Effects of fat content of high-protein milk replacer on plasma metabolite and hormone concentrations of Holstein calves in summer and winter

The objective of this study was to evaluate the effects of fat content of high-protein milk replacer on plasma metabolite and hormone concentrations of dairy calves in summer and winter. Eighty-four Holstein heifer calves (body weight at 7 d of age, 42.2 ± 2.54 kg; mean ± SD) were assigned to one of the 3 treatments: milk replacer containing 29% CP and 21% fat (LF; 4.7 Mcal of metabolizable energy/kg), 26% fat (MF; 5.0 Mcal of metabolizable energy/kg), and 32% fat (HF; 5.3 Mcal of metabolizable energy/kg) on a DM basis (n = 14 each for summer and winter). Milk replacers were offered at 600 g/d (powder basis; 3.6 L/d) from 8 to 14 d, increased up to 800 g/d (4.8 L/d) from 15 to 21 d, 1200 g/d (7.2 L/d) from 22 to 42 d, decreased down to 800 g/d (4.8 L/d) from 43 to 49 d, and 600 g/d (3.6 L/d) from 50 to 56 d, then weaned at 56 d of age. Data and samples were collected until 91 d of age. All the calves were fed a calf starter (23% CP) and chopped hay (11% CP) ad libitum from 7 d of age. Increasing the fat content of milk replacer decreased plasma urea nitrogen concentrations during the peak and weaning periods. Additionally, we observed an interaction of treatment and season for plasma urea nitrogen concentration during the step-up period; it was decreased with the fat content of milk replacer increased in winter but was not affected by treatment in summer. Calves had higher plasma total cholesterol and fatty acids concentrations with increased fat content of milk replacer during the preweaning period. Plasma insulin concentration was differently affected between summer and winter; the concentration was higher in the calves fed the 26% fat treatment than the calves fed the 21% fat treatment in the winter, but in summer, it was not affected by treatment and showed high concentration overall. In conclusion, increasing the fat content of milk replacer decreased plasma urea nitrogen concentration during the peak and weaning periods regardless of season, but only in winter during the step-up period. In addition, greater fat content of milk replacer increased plasma total cholesterol and fatty acid concentrations during the preweaning period regardless of season, but did not exert consistent effects on plasma insulin concentration.