Duodenal flow of microbial nitrogen estimated from urinary excretion of purine derivatives in calves after early weaning

Duodenal flow of microbial N (MN) was estimated from urinary purine derivatives to examine age-related changes in MN in male Holstein calves. In Exp. 1, endogenous purine derivatives were determined by measurement of purine derivatives in five calves fed nucleic acid-free milk replacer alone. In Exp. 2, the ratio of urinary excretion as purine derivatives to purines administered via the reticular groove was determined in three calves weaned at 5 wk of age. As a result, endogenous purine derivatives were constant at 705 mu mol/(kg BW(.75.)d), irrespective of the amount of milk replacer, and the ratios of purine derivatives to duodenal purines were estimated to be .549, .276, .363, and .466 at wk 1, 6, 11, and 20 after weaning, respectively. Using these variables and urinary purine derivatives, the duodenal flow of MN was estimated and its relation with N balance was examined in 15 calves weaned at 5 wk of age in Exp. 3. Digestible OM was lower at wk 1 after weaning and transiently higher at wk 6. The percentage of N absorbed to N intake, N absorbed, N retained, and estimated duodenal MN were also lower at wk 1, and rapidly increased for the first 6 wk. These findings suggest that the increases in N absorbed and N retained for the first 6 wk after weaning were due to augmentation of duodenal flow of MN and dietary N that escaped ruminal degradation.