Determination of energy and protein requirements for crossbred Holstein × Gyr preweaned dairy calves

The objective was to quantify the energy and protein nutritional requirements of Holstein × Gyr crossbred preweaned dairy calves until 64 d of age. Thirty-nine Holstein × Gyr crossbred male calves with an average initial live weight (mean ± SEM; for all next values) of 36 ± 1.0 kg were used. Five calves were slaughtered at 4 d of life to estimate the animals' initial body composition (reference group). The remaining 34 calves were distributed in a completely randomized design in a 3 × 2 factorial arrangement consisting of 3 levels of milk (2, 4, or 8 L/d) and 2 levels of starter feed (presence or absence in diet). At 15 and 45 d of life, 4 animals from each treatment were subjected to digestibility trials with total collection of feces (for 72 h) and urine (for 24 h). At 64 d of age, all animals were slaughtered, their gastro-intestinal tract was washed to determine the empty body weight (EBW; kg), and their body tissues were sampled for subsequent analyses. The net energy requirement for maintenance was estimated using an exponential regression between metabolizable energy intake and heat production (both in Mcal/EBW0.75 per d) and was 74.3 ± 5.7 kcal/EBW0.75 per d, and was not affected by inclusion of starter feed in the diet. The metabolizable energy requirement for maintenance was determined at the point of zero energy retention in the body and was 105.2 ± 5.8 kcal/EBW0.75 per d. The net energy for gain was estimated using the EBW and the empty body gain (EBG; kg/d) as 0.0882 ± 0.0028 × EBW0.75 × EBG0.9050±0.0706. The metabolizable energy efficiency for gain (kg) of the milk was 57.4 ± 3.45%, and the kg of the starter feed was 39.3 ± 2.09%. The metabolizable protein requirement for maintenance was 3.52 ± 0.34 g/BW0.75 per d. The net protein required for each kilogram gained was estimated as 119.1 ± 32.9 × EBW0.0663±0.059. The metabolizable protein efficiency for gain was 77 ± 8.5% and was not affected by inclusion of starter feed in the diet. In conclusion, the energy efficiency for gain of milk is higher than that of starter and the net protein required per unit protein gain increases with empty body weight.