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

摘要

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 3x3 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 15N 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. 15N 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 15N (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. 15N excess in milk urea differed (P<0.05) among groups (0.36, 0.74 and 1.22% of 15N 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 15N accumulated in the milk protein of the group LP (6.88, 4.50 and 2.23% of 15N dose in group LP, MP and HP, respectively). The degree of 15N incorporation into milk protein was positively correlated to 15N enrichment in bacteria (r=0.934) indicating that endogenous urea nitrogen was incorporated into bacterial protein in the rumen and afterwards consequently into the amino acids of milk protein as a main pathway of endogenous urea nitrogen incorporation into milk protein. It is concluded that in lactating goats about 50% of the ruminal bacterial N requirement can be supplied by the rumino-hepatic cycle without reduction of milk yield.