Energy requirement for primiparous Holstein × Gyr crossbred dairy cows

Our objective was to estimate the requirements of metabolizable energy (ME) and NE_M of lactating and dry cows, the efficiency of ME utilization for milk production (k_l) and tissue gain (k_g), and the use of body energy mobilization for milk production (k_t) throughout the lactation of primiparous crossbred Holstein × Gyr cows, using open-circuit respiration chambers. Twenty-nine primiparous Holstein × Gyr crossbred cows with an initial BW averaging 563 ± 40.1 kg and 2.5 ± 0.09 yr old were used throughout lactation and dry periods. The cows were kept nonpregnant throughout the study to eliminate possible confounding factors. Apparent digestibility assays, followed by calorimeter measurements, were performed 6 times throughout the lactation period. In the dry-off period, the cows were also evaluated but fed with restricted intake (DMI = 1.1% BW/d) to achieve heat production close to maintenance. After 21 d of diet adaptation, an apparent digestibility assay followed by calorimeter measurements was performed. Parameter estimates for lactation period were obtained by mixed models including lactation stage as repeated measures. For restricted feeding at dry-off and fasting period assays, the requirements were estimated by exponential regression. For whole lactation, the values of the ME requirement for maintenance (ME_M) and NE_M were 0.588 and 0.395 MJ/BW^0.75, respectively. The efficiencies of k_l, k_g, and k_t were 0.672, 0.771, and 0.814, respectively. However, ME_M and NE_M were higher in early and mid lactation than late, whereas k_l was higher in early than other lactation stages. Dry and nonpregnant cows had ME_M of 0.434 MJ/BW^0.75 and NE_M of 0.351 MJ/BW^0.75 for maintenance level, and ME_M of 0.396 MJ/BW^0.75 and NE_M of 0.345 MJ/BW^0.75 for fasting metabolism level, and efficiency of ME utilization for maintenance was 0.80. Our findings confirmed that F₁ crossbred Holstein × Gyr dairy cows have differences in energy requirement and efficiency throughout the lactation stages, suggesting the use of different values in each stage. The estimated values of energy requirement for maintenance and efficiencies for primiparous lactating crossbred Holstein × Gyr were similar to those reported in the literature in specific studies and requirements systems.