Changes in nitrogen balance with age in calves weaned at 5 or 6 weeks of age

Three experiments were conducted to examine the changes of N utilization after early weaning in calves. In Exp. 1, eight male Holstein calves were weaned at 5 wk of age and N balance was determined on the 1st, 6th, 11th, and 19th wk after weaning. In Exp. 2, to examine the effect of weaning age on N utilization, three calves were weaned at 5 wk of age and the other three at 11 wk of age. The N balance of both groups was determined on the same weeks of age (i.e., the 1st, 6th, 8th, and 11th wk after weaning in the earlier-weaned group). In Exp. 3, to examine the effect of a higher content of CP (17.9%) and TDN (75%) in concentrate on N utilization, three calves were weaned at 6 wk of age and the N balance was determined on the 1st, 5th, 10th, and 20th wk after weaning. The intake of concentrate was low in wk 1 but rapidly increased in the next 5 wk (1.74 and 1.72 times in Exp. 1 and 2, respectively). Nevertheless, the TDN intake tended to be lower in calves weaned early just after weaning compared with that in suckling calves in Exp. 2. The apparent N digestibility steadily increased with age and reached a plateau on wk 11 and 6 after weaning in Exp. 1 and 2, respectively. Conversely, the ratio of N retained:N absorbed tended to be high up to wk 6 after weaning and thereafter decreased, leading to a transient high N retained:N intake ratio on wk 6 after weaning.(ABSTRACT TRUNCATED AT 250 WORDS)