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; (H2O)-H-2-O-18) 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 (H-2) and 240 (O-18) min and half-lives were 6.3 +/- 0.6 days (H-2) and 5.6 +/- 0.4 days (O-18).
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 M-0.75 per day compared with 0.48 +/- 0.29 MJ/kg M 0.75 per day estimated by DLW. Maintenance metabolizable energy intake was 0.53 +/- 0.01 MJ/kg M 0.75 per day (Cal) and 0.50 +/- 0.01 MJ/kg M-0.75 per day (DLW). Validation of the DLW technique may empower essential, systematic appraisal of energy requirements in unrestrained working horses.
- estimating heat-production
- CO2 production
- light horses
- body water