Probiotics (growth promoters) have been used as supplements in animal feeds for more than seven decades to enhance growth and decrease feed/gain ratio. Moreover, many of the beneficial productive responses associated with the use of probiotic supplements can be directly related to their effects on the microbial population in the digestive tract. Most evidence for such effects is results of intensive studies of microbial activities in the rumen of cattle and sheep. Pronifer is a fermentation product consists of a bacterial cocktail of specific lactic acid producing bacteria as a probiotic agent, it may act through improving the balance of the intestinal micro flora. Also, pronifer improves health, performance, minimize diarrhea and increase growth rates.Moreover, yeast stimulates rumen bacteria and enhance lactate and ammonia utilization resulting in moderate pH and increases in microbial population which lead to increases in fiber digestion and protein synthesis in the rumen. Yeast culture improved feed intake, weight gain and feed conversion in growing ruminants.