Effect of varying carbohydrate and fat content of milk replacer on body composition of Holstein bull calves

The objective of this study was to determine the effect of varying concentr ations of dietary fat and carbohydrate on changes in body composition of Ho lstein bull calves fed under isocaloric and isonitrogenous intake condition s. Thirty-two calves were assigned to a randomized block design with three dietary treatments, eight calves per treatment, and one baseline group of e ight calves. Animals were reared from birth to 85 kg live BW (SEM = 0.57). All calves began treatments between 2 and 6 d of age. Three different milk replacer treatment diets were designed to deliver 14.8 (low fat, LF), 21.6 (medium fat, MF), or 30.6% of DM (high fat, HF) fat; 55.3,46.7, or 35.4% of DM lactose; and 23.5, 24.8, or 27.0% of DM CP, respectively. Gross energy values were 4.62, 5.09, and 5.77 Mcal/kg for the LF, MF, and HF milk replac ers, respectively. From d 1 to 14, calves were offered 0.24 Meal intake ene rgy/kg BW0.75, adjusted weekly based on increases in BW, and 0.28 Meal inta ke energy/kg BW0.75 from d 15 to slaughter. Dry feed was not offered. Dry m atter, energy, CP, and fat intakes were 55.2 kg, 257.6 Meal, 13.0 kg, and 8 .2 kg; 52.3 kg, 268.8 Meal, 13.1 kg, and 11.4 kg; and 46.8 kg, 270.3 Meal, 12.6 kg, and 14.3 kg for the LF, MF and HF treatments, respectively. Energy and CP intakes did not differ among treatments (P = 0.63 and 0.79, respect ively). Fat and DMI were different among treatments (P = 0.001 and 0.02, re spectively). Empty BW gains were 0.61, 0.61, and 0.65 kg/d for LF, MF, and HF, respectively, and were not different among treatments (P = 0.27). Empty body CP, water, and ash gain were not different among treatments (P = 0.65 , 0.99, and 0.57, respectively). Empty body retained energy and fat gain we re 27.2 and 57.7% greater for calves fed the HF than for those fed the LF d iet (P = 0.06 and 0.006, respectively). Fat as a percentage of the whole em pty body on a water-free basis was lower for calves consuming the LF diet ( 28.6%) than for those fed the HF diet (34.6%), whereas percentage of CP on an empty body, water-free basis was greater for calves consuming the LF die t (59.2%) than for those fed the HF diet (54.9%) (P = 0.006). The results o f this study demonstrated that, under isocaloric and isonitrogenous intake conditions, equivalent dietary energy from fat compared to carbohydrate, ab ove 15% fat, has no beneficial purpose unless additional fat deposition is required in the animal. Further, the data demonstrate significant changes i n body composition independent of growth rate.