Understanding and improving the estimation of enteric methane emissions from ruminants using the tracer gas technique

© 2014 Dr. Matthew Hervey DeightonPublications included in thesis:Deighton, M. H., O’Loughlin, B. M., Buckley, F. & Boland, T. M. (2011). A simple method for pre-calibration storage of sulphur hexafluoride permeation tubes. Animal Feed Science and Technology, 166-167, 198–200. DOI:10.1016/j.anifeedsci.2011.04.011Deighton, M. H., O’Loughlin, B. M., Williams, S. R. O., Moate, P. J., Kennedy, E., Boland, T. M. & Eckard, R. J. (2013). Declining sulphur hexafluoride permeability of polytetrafluoroethylene membranes causes overestimation of calculated ruminant methane emissions using the tracertechnique. Animal Feed Science and Technology, 183(3-4), 86–95. DOI:10.1016/j.anifeedsci.2013.04.021Deighton, M. H., Williams, S. R. O., Lassey, K. R., Hannah, M. C., Boland, T. M., Eckard, R. J. & Moate, P. J. (2014). Temperature, but not submersion or orientation, influences the rate of sulphur hexafluoride release from permeation tubes used for estimation of ruminant methane emissions. Animal Feed Science and Technology, 194, 71–80. DOI:10.1016/j.anifeedsci.2014.05.006Deighton, M. H., Williams, S. R. O., Hannah, M. C., Eckard, R. J., Boland, T. M., Wales, W. J. & Moate, P. J. (2014). A modified sulphur hexafluoride tracer technique enables accurate determination of enteric methane emissions from ruminants. Animal Feed Science and Technology, 197, 47–63. DOI:10.1016/j.anifeedsci.2014.08.003The tracer gas technique has been used to determine the enteric methane emissions of ruminant and pseudo-ruminant livestock since 1994. Since then it has become an internationally important research tool to provide data for national greenhouse gas inventories and to test experimental treatments designed to reduce methane emissions. The key advantages of the tracer gas technique are that it enables researchers to accurately determine the total daily methane emission of large numbers of individual ruminant and pseudo-ruminant animals with only minor interference of each animal’s normal daily routine. This ability to accurately determine the methane emissions of individual animals within normal farm or range environments enables research measurements and hypothesis testing to be undertaken within typical livestock production systems, including grazing systems, without alteration of diet selection, pattern of feed intake or animal behaviour. A review of the scientific literature reveals that the methodology used varies between researchers and no single methodology exists for the technique. A feature of the international adaptation of the technique has been an average between-animal coefficient of variation (CV) of methane yield of approximately 15% which is almost twice as variable as the typical CV for methane yield measured by respiration chambers. Another feature is that, on average, experiments have used 10.8 animals per treatment group. Therefore they have lacked sufficient experimental power to declare treatment differences less than 16.5% as statistically significant. In addition to high between-animal variation a significant positive bias of methane emissions determined using the SF6 technique, relative to respiration chambers, has also been reported. The present research was undertaken to improve understanding of the technique, to improve the accuracy of its implementation and to reduce the contribution of measurement error to the between-animal variation in methane yield. Thirteen hypotheses were tested relating to aspects of the SF6 technique that could influence the accuracy and variability of calculated methane yield. The research undertaken adds a significant amount of new knowledge and provides an explanation for many sources of error that have contributed to variability of published estimates of methane yield in experiments using the SF6 technique. The major findings of this thesis were: (1) that freezing at -80°C inhibits loss of SF6 from permeation tubes thereby enabling their long term storage, (2) that declining SF6 release from permeation tubes is correlated with reduced SF6 permeability of polytetrafluoroethylene membranes, (3) that the rate of SF6 release from permeation tubes changes by 2.5% per degree Celsius about 39°C, (4) that the hourly rate of methane and SF6 emissions from ruminant animals are independent, (5) that a declining rate of sample collection through capillary tubes over 24 hours causes substantial bias in gas sample collection and (6) that improved accuracy of sample collection can be achieved using orifice plates to restrict the rate of gas sample collection. It is concluded that, when well implemented, the SF6 tracer gas technique can be an accurate and versatile research tool for determination of the enteric methane emission of large numbers of individual ruminant livestock