Plasma amino acid profiles, urea and ammonia —nitrogen in one- and eight-year-old ewes fed two levels of protein

The effect of age and dietary protein level on plasma amino acid, urea and ammonia concentrations were studied using one- and eigth-year-old ewes. A diet containing 10% crude protein was fed at individual maintenance levels to six ewes of each age in trial 1, followed by feeding a similar diet containing 20% crude protein in trial 1, tyrosine, valine, glutamic and aspartic acids were higher in the plasma of young ewes, while citrulline and serine were higher in the old ewes. In trial 2, isoleucine, tyrosine, methionine, alanine, leucine, valine, glutamic acid, phenylalanine, lysine and arginine were higher in young ewes, while citrulline was higher in the old animals, suggesting a differential age-related response to higher dietary protein. Total, essential, nonessential, the ratio of essential to nonessential and branched-chain amino acids did not differ by age in trial 1, while all these values except nonessential amino acids were higher in young ewes in trial 2. Plasma urea nitrogen was higher in old ewes in both trials. No age difference occured in plasma ammonia in trial 1, while old ewes had lower plasma ammonia in trial 2. The decreased plasma amino acid concentrations and elevated urea levels in old ewes at both levels of protein intake suggested increased amino acid catabolism, perhaps reflecting age differences in cellular amino acid metabolism.