Applying the SF6 tracer gas methodology to measure enteric methane emissions in grazing Brahman heifers

Context Sulfur hexafluoride (SF6) is a non-toxic, synthetic gas, which is used as a chemical tracer to contextualise the source of other gases. In livestock research, the SF6 tracer gas technique is regaining favour as a useful, viable, and robust approach to measuring enteric methane emissions, especially when respiratory chambers are not an option. This paper outlines the methodology used by the authors as part of a concept exploration study assessing the potential of using the saliva microbiome as a proxy for the rumen microbiome and thus an indicator of methane emissions. In this context, the SF6 technique was implemented to measure enteric methane emissions from tropical beef heifers in a grazing system. Aims To facilitate the use of the SF6 technique by others, this paper describes its implementation for Brahman heifers in a grazing system, emphasising its practical adaptation for long-term use under tropical conditions with Bos indicus cattle and methodological insights gained over a full reproductive cycle. Methods This paper describes the methodology applied by the authors, emphasising important lessons learned regarding five critical points of this technique. First, cattle were trained to wear the sampling gear imported from Institute of Animal Science (IZ, Instituto de Zootecnia), São Paulo, Brazil. Second, permeation tubes charged with appropriate SF6 amounts were manufactured, calibrated, and deployed. Third, the sampling gear was optimised for grazing conditions and the size of the cattle. Fourth, the gas sampling procedure, including sampling background SF6 from the environment, was standardised. Fifth, the gas chromatography method for measuring methane and tracer amounts of SF6 was optimised. Key results Training of the animals is an essential step that should always be considered when implementing the SF6 technique. Subsampling the gas from canisters into glass vials for analyses should be conducted open air to avoid contamination issues. Inside the rumen, permeation tubes have a shelf life. Conclusions Having all the components of this methodology in-house facilitated the process of implementing the methodology, supported by collaborations with researchers at IZ and Agriculture Victoria who were already experts in this methodology. Implications Now that the SF6 technique is readily available at Gatton, through QASP and the analytical facility, further research can be conducted on the methane emissions of cattle raised under tropical conditions, expanding the applicability of this approach to grazing Bos indicus heifers measured through their first pregnancy cycle.