AimsTo monitor the effect of a live yeast additive on feedstuff colonization by targeted fibrolytic micro-organisms and fibre degradation in the cow rumen. Methods and ResultsAbundance of adhering fibrolytic bacteria and fungi on feedstuffs incubated in sacco in the cow rumen was quantified by qPCR and neutral detergent fibre (NDF) degradation was measured. Saccharomyces cerevisiae I-1077 (SC) increased the abundance of fibre-associated Fibrobacter succinogenes on wheat bran (WB) and that of Ruminococcus flavefaciens on alfalfa hay (AH) and wheat silage (WS). The greatest effect was observed on the abundance of Butyrivibrio fibrisolvens on AH and soya hulls (SH) (P<0001). Fungal biomass increased on AH, SH, WS and WB in the presence of SC. NDF degradation of AH and SH was improved (P<005) with SC supplementation. ConclusionsLive yeasts enhanced microbial colonization of fibrous materials, the degree of enhancement depended on their nature and composition. As an effect on rumen pH was not likely to be solely involved, the underlying mechanisms could involve nutrient supply or oxygen scavenging by the live yeast cells. Significance and Impact of the StudyDistribution of this microbial additive could be an interesting tool to increase fibre digestion in the rumen and thereby improve cow feed efficiency.