The study investigated how different milk feeding levels and the composition of the starter feed interact to influence growth performance, rumen fermentation, certain blood parameters and the excretion of purine derivatives in the urine of dairy calves. A total of 72 female Holstein dairy calves (mean BW: 40.6 ± 1.9 kg) were randomly assigned to a complete randomized design with a 2 × 3 factorial arrangement. The starter diets differed in crude protein (CP, 20% vs. 24%, DM basis) and total crude fat (Ether extract; 3.0% vs. 5.5%, DM basis). The 6 treatment groups were: moderate milk with 20% CP and 3% CF (MM-CON), moderate milk with 24% CP and 3% CF (MM-HCP), moderate milk with 20% CP and 5.5% CF (MM-HCF), high milk with 20% CP and 3% CF (HM-CON), high milk with 24% CP and 3% CF (HM-HCP) and high milk with 20% CP and 5.5% CF (HM-HCF). Calves had free access to a starter feed and water and were weaned on d 53 but remained in the study until d 73. Calves fed the HM-HCF diet had the lowest starter feed intake both pre-and post-weaning, while calves fed the HM-HCP and HM-CON diets had the greatest pre-weaning ADG. Feed efficiency and protein intake were also lowest in HM-HCF calves prior to weaning, while fat intake was consistently greater in calves fed the HF diet. Calves fed the HM-HCP diet had the largest body size, heart girth and withers height at weaning and at the end of the study, while the HM-HCF calves had the smallest. Rumen fermentation profiles showed that total VFA concentrations were lowest in HM-HCF calves, along with an increased acetate to propionate ratio. In contrast, HM-HCP calves had a greater molar proportion of propionate pre- weaning and butyrate post- weaning, along with a greater proportion of branched-chain VFA in high-CP diets, indicating more efficient microbial activity in the rumen. Urinary analyses were consistent with these results, as HM-HCF calves had the greatest creatinine levels and lowest urine volumes, while HM-HCP calves showed the greatest excretion of MPS markers, allantoin and purine derivatives. Blood metabolite analysis showed that glucose levels were greater in HM-fed calves pre- weaning, while BHB levels were lowest in HM-HCF calves. Overall, diets with high milk allowance, high CP (24% of DM) and moderate fat (3% of DM) promoted growth, rumen function and metabolic efficiency, while high fat starter diets (5.5% of DM) may limit feed intake and growth of dairy calves.
