Effect of age and dietary protein level on ovine nitrogen metabolism—I. Evidence of comparable metabolizable nitrogen availability

Seven 8-month old and seven 9-yr old ewes were utilized to study the effects of age and dietary protein level on nitrogen and organic matter digestibilities and urinary and plasma nitrogenous constituents. Animals were fed individual maintenance levels, based on metabolic body size (W0.75), of a diet containing 10% crude protein in trial 1 and isocaloric amounts of a diet containing 20% crude protein in trial 2. No age-related differences occurred in either trial for apparent digestible nitrogen (ADN) or organic matter. Nitrogen retention (g/kg W0.75/day) was 0.09 ± 0.02 and 0.10 ± 0.01 in trial 1 and 0.09 ± 0.03 and 0.14 ± 0.03 in trial 2 for young and old ewes, respectively. Total urinary N as a percentage of ADN did not differ by age in either trial, and the relative proportions of urinary urea, ammonia-N and creatinine-N were comparable. Plasma urea and ammonia nitrogen were also similar for the two age groups in both trials. Asparagine, alanine, valine, leucine and total branched chain amino acids were significantly higher while arginine was lower in the plasma of young ewes in trial 1. In trial 2, glutamine, citrulline and arginine were significantly lower in the plasma of young ewes. Concentrations of essential, nonessential, branched and total amino acids were negatively correlated with protein retention in trial 1, while no significant correlations were observed in trial 2. These data suggest that in ewes of different ages fed at maintenance, availability of metabolizable nitrogen is comparable as indicated by no significant differences observed in digestion, absorption or excretion of nitrogen-containing metabolites measured in these studies. Thus, the differences seen in various plasma parameters reflect differences in metabolic processes due to age.