|Hall, Mary Beth|
Submitted to: Journal of Dairy Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/23/2005
Publication Date: 3/15/2006
Citation: Firkins, J.L., Hristov, A.N., Hall, M., Varga, G.A., St-Pierre, N.R. 2006. Integration of ruminal metabolism in dairy cattle. Journal of Dairy Science. 89:E31-51E. Interpretive Summary: Absorbed amino acids and protein are essential to meeting the true protein requirements of the lactating cow. Accurately predicting their supply can improve our ability to formulate diets to more closely meet cow nutrient requirements. In this review, we identified limitations to the approach used by the 2001 Dairy NRC in predicting how well met these protein requirements were for supporting milk protein production. Current research and future insight were provided to supplement previous reviews of factors influencing microbial protein synthesis and volatile fatty acid production in the rumen, both of which are important nutrient sources to the cow. Also, considerations for improvement in both accuracy and precision of predictions were discussed to help researchers improve the integration of ruminal metabolism as we strive to increase efficiency of conversion of dietary carbohydrates and protein into milk and milk components.
Technical Abstract: Our objectives were focused toward identification of nutrients or dietary factors that are most critical for advancing our knowledge of, and improving our ability to predict, milk protein production. The Dairy NRC (2001) model is sensitive to prediction of microbial protein synthesis (MPS), which is an important component of many parameters used to meet metabolizable protein or amino acid requirements. We discussed important considerations for microbial marker methodology. Statistical formulas and examples documented why the use of a simple algebraic formula from a previous study measuring duodenal flow of purine bases and urinary excretion of purine derivatives greatly underestimates the true precision of predicted duodenal purine bases in a future study. Considerations of specific carbohydrate source and needs for rumen-degraded protein components were provided to help explain microbial interactions. Further variation will be explained with emerging technology such as real-time PCR quantification of targeted microbial populations. For example, such methods might help explain the variation in ruminal outflow of biohydrogenation intermediates that modify milk fat production. Finally, MPS can be better integrated with the production of volatile fatty acids (VFA), which do not necessarily reflect VFA molar proportions in the rumen. The gut and splanchnic tissues metabolize varying amounts of VFA, and propionate has important hormonal responses influencing milk protein percentage. Integration of ruminal metabolism with that in the mammary and peripheral tissues can be improved to increase the efficiency of conversion of dietary nutrients into milk components for more efficient milk production with decreased environmental impact.