The aim of this study was to estimate the nitrogen requirements of Holstein suckling calves at 2, 6 and 8 weeks of age. Newborn healthy calves, randomly divided into 2 groups (n=16 per group), were fed with either a starter based on corn or barley. The calves received pasteurized cow’s milk in accordance to the herd schedule. The calves were housed in individual metabolic cages and urinary and fecal nitrogen contents were measured in samples collected for four days at 2, 6 and 8 weeks of age. Nitrogen requirement was estimated using the regression analysis. Nitrogen requirement for maintenance at week 2 significantly (P=0.037) differed between the starters, but not at 6 and 8 weeks. However, the nitrogen requirement for maintenance at 8 weeks of age tended to be differ between male and female calves (P=0.075), but not at weeks 2 and 6. Daily nitrogen requirements for maintenance based on the metabolic weight decreased as the calves aged. Nitrogen requirement for growth was affected by the type of grain in the diet at weeks 2 (P=0.068), 6 (P<0.05) and 8 (P<0.05). No effect of sex was found on nitrogen requirements for growth (P>0.05). پرونده مقاله