Energy expenditure estimation from heart rate: validation by long-term energy balance measurement in cows

The use of heart rate (HR) measurements to estimate energy expenditure (EE) was evaluated, by comparing HR method with energy balance measurements during the reproductive cycle of mature beef cows. Six beef cows, 7- to 10-years-old and 607+/-29 kg body weight (BW) were used. Intake was measured individually. Organic matter digestibility of the diets was measured, and metabolizable energy (ME) and ME intake (MEI) were calculated from digestibility values. EE was estimated from daily HR measurements and oxygen consumption, calibrated against oxygen consumption per heart beat (O-2 pulse). Retained energy (RE) in cows' BW was calculated on the basis of BW changes, on the assumption that all changes derived from gain or loss of fat tissue. The energy content of the newborn calf and accompanying uterine tissues was calculated from the calf live weight at birth and published data. Milk production was measured on three occasions, as the weight increment of calves following suckling. Milk energy content was measured and the energy retained for milk production was calculated. RE for the entire trial, that is, energy in BW changes, plus the energy in the calf and accompanying uterine tissues, plus the energy in milk, was 4.53+/-1.09% of MEI. For the 1-year trial of the six cows, the MEI recovery, i.e. the ratio of MEI to (EE + RE), was 1.06+/-0.026, and the ratio of EE calculated by the difference (MEI - RE) to EE measured by HR was 1.06+/-0.028, which were not significantly different from the expected value of 1.0. Lactation increased dry matter intake (DMI), MEI, HR and EE. Late pregnancy increased HR and total EE but did not significantly increase EE per kg BW0.75, when BW included the uterine content. Reproductive state did not affect the O-2 pulse. It is concluded that: (a) energy expenditure of beef cows during the reproductive cycle can be calculated from HR and O-2 pulse; (b) the combination of EE and MEI measurements enables estimation of the feed energy available for production by non-constrained cattle. A significant correlation (P < 0.001) was found between the EE, estimated from the HR and O-2 pulse, and the MEI; this can be used in the reciprocal calculation to estimate MEI from EE measurement. The suggested method offers significant potential for future use in estimating the energy status of farm animals and in supporting management decisions. (C) 2002 Elsevier Science B.V. All rights reserved.