Energy and nitrogen utilization of high versus low producing dairy cows.

Six high and 6 low producing cows were fed corn silage, alfalfa hay, corn, and soybean meal diets to evaluate the cow's ability to metabolize energy and N. High producers consumed more feed and gave more milk than low producers. Energy digestibility (.70), conversion of digestible to metabolizable energy (.90), and absorption of N (.70) were not different between groups. For low producers, heat production (249 kcal/kg BW.75) and loss of metabolizable energy as heat (.62) were greater than for high producers (238 kcal/kg BW.75 and .53), suggesting a less efficient intermediary metabolism of the former. Body composition was unaffected by treatment. Body fat mobilized during periods 1, 2, and 3 was replaced during period 4. Change in body energy (fat) and change in energy balance (calorimetric) data were in the same direction, but absolute values did not agree; change in energy as body fat lagged behind change in energy balance determined by calorimetry. Body protein was used in early lactation and was replaced immediately in spite of negative energy balance; this suggests that protein (amino acids) may play a significant role in meeting short-term energy needs during the first few weeks of lactation.