Efficiency of endogenous urea 15N nitrogen incorporation into bacterial and milk protein of goats fed diets with three different protein levels

The aim of the study was to estimate the magnitude of endogenous blood urea nitrogen (EBUN) incorporation into bacterial protein produced in the rumen and into milk protein of goats fed low (LP), medium (MP) or high protein (HP) diets arranged in a 3 x 3 Latin Square design. Three Alpine goats of about 35 kg body weight fitted with cannula into the rumen and a catheter into the jugular vein were fed three isoenergetic diets containing 11(LP), 13(MP) or 16(HP)% crude protein in dry matter (DM). Goats were continuously infused N-15 urea into the jugular vein for 6 days. Daily milk yield was 1575, 1492 and 1770 g. Concentrations of milk urea were 139, 342 and 451 mg/l, whereas plasma urea was 178, 356 and 667 mg/l in LP, MP and HP groups, respectively. N-15 excess in urinary N was higher in the HP group, and in faecal N was higher in the LP group, respectively (P<0.05). In rumen bacteria mass enrichment of N-15 (atom% excess) decreased when the level of ammonia nitrogen in the rumen fluid increased with the level of protein in the diet. These values indicate a most efficient utilization of recycled urea nitrogen in the rumen for bacterial protein synthesis in goats consuming the diet with the lowest protein level. N-15 excess in milk urea differed (P<0.05) among groups (0.36, 0.74 and 1.22 % of N-15 dose in LP, MP, and HP groups, respectively), indicating dependence on the level of protein in the diet. The highest (P<0.05) amount of infused N-15 accumulated in the milk protein of the group LP (6.88, 4.50 and 2.23 % of N-15 dose in group LP, MP and HP, respectively).