The estimation of energy expenditure (EE) of grazing animals is of great importance for efficient animal management on pasture. In the present study, a method is proposed to estimate EE in grazing animals based on measurements of body acceleration of animals in combination with the conventional Agricultural and Food Research Council (AFRC) energy requirement system. Three-dimensional body acceleration and heart rate were recorded for tested animals under both grazing and housing management. An acceleration index, vectorial dynamic body acceleration (VeDBA), was used to calculate activity allowance (AC) during grazing and then incorporate it into the AFRC system to estimate the EE (EE derived from VeDBA [EEVeDBA]) of the grazing animals. The method was applied to 3 farm ruminant species (7 cattle, 6 goats, and 4 sheep). Energy expenditure based on heart rate (EEHR) was also estimated as a reference. The result showed that larger VeDBA and heart rate values were obtained under grazing management, resulting in greater EEVeDBA and EEHR under grazing management than under housing management. There were large differences between the EE estimated from the 2 methods, where EEHR values were greater than EEVeDBA (averages of 163.4 and 142.5% for housing and grazing management, respectively); the EEVeDBA was lower than the EEHR, whereas the increase in EEVeDBA under grazing in comparison with housing conditions was larger than that in EEHR. These differences may have been due to the use of an equation for estimating EEHR derived under laboratory conditions and due to the presence of the effects of physiological, psychological, and environmental factors in addition to physical activity being included in measurements for the heart rate method. The present method allowed us to separate activity-specific EE (i. e., AC) from overall EE, and, in fact, AC under grazing management were about twice times as large as those under housing management for farm ruminant animals. There is evidence that the conventional energy system can predict fasting metabolism and the AC of housed animals based on accumulated research on energy metabolism and that VeDBA can quantify physical activity separately from other factors in animals on pasture. Therefore, the use of the VeDBA appears to be a precise way to predict activity-specific EE under grazing conditions, and the method incorporating acceleration index data with a conventional energy system can be a simple and useful method for estimation of EE in farm ruminants on pastures.