Submitted to: Animal Feed Science and Technology
Publication Type: Review Article
Publication Acceptance Date: 2/15/2008
Publication Date: N/A
Technical Abstract: The ability of most ruminal bacteria to use ammonia as a nitrogen source for growth is a highly advantageous characteristic of ruminal fermentation, but bacteria typically grow faster and more efficiently if amino nitrogen (N) sources (ruminally degraded protein, peptide or amino acids) are also available. Because bacterial protein is the major amino acid source for dairy cattle, the efficiency of bacterial protein synthesis in the rumen can have a profound effect on animal productivity, but the impact of ruminal amino nitrogen on animal performance is far from clear-cut. If ruminally available amino nitrogen is simply deaminated, any potential benefit is eliminated, and this latter avenue of ruminal nitrogen metabolism has confounded diet formulation. Most systems of diet formulation are based on empirical regressions that have attempted to predict microbial flow from the rumen from one or more dietary characteristics, but the correlation coefficients have been low. These empirical approaches have been further complicated the fact that microbial protein is difficult to differentiate from feed protein, and observations made with ‘microbial markers’ have in most cases been less than reliable. It is now clear that bacteria have three avenues of energy utilization: 1) growth, 2) maintenance and 3) spilling, but these latter two mechanisms of non-growth ATP turnover have typically been ignored. Amino nitrogen availability can affect the magnitude of non-growth energy dissipation in bacteria, but it is not the only regulatory factor. This review will make the argument that a truly mechanistic model of ruminal fermentation and bacterial growth must address non-growth energy dissipation.