Enteric methane emission rates determined by the SF6 tracer technique: Temporal patterns and averaging periods

• Published in: Animal feed science and technology 2011-06, Vol.166-167, p.183-191
• Main Authors: Lassey, K.R., Pinares-Patiño, C.S., Martin, R.J., Molano, G., McMillan, A.M.S.
• Format: Article
• Language: English
• Subjects: Animal productions; Averaging period; Biological and medical sciences; detection limit; digestive tract; emissions; excretion; Feed and pet food industries; feeds; Food industries; foods; Fundamental and applied biological sciences. Psychology; gas chromatography; gases; greenhouse gases; Intensive breath sampling; Methane; mouth; nose; rumen; Ruminant livestock; SF6 tracer technique; sheep; sulfur hexafluoride; Terrestrial animal productions; tracer techniques; Vertebrates
• ISSN: 0377-8401; 1873-2216
• DOI: 10.1016/j.anifeedsci.2011.04.066

The sulphur hexafluoride (SF6) tracer technique has been widely applied to determine CH4 emission rates by ruminants since its development in the mid-1990s. It remains the only viable method for determining emission rates from individual grazing animals. Essential parts of the method for each participating animal are pre-insertion into the rumen of a source of SF6 with known release rate and breath sample collections near the nose and mouth for CH4 and SF6 analysis. Breath samples are accumulated over an ‘averaging period’ of usually 24h to yield estimates of CH4 emissions. As a tracer, SF6 is biologically inert and has a very low detection limit (i.e., 10−12), enabling release rates of a few tens of μl/h to be sustained for many months by an initial SF6 charge of ∼1g. Any departure from a uniform SF6 collection rate, such as through SF6 interactions in the digestive tract, could introduce variability into the inferred CH4 emission rate, which has the potential to explain reports of higher variability in CH4 emission rates estimated with this technique compared with whole animal chamber techniques. Our study examined SF6 and CH4 excretion rates for their variability using a novel automated gas chromatography system that isolated and analysed 20min breath samples collected successively for 6d from each of 9 housed sheep. We found that that SF6 was not excreted into the breath stream at a uniform rate, but its daily pattern of excretion was strongly correlated with that of CH4, suggesting that some SF6 is retained within the digestive tract and later ventilated with eructated gases following feeding. Methane emission rates can be estimated for different averaging periods through different combinations of the 20min data. Methane emission rate estimates for each sheep are independent of averaging period between 3h and 6d, although inter-period variability is highest for averaging periods less than 1d. Improved understanding of the SF6 tracer technique supports it as a reliable unbiased estimator of enteric CH4 emission rate in ruminants. This paper is part of the special issue entitled: Greenhouse Gases in Animal Agriculture – Finding a Balance between Food and Emissions, Guest Edited by T.A. McAllister, Section Guest Editors; K.A. Beauchemin, X. Hao, S. McGinn and Editor for Animal Feed Science and Technology, P.H. Robinson.