Yeast culture (YC) based on Saccharomyces cerevisiae is gaining increasing acceptance as a feed additive for ruminants. Production responses to YC have been reported in growing and lactating animals. It has been suggested that these responses are at least partly due to enhanced microbial growth in and microbial nitrogen flow from the rumen (Williams and Newbold, 1990). The aim of the present study was to investigate the effect of YC on rumen fermentation, microbial growth and liveweight gain in bulls fed isonitrogenous diets containing either urea or soya.

Thirty two Limousin cross Friesian bulls (liveweight 344 kg) were allocated to treatments on the basis of previous growth rate and initial liveweight. Animals received mixed diets of barley/urea (92% rolled barley, 5% cane molasses, 1.5% urea and 1.5% minerals/vitamin mix) or barley/soya (86.75% rolled barley, 5% cane molasses, 0.75% urea, 6% soyabean meal and 1.5% minerals/vitamin mix) plus or minus 1.5 kg/ tonne YC (Yea-sacc1026 , Alltech, UK) (Table 1). YC was added to the diet prior to mixing. All rations were offered ad libitum twice daily until slaughter (460 kg). Rumen fluid samples were withdrawn by stomach tube 5 and 10 weeks after the beginning of the trial. The outflow of microbial nitrogen from the rumen was estimated from the appearance of purine derivatives in the urine as described previously (Chen et al., 1990).