Maximizing microbial protein synthesis in the rumen.

Microbial protein supply to the duodenum should be maximized for efficient use of feed protein and energy. High producing ruminants often are fed significant concentrations of cereal grains and fat in their diets. Increasing starch in the diet decreases ruminal pH, which often decreases extent of ruminal fiber digestion and also may decrease efficiency of microbial protein synthesis because of energy-spilling reactions. In contrast, higher grain feeding increased efficiency of microbial protein synthesis in some studies because ruminal passage rate was increased. Ruminal degradation of carbohydrates and protein must be synchronized for optimal microbial efficiency, but the microbes appear to withstand transient periods of asynchronous nutrient supply in many cases. Protozoa extensively prey upon bacteria, and a higher proportion of protozoa than bacteria lyse within the rumen, recycling significant amounts of protein. Feeding moderate amounts of unsaturated fat appears to reduce, especially on relatively low forage diets, protozoal numbers and the extent of intraruminal recycling. Indirect and direct calculations have been derived from models to estimate microbial protein transactions within the rumen; however, models need further definition and more detailed inputs from published literature to further their predictive ability.