The experiment was designed to investigate the effects of saccharomyces cerevisiae supplementation on the efficiency of rations containing different levels of concentrate in male Holstein calves. 36 male Holstein calves (average body weight of 175. 9 ± 5.54 kg) were allocated into six treatments of 60, 70 and 80 percent of concentrate with or without yeast culture based on a 2 x 3 factorial design. The performance parameters (daily feed intake and gain) were recorded during 150 days of fattening period. At the end of the experimental period, the rumen content of animals were sampled by esophagus tube at 0, 3 and 6 hours after morning feeding and ruminal parameters (pH, VFA and NH3-N) and microbial population (total and cellulolytic bacteria, lactate utilizing bacteria and protozoa) were determined. All data were analyzed based on the complete randomized design in which the performance data were adjusted by covariance before analysis. Yeast supplement resulted the increase of the pH ,total volatile fatty acids and population of lactate utilizing bacteria and decrease of the level of NH3-N in the rumen fluid samples (p<0.05). The increase in the total number of rumen bacteria and protozoa was not significant. Feed intake and daily gain were higher in calves with yeast supplemented rations and 80 percent cocentrate (p<0.05). It can be concluded that the supplementation of yeast (Saccharomyces cerevisiae, S.C.47) could have a beneficial effect on the rumen fermentation and microbial populations in fattening calves. This in tum may results in an improvement of the animal performace especially when feeding high concentrate diets.
Key words: fattening calves, Saccharomyces cerevisiae, rumen fermentation, microbial population.