Impact of reducing starter feed crude protein content with different milk allowances on the performance and nitrogen balance in dairy calves.

Associate milk allowance and starter feed for dairy calves to achieve high performance and adequate nitrogen (N) balance can be challenging. Thus, this study aimed to evaluate the effects of reducing the CP content of the starter feed on the performance, mammary gland development, and N balance of dairy calves in the preweaning, weaning, and postweaning phases. The study was conducted according to a completely randomized design in a 3 × 2 factorial scheme, using 3 milk volumes (4, 6, or 8 L/d) and 2 strategies for starter feed supply (fixed or decreasing CP content). The first strategy consisted of feeding the animals a fixed starter feed with 18% CP (DM basis) throughout the preweaning phase (from 4 to 73 d), and in the second strategy, the animals were fed a starter feed with 24% CP (DM basis) from 4 to 24 d of age, 18% CP (DM basis) from 25 to 45 d of age, and 14% CP (DM basis) from 46 to 73 d of age. Therefore, 60 female crossbred calves were distributed through the 6 treatments formed: 4 L/d of milk and starter feed with fixed CP (FCP) content; 6 L/d of whole milk + FCP; 8 L/d of whole milk + FCP; 4 L/d whole milk and starter feed with decreasing CP (DCP) content; 6 L/d whole milk + DCP; 8 L/d milk + DCP. Calves receiving DCP had lower intake and fecal excretion on d 66. Moreover, younger calves (24 d old) had higher N utilization efficiency than calves at 45 and 66 d old, but when fed 8 L/d of milk, all calves had the similar N-use efficiency regardless of age. Additionally, calves receiving 8 L/d of milk had higher N intake and retention and lower N excretion. In the preweaning phase, animals fed FCP showed higher performance. In the weaning phase, higher ADG and feed efficiency were observed in animals fed 4 L/d whole milk compared with 8 L/d whole milk, and calves fed FCP had a higher BW. In the postweaning period, calves in the DCP group showed better feed efficiency, but animals fed FCP had higher ruminal ammonia-N and propionate concentrations. Moreover, calves on 4 and 6 L/d and FCP had higher blood urea N levels. Finally, for the mammary gland, age affected area, perimeter, circularity, and eccentricity, which increased in size during the 80 d of the trial. Female calves receiving 4 L/d of whole milk had a smaller area than the ones with 6 L/d, and, for perimeter, 4 L/d treatment promoted reduced parenchyma growth compared with 6 and 8 L/d treatment. To conclude, animals receiving 6 L/d of whole milk showed good performance, mammary parenchyma growth, greater N efficiency, and reduced N excretion. The FCP strategy improved performance, whereas the DCP strategy increased feed efficiency postweaning and tended to reduce fecal N excretion. However, it should be noted that this reduction was modest at the individual level, and the use of multiple starters on the farm should be carefully evaluated.