Energy Metabolism in Jersey Cows: Improving Our Understanding of Energy Requirements and Utilization

Energy is the most limiting nutrient for high producing lactating dairy cows and improving our understand of factors affecting energy supply should improve our ability to estimate net energy of lactation (NEL), milk production, and tissue accretion. To achieve this object, firstly, three experiments were completed to determine 1) maintenance energy requirements; 2) factors affecting heat production; and 3) the relationship between urinary N and energy excretion. Three additional experiments using lactating Jersey cows were completed to evaluate energy utilization of diets varying in 1) supply of Lys and His; 2) fatty acids (FA) and starch; and 3) FA, starch, and Lys. Maintenance energy requirement was 0.102 ± 0.0071 Mcal/kg of metabolic body weight (MBW). Heat production was most effectively explained by MBW and dry matter intake (DMI), and heat production associated with milk protein was two-fold that associated with milk fat. Urinary energy excretion was linearly associated with urinary N. Milk protein yield increased when rumen-protected His was added to a diet containing hydrolyzed feather meal. When adding rumen-protected Lys, milk protein yield was unaffected, but N balance increased. Feeding a high-starch compared to a high-fat diet increased milk protein yield, utilization of dietary N for milk secretion, and tissue energy deposition at fat. Although diets were formulated to be isoenergetic, the high-starch diet had a greater measured NEL (1.83 vs. 1.67 ± 0.036 Mcal/kg of DM) which indicates that energy models need improved. In the final experiment, although increasing FA quadratically increased ME content, ME supply decreased at high FA. Increasing supplemental Lys increased milk protein content at low dietary starch, but not at high dietary starch. Increasing supplemental Lys increased N balance at high supplemental Lys. Increasing starch increased conversion of dietary N into milk N. Results suggest that NEL supply is interrelated to protein metabolism, energy from fat or starch differentially affect milk production and N utilization, and Lys may be preferentially utilized by muscle tissue