Amino acid nutrition: Demand and supply in forage-fed ruminants

ABSTRACT Amino acid requirements of cattle, particularly grazing cattle, scarcely have been studied. Limiting amino acids for growing cattle include methionine, lysine, histidine, leucine, and valine. Grazing cattle rarely demonstrate large performance responses to supplemental protein (provided ruminal N needs are met) due to the relationships between dietary energy supply, amino acid supply, and amino acid requirements. Amino acid requirements are related to dietary energy supply because protein deposition can be considered an energy-dependent process. Amino acid supply also is an energy-dependent process as a result of ruminal synthesis of microbial protein. Thus, supply and demand for amino acids are both linked to dietary energy such that, under most grazing situations, deficiencies of amino acids are unlikely to be severe. Growing cattle fed grass silages may experience limitations in amino acid supply and, thus, are capable of responding to supplementation with ruminally undegraded protein. Predictions from the Cornell Net Carbohydrate and Protein System would support these conclusions.

Implications

Amino acid requirements of grazing cattle scarcely have been studied. Results from studies that have evaluated supplementation of ruminally undegraded protein to grazing cattle can be used to predict situations in which basal supplies of amino acids are limiting. Responses to ruminally undegraded protein in forage-fed cattle are generally small if ruminally degradable protein needs have been met; this can be attributed to the relationships between dietary energy supply, amino acid supply, and amino acid requirements. Protein deposition, which impacts amino acid requirements, and ruminal synthesis of microbial protein, which impacts amino acid supply, are both energy-dependent processes. Thus, under normal feeding conditions, deficiencies of amino acids are unlikely to be severe. Diets based on grass silages are likely to provide inadequate supplies of amino acids for growing cattle due to low microbial protein production and low ruminally undegraded protein content in the silages.

Key Words: Amino Acids, Forage, Nutrient Requirements, Ruminants

© 2001, by the American Society of Animal Science. All rights reserved.

J. Anim. Sci. 2001. 79:E180-E189