It has been suggested that the mechanism by which yeast cultures (YC), based on Saccharomyces cerevisiae, improve ruminant production is by stimulating rumen fibre digestion and microbial protein flow from the rumen. Both of these effects have in turn been related to the increases in bacterial growth in the rumen observed when YC is added to the diet. However, although it is has been shown that the effectiveness of YC in improving productivity is influenced by the composition of the diet fed (Williams and Newbold, 1990), little is known about how the ability of YC to stimulate bacterial numbers in the rumen is affected by diet composition.

The effect of diet on the stimulation of bacterial numbers in the rumen by YC was investigated in the rumen simulation technique (Rusitec). Grass silage was prepared from a sward of perennial rye-grass, without the use of a preservative. Fresh grass was collected from the sward prior to ensilage and stored at -20°C. Grass (5.96 g dry matter (DM) /day) and grass silage (6.54 g DM/ day) were compared alone or supplemented with 500 mg / day YC (Yea-sacc, Alltech), in triplicate, in an experiment lasting 21 days.

Four rumen cannulated sheep, allowed ad lib access to grass silage, were used to investigate further the effects of YC on bacterial numbers in the rumen in vivo. The effects of YC (4 g/ day) were investigated in a 2 × 2 factorial design with 28 day periods.