A modified sulphur hexafluoride tracer technique enables accurate determination of enteric methane emissions from ruminants

Abstract

The sulphur hexafluoride (SF6) tracer technique enables determination of enteric methane emissions from large numbers of individual ruminant animals. The objective of this research was to identify and correct substantial errors within the SF6 technique. Six experiments were undertaken using respiration chamber, laboratory or SF6 techniques. Experiment 1 used respiration chambers to demonstrate that the daily pattern of methane emissions from dairy cows was related to their pattern of feed intake. In contrast, the daily emission of SF6 from these cows was constant and independent of the pattern of methane emission. This finding supports the contention that in order to accurately determine daily methane emissions using the SF6 technique, it is necessary that gases are collected continuously at a constant rate for 24h. Since development of the SF6 technique in 1993, it has been propounded that capillary-tube flow restrictors achieved a constant rate of sample collection into evacuated gas collection canisters. Laboratory experiments 2, 3, 4 and 5 demonstrated that, when capillary-tube flow restrictors are used, the rate of sample collection declined and caused a bias of up to 15.6% in calculated methane emissions. This error was caused by an interaction between the declining sample collection rate and the pattern of an animal's methane emission over 24h. In contrast, orifice plate flow restrictors maintained a constant sample collection rate at canister pressures <0.31atm and thereby minimised the decline in sample collection rate. Experiment 5 also demonstrated that sample collection using orifice plate flow restrictors, combined with initial (<0.03atm) and final (<0.49atm) canister pressures, substantially reduced measurement error. Accuracy of the modified SF6 technique, incorporating orifice plate flow restrictors for 24h sample collection, was validated in Experiment 6. The mean (S.D.) methane yield (g CH4/kg DMI) of eight cows did not differ (P=0.135) when determined using the modified SF6 technique 22.3 (1.44) or chambers 21.9 (1.65). In addition, the between-animal coefficient of variation for methane yield determined using the modified SF6 technique (6.5%) was similar to that determined using chambers (7.5%). Consequently the modified SF6 technique enables the statistical power of experiments to be increased or their size decreased. We conclude that the modified SF6 technique reduced error associated with SF6 release, sample collection and analysis. It is recommended that the modified SF6 technique should be used in preference to the original SF6 technique for determination of enteric methane emissions from ruminants.