An experiment was conducted to evaluate the effects of different levels of yeast (Saccharomyces cerevisiae SC47) on lactating performonce of Holestein dairy cows. In this experiment a balanced change-over design with twelve cows (in early lactation stage), four rations (1-4), three periods (28 day per period), three blocks and four cows per block was employed. Ingredients of the basal diet were alfalfa hay (23.65%), corn silage (17.2%) and concentrate (59.15%) on a dry matter basis. The experimental diets 1 through 4 contained 0, 3, 6 and 12 grams of yeast per day, respectively. The rations were fed to cows as total mixed ration (TMR), but yeast was top-dressed on the p.m allotment of the ration. The cows were fed individually ad libitum and milked three times a day. Daily milk yield was recorded while milk samples taken twice per week for determination of milk composition; also sampels of rumen liquor and blood were taken at the end of each period. In this experiment it was shown that dry matter intake and milk yield in cows were not affected by experimental diets. But milk compostion including fat, solid non fat and total solid percent were increased through feeding yeast culture (p<0.05). The concentration of milk lactose and protein were not affected through use of yeast culture (p>0.05). The averages of fat corrected milk yield for 3.2% fat with respect to the rations 1-4 were 33.25, 30.92, 33.48 and 33.55; for 3.5% fat 31.61, 29.35, 31.81, and 31.9, and for 4% fat 29.29, 27.25, 29.47 and 29.53 kg per day. The diffrences between milk yield averages were not significant. The pH of cows rumen liquor was not significantly different. Also concentration of glucose, calcium, phosphorus, sodium, cholesterol, triglycerides, and total protein in plasma of cows receiving different diets were not different (p>0.05) while concentrations of potassium (p<0.05) and magnesium (p<0.05) in plasma of cows receiving diet 2-4 were lower than those in control group and statistically significant (P<0.05).
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