Evaluations of the energy requirements of working animals have been confounded by the constraints of indirect calorimetric techniques (Cal). This study sought to investigate a non-restrictive methodology, the doubly labelled water (DLW; 2H218O) technique, for the measurement of energy expenditure in free ranging equids. Six pony geldings were intravenously injected with DLW in two sequential studies that permitted first, isotope equilibration and half-lives to be determined and then second, heat production (HP) to be measured simultaneously by DLW and traditional (Cal) techniques.

In study 1, three animals were injected with DLW, three animals were untreated controls. Blood samples were collected every 30 min for 12 h and thereafter at 24 h intervals for 14 days. Isotopes equilibrated throughout the body water pool within 300 (2H) and 240 (18O) min and half-lives were 6.3 ± 0.6 days (2H) and 5.6 ± 0.4 days (18O).

In study 2, HP was simultaneously determined by Cal and DLW over a 4-day period. Animals (no. = 6) were assigned to pairs and in successive weeks two ponies were injected with DLW and confined to metabolism chambers 12 h later. Cal HP was 0.51 ± 0.02 MJ/kg M0.75 per day compared with 0.48 ± 0.29 MJ/kg M0.75 per day estimated by DLW. Maintenance metabolizable energy intake was 0.53 ± 0.01 MJ/kg M0.75 per day (Cal) and 0.50 ± 0.01 MJ/kg M0.75 per day (DLW). Validation of the DLW technique may empower essential, systematic appraisal of energy requirements in unrestrained working horses.