Digestion processes in the rumen can be manipulated by the addition of direct feed microbial to enhance feed digestion, to improve the performance of animals, to promote the health status of animals. A number of microbial feed additives are used in ruminants for these purposes. Yeast products such as Saccharomyces cerevisiae appear to be more useful in manipulating rumen metabolism. What is the role of yeast and how does it affect dairy animals' performance? This is demonstrated by a practical experiment in this book.