. 2008. Estimation of carbon dioxide production and energy expenditure of grazing cattle by the sulphur hexafluoride (SF 6 ) tracer gas technique. Can. J. Anim. Sci. 88: 651Á 658. The purpose of the study was to explore the suitability of the sulphur hexafluoride (SF 6 ) tracer gas technique to estimate CO 2 production and energy expenditure (EE) for grazing animals on pastures typical of western Canada. During each year of a 3-yr study, CO 2 emissions were collected from 60 yearling steers on grass pastures receiving one of three fertility treatments: no manure, liquid hog manure applied as a split application (74 kg available N ha(1 ) in the spring and fall, and a single application of liquid hog manure applied at a rate of 155 kg available N ha(1 each spring. The mean CO 2 production estimate across all treatments was 542492218 L d(1 (mean9SD) with a range of 1099 to 11548 L d(1. When compared across three grazing periods in June, July and August, steers produced more (PB0.05) CO 2 in June than in either July or August. Metabolic body weight (BW 0.75 ), average daily gain (ADG), standing forage biomass, and forage neutral detergent fibre (NDF) and crude protein (CP) contents explained 33% of the variation in CO 2 production. Energy expenditure (EE) was estimated from CO 2 production by adopting an energetic equivalent of 22 kJ L(1 CO 2 and EE was calculated for animals that were not losing body weight at time of measurement. The average EE estimate was 1.290. . Energy expenditure declined as ADG and forage biomass declined, but EE was not influenced (P0.05) by fertility treatment. The EE estimate was higher (P B0.05) early in the grazing season (June) and declined as the grazing season progressed. The SF 6 tracer gas technique was able to estimate EE of individual animals without interfering with herd dynamics. The technique was also able to show differences in EE in response to pasture conditions. The results of this study suggest that the SF 6 tracer gas technique shows potential as a simple and non-invasive method of estimating CO 2 production and EE for grazing animals. Further validation under different grazing conditions and with animals undertaking different degrees of activity is required. Comparisons with other field techniques of estimating EE are also important.Key words: Energy expenditure, carbon dioxide, sulphur hexafluoride, grazing animals Stewart, A. A., Undi, M., Wilson, C., Ominski, K. H. et Wittenberg, K. M. 2008. Estimation de la production de dioxyde de carbone et de la de´pense d'e´nergie par les bovins en paissance par de´tection de l'hexafluorure de soufre (SF 6 ). Can. J. Anim. Sci. 88: 651Á658. L'e´tude devait e´tablir si la de´tection de l'hexafluorure de soufre (SF 6 ) permet d'estimer la production de CO 2 et la de´pense d'e´nergie des animaux mis a`l'herbe dans les paˆturages caracte´ristiques de l'Ouest canadien. Chaque anne´e pendant trois ans, les auteurs ont recueilli les e´missions de CO 2 de 60 bouvillons d'un an mis a`l'herbe sur des paˆturages bonifie´s de trois fac¸on...