Effect of animal activity and air temperature on heat production, heart rate, and oxygen pulse in lactating Holstein cows

A linear relationship between heart rate (HR) and oxygen consumption (VO 2 ) has been reported in homeothermic animals, indicating that is possible to estimate heat production through HR measurements. This relationship may depend on the animal activity and environmental conditions. The main objective of the present study was to evaluate the effect of the air temperature and animal posture and activity on heat production and VO 2 in relation to HR. In addition, as a secondary objective, the energy cost of eating and ruminating versus idling and standing versus lying down was determined. Twelve Holstein lactating cows were housed inside climate-controlled respiration chambers for 8 d, where the air temperature was gradually increased from 7 to 21°C during the night and from 16 to 30°C during the day with daily increments of 2°C for both daytime and nighttime. During the 8-d data collection period, HR and gaseous exchange measurements were performed, and animal posture and activity were recorded continuously. The oxygen pulse (O 2 P), which represents the amount of oxygen that is consumed by the cow per heartbeat, was calculated as the ratio between VO 2 and HR. Results showed that heat production and VO 2 were linearly and positively associated with HR, but this relationship largely varied between individual cows. Within the range tested, O 2 P was unaffected by temperature, but we detected a tendency for an interaction of O 2 P with the temperature range tested during the night versus during the day. This indicates that the effect of air temperature on O 2 P is nonlinear. Standing and eating slightly increased O 2 P (1.0 and 2.5%) compared with lying down and idling, respectively, whereas rumination increased O 2 P by 5.1% compared with idling. It was concluded that the potential bias introduced by these effects on the O 2 P for the application of the technique is limited. The energy cost of eating and ruminating over idling was 223 ± 11 and 45 ± 6 kJ/kg 0.75 per day, respectively, whereas the energy cost of standing over lying down was 53 ± 6 kJ/kg 0.75 per day. We concluded that O 2 P in dairy cows was slightly affected by both animal posture and activity, but remained unaffected by air temperature within 8 to 32°C. Nonlinearity of the relationship between the O 2 P and air temperature suggests that caution is required extrapolating O 2 P beyond the temperature range evaluated in our experiment.