The variability of the ratio of oxygen consumption to heart rate in cattle and sheep at different hours of the day and under different heat load conditions

The oxygen consumption (VO2) per heart beat (O2 pulse, O2P) can be used for the estimation of energy expenditures as a function of the heart rate (HR) of animals. The magnitude of error of such estimation is derived from the variability of O2P in different environment conditions. Three experiments were carried out to define changes in O2P, attributable to changes in the time of day or in the heat load. In Exp. 1 and 2, calves (n=8) and lambs (n=7), respectively, were measured simultaneously for HR and VO2 five and six times, respectively, during a 24-h period. In Exp. 3, high-yielding dairy cows (n=20) were measured simultaneously for HR and VO2 four times during a 4-month summer season, throughout a wide range of heat load (HL) conditions. The measurements of each animal in all the experiments lasted 10 min. Significant individual variability of HR, VO2 and O2P was observed in all three experiments. The time of day, or the HL, affected HR and tended to affect VO2, but had no effect on O2P in Exp. 1 and 2. On the other hand, both HR and O2P were strongly affected by HL in Exp. 3, and the HL effect on O2P was not linear. We suggest that the difference in response of O2P to HL between milking cows and growing animals could be explained by the higher metabolic rate of the cows. The O2P of growing animals could be used for long-term energy expenditure calculations, whereas the O2P of high-yielding dairy cows should be corrected for the prevailing HL conditions before it can be used for such calculations.