Technical Abstract: Protein is often the most expensive nutrient for high producing ruminants. Microbial protein is usually the major source of amino acids entering the duodenum, and microbial protein is generally less expensive than feed protein. The National Research Council ignores the fact that bacterial protein yields depend on many factors including carbohydrate source, nitrogen source, growth rate, dilution rate and pH. The rumen sub-model of The Cornell Net Carbohydrate and Protein System for Evaluating Cattle Diets (CNCPS) adjusts microbial protein yields in relation to the supply of peptides and amino acids in the rumen, but the mechanism of this adjustment was not clear. Because amino acid-dependent changes in growth rate and yield were 10-fold greater that the maintenance of energy-limited cultures, it appeared that ruminal bacteria had mechanisms of spilling energy. The extent to which energy spilling affects microbial protein production in vivo has not yet been determined, but it is easy to envision diets that would promote energy spilling. Water soluble carbohydrates can accumulate in the period soon after feeding, and many starch sources also have very high rates of fermentation. If the diet is formulated with little, or no, ruminally degraded amino acids, the catabolic rate could exceed the anabolic capacity of ruminal bacteria.