Respiration chamber
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Respiration chambers are calibrated to be accurate and precise, and are the gold standard for benchmarking new methods. Only respiration chambers measure total emissions from the animal via the oral, nasal and anal routes; all other methods ignore emissions via the anus and only measure CH4 emitted in breath. Breath measurements are justified because 99% of CH4 is emitted from the mouth and nostrils, and only 1% via the anus (Murray et al., 1976[1]). A single animal (or occasionally more) is confined in a chamber for between 2 and 7 days. Concentration of CH4 (and other gases if required) is measured at the air inlet and outlet vents of the chamber. The difference between outlet and inlet concentrations is multiplied by airflow to indicate CH4 emissions fluxes. In most installations, a single gas analyser is used to measure both inlet and outlet concentrations, often for two or more chambers. This involves switching the analyser between sampling points at set intervals, so concentrations are actually measured for only a fraction of the day. If the sampling points acquisition frequency is high it enables to draw the diurnal pattern of methane emission, comparable to the GreenFeed system. Respiration chambers vary in construction materials, size of chamber, gas analysis equipment and airflow rate, all of which can influence results. Validation of 22 chambers at six UK research sites revealed an uncertainty of 25.7% between facilities, which was reduced to 2.1% when correction factors were applied to trace each facility to the international standard CH4 (Gardiner et al., 2015[2]). The main sources of uncertainty were stability and measurement of airflow, which are crucial for measuring CH4 emission rate. The authors concluded, however, that chambers were accurate for comparing animals measured at the same site. This is an added challenge to benchmarking alternative methods with respiration chambers if respiration chambers themselves have not been benchmarked with respiration chambers at other facilities. It should be noted that substantial errors can occur if appropriate calibration procedures are not followed (Gardiner et al., 2015[2]). For large-scale evaluation of CH4 emissions by individual animals, respiration chambers are challenging with only a single study in growing Angus steers and heifers exceeding 1000 animals and finding CH4 production to be moderately heritable h2 = 0.27 ± 0.07 (Donoghue et al., 2016[3]). Installation and running costs are high, as only one animal is normally measured at once. If we assume that the monitoring time is three days per animal, and chambers are run continuously, then maximum throughput would be approximately 100 animals per chamber per year. In practice, throughput is likely to be 30 to 50 animals per year. Cows are social animals and confinement in a chamber may ultimately influence their feeding behaviour resulting in less feed consumed and in a different meal pattern compared with farm conditions. Altered feeding pattern or level is not a problem for metabolic studies evaluating feeds but can be a problem when evaluating individual animals. Furthermore, the representativeness of respiration chambers to grazing systems has been called into question (Pinares-Patiño et al., 2013[4]). However, promising developments have led to more animal friendly respiration chambers constructed from cheaper, transparent materials. These lower the cost and reduce the stress of confinement with minimal disruptions to accuracy, precision and no drop in feed intake of the cows (Hellwing et al., 2012[5]). Where an alternative method may be cheaper, less invasive, easier to implement, or have a wider scope of application, it is of value to assess the relative accuracy, precision and correlation with the gold standard to assess the relative worth of the alternative method (Barnhart et al., 2007[6]). All methods measure CH4 with some level of error, so the ‘true value’ of an individual is not known. However, when the level of measurement error increases, so too does the imprecision. When comparing two methods where one or both methods has high imprecision a phenomenon known as ‘attenuation of errors’ occurs (Spearman, 1904[7]). The increased measurement error biases the correlation between the two methods downwards and reduces the efficacy of detecting significant differences in accuracy (Adolph and Hardin, 2007[8]). Or in terms of linear regression terms, when the observed CV of an alternative method is higher than that of the gold standard method, the slope of regression between the methods is decreased and the intercept is biased upwards.
| Method | Purchase cost | Running costs | Labour | Repeatability | Behaviour alteration | Throughput |
|---|---|---|---|---|---|---|
| Respiration chamber | High | High | High | High | High | Low |
| SF6 technique | Medium | High | High | Medium | Medium | Medium |
| Breath sampling during milking and feeding | Low | Low | Low | Medium | None | High |
| GreenFeed | Medium | Medium | Medium | Medium | Medium | Medium |
| Laser methane detector | Low | Low | High | Low | Low-Medium | Medium |
Table 3. Summary of the main features of methods for measuring CH4 output by individual animals.
- ↑ Murray, R.M., Bryant, A.M., and Leng, R.A.. 1976. Rates of production of methane in the rumen and large intestine of sheep. Br. J. Nutr. 36:1-14.
- ↑ 2.0 2.1 Gardiner, T.D., Coleman, M.D., Innocenti, F., Tompkins, J., Connor, A., Garnsworthy, P.C., Moorby, J.M., Reynolds, C.K., Waterhouse, A., and Wills, D. 2015. Determination of the absolute accuracy of UK chamber facilities used in measuring methane emissions from livestock. Measurement 66: 272-279.
- ↑ Donoghue, K.A., Bird-Gardiner, T., Arthur, P.F., Herd, R.M., and Hegarty, R.F. 2016. Genetic and phenotypic variance and covariance components for methane emission and postweaning traits in Angus cattle. J. Anim. Sci. 94:1438–1445. doi:10.2527/jas2015-0065.
- ↑ Pinares-Patiño C.S., Hickey, S.M., Young, E.A., Dodds, K.G., MacLean, S., Molano, G., Sandoval, E., Kjestrup, H., Harland, R., Pickering, N.K., and McEwan, J.C. 2013. Heritability estimates of methane emissions from sheep. Animal 7: 316–321.
- ↑ Hellwing, A.L.F., Lund, P., Weisbjerg, M.R., Brask, M., and Hvelplund. T. 2012. Technical note: test of a low-cost and animal-friendly system for measuring methane emissions from dairy cows. J. Dairy Sci. 95:6077–85. doi:10.3168/jds.2012-5505.
- ↑ Barnhart, H.X., Kosinski, A.S., and Haber, M.J. 2007. Assessing Individual Agreement. J. Biopharm. Stat. 17:697–719. doi:10.1080/10543400701329489.
- ↑ Spearman, C. 1904. The Proof and Measurement of Association between Two Things. Am. J. Psychol. 15:72–101.
- ↑ Adolph, S.C., and Hardin, J.S. 2007. Estimating phenotypic correlations: Correcting for bias due to intraindividual variability. Funct. Ecol. 21:178–184. doi:10.1111/j.1365-2435.2006.01209.x.
