Amino acid metabolism in ruminants

All animals require amino acids (AA) which are the building blocks of proteins required for optimal growth, reproduction, lactation, and maintenance. In ruminants, proteins and AA are first subject to microbial degradation in the rumen making it difficult to predict the quality and quantity of AA that are absorbed by the animal. In ruminants, absorbed AA comes from microbial protein synthesis in the rumen and from dietary amino acid sources that are undergraded in the rumen. Production of microbial protein alone is insufficient to supply adequate amounts of amino acids for optimal production. In cattle, methionine and lysine are generally the first limiting AA for production. Improving the quality of absorbable AA can be accomplished through several means. For example, feeds can be chosen whose proteins are relatively resistant to rumen degradation. However, this does not ensure overcoming the limiting AA. A better method would be to predict which AA were limiting and then to supply these AA in a rumen protected form. The future of understanding requirements of AA for ruminants will rely on the ability of models to accurately predict the need for supplemental AA.