Estimation of the energy cost of activities in grazing dairy cows using the oxygen pulse–heart rate method

Grazing dairy production systems have gained interest due to the increasing consumer preferences associated with environmental care and animal welfare perception. Nonetheless, grazing dairy cows have lower feed conversion than those fed indoors with a TMR, partly because of increased maintenance energy costs associated with grazing and walking activity. However, to our knowledge, the energy cost of grazing has not been quantified for dairy cows. The objectives of the present work were to evaluate the effects of grazing on the oxygen pulse (O₂P; mL O₂ consumed per heartbeat; experiment 1) and to quantify the energy cost of grazing in dairy cows using the oxygen pulse–heart rate (O₂P-HR) technique (experiments 1 and 2). In both experiments, heat production (HP) was measured using the O₂P-HR technique and synchronized with animal activity records. In experiment 1, 15 dairy cows were used to measure the O₂P when resting and then immediately after the first grazing session to assess the effect of grazing on O₂P. Grazing activity increased HR and oxygen consumption compared with when the cows were resting. However, the increase in both variables was proportional; therefore, O₂P was similar, indicating that the change in HR was the main component in the response to an increased O₂ demand due to grazing. Hence, it is possible to use the O₂P-HR method to measure HP during grazing in dairy cows. The energy cost of grazing was estimated by the difference in HP between when the cow was grazing and when it was not grazing (i.e., idling or ruminating) and ranged from 6.63 ± 1.12 to 7.85 ± 0.68 kJ/kg^0.75 per hour and 1.31 ± 0.28 to 1.59 ± 0.14 kJ/kg per hour, respectively. In addition, the energy cost of walking was calculated as the difference in HP when the cow was walking to and from the pasture and the milking parlor versus when it was idling or ruminating. The energy cost of walking was 24.03 ± 1.12 kJ/kg^0.75 per hour or 4.72 ± 0.28 kJ/kg per hour. Hourly energy expenditure (kJ/h) was similar between grazing and consuming TMR at the feed bunk, indicating that the extra energy required by dairy cows when fed in a grazing system is mostly explained by the longer time spent to harvest the pasture and the energy cost of walking between the pasture and the milking parlor compared with confined cows fed TMR. Finally, the results of this study suggest that O₂P is not affected by grazing and therefore that the O₂P-HR technique is a promising methodology to estimate the energy cost of grazing animals without substantially affecting their behavior within the production system.