Accounting for Ruminal Deficiencies of Nitrogen and Branched- Chain Amino Acids in the Structure of the Cornell Net Carbohydrate and Protein System

The rumen typically operates as an energy-limited, nitrogen-excess system, but diets can be so low in degradable crude protein that microbial growth is limited by N. Ruminal bacteria respond differently to N sources and N-limitation. Cellulolytic ruminal bacteria need ammonia as an N source, have little capacity to utilize amino-N, are unable to ferment fiber when ammonia is depleted, and do not produce ammonia from amino-N sources (1, 2, 3). Some hemicellulose-digesting ruminal bacteria are stimulated by amino-N, but even these species are primarily dependent on ammonia as an N source (4). Many nonfiber digesting ruminal bacteria prefer amino-N to ammonia (2, 5), and some can even produce ammonia from amino-N source (6).