Responses to incremental nutrient supply on energy and protein metabolism in preweaning dairy calves

Recently reviewed development objectives and feeding practices in young dairy calves require an adaptation of nutrient recommendations set for milk replacer (MR) composition. Nutrient requirements of calves younger than 21 d of age, and those of calves fed with high levels of MR are insufficiently quantified. The efficiency at which macronutrients are utilized, particularly protein, substantially diminishes with age, and little data exists for the first weeks of life. In addition, in older preruminant and ruminant calves, protein and energy can be simultaneously limiting for protein gain. Whether this also applies to calves in the first weeks of life is unknown. Therefore, this study aimed to quantify the responses in protein and fat gain to incremental supply of protein, fat, or lactose to MR in very young calves. Thirty-two groups of 3 mixed-sex Holstein-Friesian newborn calves (3.4 ± 1.6 d of age), were randomly assigned to 1 of 4 dietary treatments applied for 19 d: a basal MR (23.3% crude protein, 21.2% crude fat, and 48.8% lactose, percentages of dry matter), provided at 550 kJ/kg of metabolic body weight (BW^0.85) per day (CON; n = 24), or the basal MR incrementally supplied with 126 kJ of digestible energy per BW^0.85 per day as milk fat (+FAT; n = 23), lactose (+LAC; n = 24), or milk protein (+PRO; n = 23). Calves were fed MR in 2 daily meals and had ad libitum access to water, but did not have access to calf starter nor any other solid feed. After 2 wk of adaptation to their respective diets, groups of calves were placed for 1 wk in an open-circuit respiration chamber for nitrogen and energy balance measurements (5 d). The incremental nutrient efficiencies indicate what percentage of extra intake of nutrients is retained. In this study, we observed that with every 100-g increase in protein intake, 52% was converted into protein deposition, and 44% contributed to heat production. Similarly, a 100-g increase in fat intake resulted in 67% being stored as fat, 22% being released as heat, and only 5% being retained as protein. Likewise, a 100-g increment in lactose intake led to 49% being stored as fat, with 38% being released as heat. Additional protein intake was not deposited as fat; extra energy intake (fat and additional lactose) increased postabsorptive N efficiency in young calves.