Heart rate measurements as an index of energy expenditure and energy balance in ruminants: A review

A major part of the ME consumed by ruminants (MEI) is dissipated as heat. This fraction, called heat production or energy expenditure (EE), is assayed largely by measuring O₂ consumption (VO₂). Conventional measurement of EE in controlled conditions in chambers does not reflect the complexity of natural, environmental, and social conditions of free-ranging animals. In mammals, most of the measured VO₂ is transferred to the tissues through the heart; therefore, regression of heart rate (HR) against VO₂ can be used to estimate the EE of free-ranging animals. The present article reviews the current knowledge on the use of HR for estimating EE. Energy expenditure can be determined from HR measurements, recorded daily over the course of several days, multiplied by the VO₂ per beat. When an animal does not perform significant exercise, a constant value of VO₂ per beat [O₂ pulse (O₂P)] measured over a short period (10 to 15 min) is used; during exercise, O₂P increases, and the regression equation of VO₂ against HR is used. Under extreme heat load, HR increases to improve heat dissipation, and O₂P decreases; therefore, the effect of heat load on O₂P needs to be taken into account. Cold stress that doubles heat production does not affect O₂P. Heart rate and EE are highly correlated with MEI, but there is significant individual variation in the relationship; therefore, the daily change in the HR of individual animals can be used as an indicator of changes in the individual energy status of a ruminant, and the average HR of the group can serve in the estimation of the energy status of the group. When O₂P is measured, the average group EE is an indication of the energy balance of the whole group. Because the MEI of nondraft animals is the sum of EE and retained energy (RE), the MEI of free-ranging ruminants can be determined by measurement of EE by the HR method and adding the RE. Similarly, the RE can be determined without slaughtering the animals from measurements of EE and MEI. Soon when devices for automatic HR monitoring of domestic ruminants become available at a reasonable price, continuous monitoring of HR might provide producers with a sensitive tool for identifying changes in the energy status of their animals. This will also significantly help to shorten the time needed to identify health problems of individual animals.