Almost half of New Zealand's greenhouse gas emissions arise from agriculture and enteric methane (CH 4 ) emissions arising from ruminant animals constitute 30% of total CO 2 -e emissions. Enteric CH 4 emissions have increased by 9% since 1990. Extensive research has been undertaken to develop reliable methods for measuring enteric CH 4 emissions. New Zealand studies using the SF 6 tracer technique suggest that on average this technique yields similar values to the 'gold' standard of calorimetry, but with a larger variance. National inventory estimates based on results obtained using the SF 6 technique will therefore overestimate the uncertainty. Mitigating emissions can be achieved by changing feed type but there are practical and cost barriers to the use of alternative feeds. Forages containing condensed tannins do reduce emissions but are agronomically inferior to the forages currently used. Rumen additives have shown some success in-vitro but results from in-vivo trials with both monensin and fumaric acid have been disappointing. The development of methods for directly manipulating rumen microorganisms are at an early stage and work to develop vaccines that can inhibit methanogenesis has yielded mixed results. The successful identification of sheep with contrasting CH 4 yields raises the possibility that, in the long term, a breeding approach to CH 4 mitigation is feasible.
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Clark et al. (2010) studied this question.