Why is it possible that glyphosate in environmental concentrations affects animals?

Glyphosate was taken to be a very good herbicide, because it disturbs the function of shikimic acid pathway via an inhibition of the enzyme 5-enolpyruvylshikimate-3-phosphate (EPSP) synthase. This pathway is unique to plants and some micro-organisms. Thus, the toxicity of glyphosate to animals is expected to be low. Indeed, most studies on animal cells and also laboratory animals showed quite low toxicity of glyphosate to animals. Thus, for a long time it has been thought that the compound can be used without any worries about effects to animals at the concentrations needed for controlling weeds. The compound is quite stable with an estimated half-life of up to 200 days in soil. This means that yearly application increases its level through the years. In water the half-life appears to be somewhat shorter, about 3 months.

However, information about effects on animals started accumulating. This led to strong polarization between proponents and opponents of glyphosate use. If one examines the question based on scientific grounds, this should not be the case. One should try to resolve the two opposite findings: little toxicity in controlled laboratory experiments, especially on animal cells, and effects on animals, especially in field experiments. It should be pointed out here that the reported effects on animals, e.g. the compound causing cancer, have used very high environmentally non-relevant concentrations. Thus, I find the Californian court decision that Bayer-Monsato should pay a multi-million dollar sum to plaintiffs for causing their cancer somewhat surprising.

The question really boils down to the effects of glyphosate on the microbiota of animals, and the importance of microbiota and its changes on the well-being and function of animals. It is clear that environmental concentrations of glyphosate can affect some microbes. It is also clear that the compound is taken up by animals, so regardless of their location in the animal body, the micro-organisms will be exposed to glyphosate. Studies about the importance of microbiota on animal functions are still in their infancy., but already even interactions between microbiota and neural functions have been described: gut microbes can affect sleep quality and depression. The direct effects of glyphosate on the microbiota of animals can thus affect animal functions indirectly.

In addition to the effects of glyphosate itself on the microbiota, the commercial formulations of glyphosate contain additives. The additives are trade secrets, but it appears that they can be much more acutely toxic to animals than the major active compound. However, since their structure is not known, proper toxicity evaluations by independent scientists are impossible.