Application of ¹³CO₂ measurements to the estimation of energy expenditure in goats

T. Sahlu, S. P. Hart, P. D. Klein, N. Jacquemet, and H. Carneiro

The use and limitations of ¹³C stable isotopes as tracers in animal experimentation was investigated using ¹³C-labeled bicarbonate to estimate CO₂ production and energetic requirement of free-ranging goats. Goats were fed wheat hay (Triticum aestivum; C₃) or Old World Bluestem hay (Bothriochloa caucasica; C₄; OWBS) ad libitum for 21 d. Parotid salivary ducts were cannulated for saliva collection. Isotopic bicarbonate (¹³C) was infused at a constant rate into the peritoneal cavity. In exp. 1, saliva samples were obtained at successive 2-h periods for 20 h to establish the time to achieve equilibrium. The half-life of bicarbonate was 2.9 h for animals consuming the wheat diet and 4.1 h (P < 0.06) for animals consuming OWBS diet. The time required to reach equilibrium was 12 h for animals on the wheat diet and 16 h for animals on OWBS diet. The error in the technique arising from animals consuming C₃ vs. C₄ species (which have different endogenous ¹³C contents) was studied in exp. 2. If species composition (C₃ vs C₄) of the diet remains constant within 10%, the error is negligible. In exp. 3, estimation of CO₂ production from continuous saliva samples had a lower coefficient of variation (6.3 vs 10.0%) than measurements based on spot serum samples. Energy expenditure of goats in stalls consuming hay ad libitum was similar to that of ad libitum-fed sheep at similar body weights. The ¹³C methodology appears to be a promising method for measuring energy expenditure of grazing animals.