphosate residues through their roots or show phytotoxic effects?
Last update: 10 October 2012