AimsThis study was performed in a well-established invitro model to investigate whether the application of a glyphosate-containing herbicide might affect the bacterial communities and some biochemical parameters in a cow's rumen. Methods and ResultsThe test item was applied in two concentrations (high and low) for 5days. In a second trial, fermentation vessels were inoculated with Clostridium sporogenes before the high dose was applied. Effluents were analysed by biochemical, microbiological and genetic methods. A marginal increase in short-chain fatty acid production and a reduction in NH3-N were observed. There were minor and rather equivocal changes in the composition of ruminal bacteria but no indications of a shift towards a more frequent abundance of pathogenic Clostridia species. Clostridium sporogenes counts declined consistently. ConclusionsNo adverse effects of the herbicide on ruminal metabolism or composition of the bacterial communities could be detected. In particular, there was no evidence of a suspected stimulation of Clostridia growth. Significance and Impact of the StudyAntibiotic activity of glyphosate resulting in microbial imbalances has been postulated. In this exploratory study, however, intraruminal application of concentrations reflecting potential exposure of dairy cows or beef cattle did not exhibit significant effects on bacterial communities in a complex invitro system. The low number of replicates (n=3/dose) may leave some uncertainty.