Amino acid imbalance in ruminant lambs

Abstract

In a series of 14 experiments young Merino x Dorset Horn or Merino x Suffolk wethers were fed on wheaten straw or wheaten hay supplemented with a mineral mixture and, in some cases, urea and/or molasses. The diets were contrived to provide between 8 and 12% of digestible energy as protein digested in the intestines. A mixture of amino acids estimated to provide suitable proportions of essential amino acids and adequate non-essential amino acids was developed. With each diet, either the complete amino acid mixture, or a mixture from which one essential amino acid was excluded (imbalanced mixture), was infused per abomasum. In several experiments feed intake was depressed by imbalanced mixtures in which methionine, threonine, isoleucine and lysine were the respective deletions from the mixture, but was elevated by the infusion of the complete amino acid mixture. In each experiment an imbalanced infusion resulted in a decrease in plasma concentration of that amino acid excluded from the mixture to levels only 15–50% of control (pre-infusion) levels. All other essential amino acids were increased in concentration in plasma, reaching 1.5 to 6 times the concentrations in pre-infusion conditions. Infusions of greater amounts of amino acids resulted in greater changes in the plasma amino acid concentration. These results indicated that, although ruminant lambs ingesting herbage diets are unlikely to be subjected to an effective amino acid imbalance, they have the physiological capacity to respond to amino acid imbalances. This needs to be considered when rumen bypass of amino acids or proteins is being considered in practical or experimental circumstances.