Assessment of the sulphur hexafluoride (SF6) tracer technique using respiration chambers for estimation of methane emissions from sheep

Abstract

Attempts to evaluate the sulphur hexafluoride (SF6) tracer technique to estimate CH4 emissions from ruminants have yielded mixed results. These studies either used SF6 permeation tubes with a long history of use in animals, involved small number of animals or used partial animal enclosure. Our study was conducted with a relatively large number of experimental sheep and controlled variables regarding the permeation rate (PR) of SF6. Twenty four sheep housed in a covered yard and fed lucerne silage to achieve common feed intakes among individuals in the study were administered fresh SF6 permeation tubes. Following 10d acclimatisation in pens, sheep were staggered in 3 groups of 8 each in order to match availability of 8 respiration chambers. Each group were transferred to individual metabolic crates and habituated to breath collection harnesses for 3d before breath samples were collected daily over 6 consecutive d for CH4 emission estimation using the SF6 ‘Tracer’ technique. Sheep were then brought into respiration chambers for CH4 measurements over 4 consecutive d (‘Chamber’). During sheep occupation, chamber inlet and outlet gas streams were sampled into evacuated yokes, as for the tracer technique procedures. Samples were analysed for CH4 and SF6 mixing ratios by gas chromatography as for the Tracer technique, which were then used to estimate CH4 emissions using tracer technique procedures (i.e., Tracer in chamber). Paired t-tests based on within sheep data were used for pairwise comparisons of CH4 emission estimates between techniques. Daily CH4 emissions for the Tracer, Chamber and Tracer in chamber procedures were 14.8±2.4, 13.9±1 and 16.1±2.8g, respectively. Although Tracer and Chamber emission estimates did not differ, Tracer estimates were associated with much larger among- and within-animal variability than Chamber values, and the relationship between Chamber and Tracer estimates was poor. Rate of recovery of SF6 from chamber gases calculated by dividing the calculated daily emission of SF6 (i.e., net mixing ratio of SF6×chamber ventilation rate) by the known PR of SF6 was 10% lower than that for CH4. In sheep, the average CH4 emission estimate using the SF6 tracer technique matches that obtained from chambers, but the correlation between estimates is poor, possibly due to a mismatch in routes of excretion of tracer and trace gases.This article 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.