352 A Century and a Half of Nitrogen and Protein Metabolism in Ruminants

Abstract

Abstract By 1870, we knew plants and animals contained protein, and the participation of protein and urea in animal and human N metabolism was recognized. Nitrogen balance studies had been conducted, and the atomic theory was established. By 1900, research established the concepts of energy and protein metabolic interactions, the role for urea in ruminant metabolism, amino acids as constituents of protein molecules, and enzyme-catalyzed reactions. Research stations were publishing research results and feeding recommendations. By 1950, the concept of biological value was established, and we knew ruminal microorganisms use dietary NPN, degrade dietary protein, and form microbial protein as a source of amino acids for ruminant tissue protein. By 2000, hypothesis-driven, statistically valid experimental designs created quantitative information on metabolizable protein and essential amino acid requirements. Use of surgically altered animals, isotopically labelled molecules, and standardized laboratory analyses created quantitative information on ingestion, ruminal degradability of dietary protein, digestion, absorption, and metabolism of N-containing compounds. Research showed that ruminants respond to manipulation of postruminal amino acid supply. We had elucidated protein composition and structure, the role of protein turnover to determine the concentration of cellular proteins, and the quantitative significance of protein turnover to whole body protein metabolism. The internet and the advent of international symposia provided links among researchers around the globe. By 2020, proteomics and metabolomics enhanced description of underlying control mechanisms related to amino acid metabolism. Research quantified integration of amino acid supply and use among body tissues. Multidisciplinary research teams had created empirical and mechanistic models. Those models currently use or estimate rates and daily amounts of ruminal (in)degradability of dietary protein, ammonia production, urea recycling, microbial protein synthesis, postruminal protein digestion, metabolic fecal nitrogen, and amino acid absorption and metabolism. They predict use of metabolizable amino acids for maintenance, reproduction, postnatal growth, and lactation.