Yeast in Dairy Cattle Diets

Yeast is a unicellular fungi that does not reproduce via asexual spore production (Phaff, 1966). The most commonly fed yeast in dairy diets is Saccharomyces cerevisiae (SC); a facultative anaerobic yeast often referred to as brewers or bakers yeast. The most common yeast products fed to ruminants are live cells or yeast culture mixtures. A yeast culture is a yeast-fermented product that contains live and dead yeast, the culture media the yeast cells were grown on, and the metabolic by-products produced by the yeast during fermentation. The process involves inoculation of a culture media (generally liquid and cereal grain raw ingredients) with live yeast cells, fermentation of the media, and drying of the fermented media. Live cell yeast products consist solely of live dried yeasts that are mixed with a carrier for feeding. Yeast is supplemented in dairy diets to improve animal performance and is considered a ‘natural’ alternative to using antibiotics. When fed to lactating dairy cows, improved milk production is the most consistent benefit reported, however increased dry matter intake (DMI) and milk fat percentage have been shown. Supplementing calves with yeast has been found to improve body weight gain and health. How yeast directly improves animal health and performance is not known although a variety of mechanisms have been suggested. These include changes in the rumen microbial population, rumen fermentation, intestinal nutrient flow, and diet digestibility. Dairy cattle research on feeding yeast has shown inconsistent responses. One explanation for this inconsistency is the wide variation in conditions across studies. This includes differences in inclusion level, type of diet fed, feed intake and the use of additional feed additives along with animal factors such as age, physiological stage, health and stress status; all of which may affect DFM efficacy (Wagner et al., 1990).