The 13C-bicarbonate method: its suitability for estimating the energy expenditure in grazing goats

Abstract

Two main experiments were carried out with adult goats of the Granadina breed to obtain information on the suitability of the carbon dioxide entry rate (CER) technique using 13C-bicarbonate to estimate CO 2 production and subsequently the energy expenditure of animals performing variable physical activity. The purpose was to validate this method for application to the grazing animal. The goats were accustomed to spending long periods of time in an open-circuit respiration chamber and 5 days prior to the start of the experiments they were prepared with catheters into the jugular vein (infusion of 13C-bicarbonate) and the parotid salivary duct (sampling). Bicarbonate from saliva samples was converted to BaCO 3 and the 13C/ 12C ratio were determined in a gas isotope ratio mass spectrometer. Experiment 1 was performed to determine the effect of exercise on the levels of 13C natural abundance. Saliva samples were collected from each of six animals over 2 h periods to measure the natural abundance of 13C (at% 13C) in non-exercised goats prior to infusion. Subsequently, each goat was walked on a treadmill at zero gradient for 30 min at 20 m min −1. The treadmill belt was stationary during periods of 45 min pre and post the walking activity. Saliva samples were taken during each 2 h period (each comprising 30 min walking preceded and followed by 45 min standing), which were replicated at least four times. The average natural abundance of 13C was 1.09656 and 1.09363% for the resting and intermittently exercised animals, respectively ( P<0.05). Experiment 2 was performed to establish the relationships between values of CO 2 production obtained by calorimetry and the CER technique in animals standing at rest or in the intermittent locomotion protocol assayed in Experiment 1. Five animals were individually placed into an open-circuit respiration chamber and continuously infused with 13C-bicarbonate for at least 15 h before starting saliva collection. Samples of saliva were taken over 2 h periods and concomitant measurements of CO 2 production were made. Based on the ratio of CO 2 production/CER, apparent mean recoveries of 13CO 2 between resting and intermittent locomotion conditions were not significantly different (0.723±0.0437 and 0.779±0.0409, respectively). The absolute entry rate was slightly affected by the goats’ physical activity. The results show that the 13C-bicarbonate method appears to be a promising technique for measuring energy expenditure of the grazing goat.