Abstract. The methods for estimating methane emissions from cattle as used in the Australian national inventory are based on older data that have now been superseded by a large amount of more recent data. Recent data suggested that the current inventory emissions estimates can be improved. To address this issue, a total of 1034 individual animal records of daily methane production (MP) was used to reassess the relationship between MP and each of dry matter intake (DMI) and gross energy intake (GEI). Data were restricted to trials conducted in the past 10 years using open-circuit respiration chambers, with cattle fed forage-based diets (forage >70%). Results from diets considered to inhibit methanogenesis were omitted from the dataset. Records were obtained from dairy cattle fed temperate forages (220 records), beef cattle fed temperate forages (680 records) and beef cattle fed tropical forages (133 records). Relationships were very similar for all three production categories and single relationships for MP on a DMI or GEI basis were proposed for national inventory purposes. These relationships were MP (g/day) = 20.7 (AE0.28) · DMI (kg/day) (R 2 = 0.92, P < 0.001) and MP (MJ/day) = 0.063 (AE0.008) · GEI (MJ/day) (R 2 = 0.93, P < 0.001). If the revised MP (g/day) approach is used to calculate Australia's national inventory, it will reduce estimates of emissions of forage-fed cattle by 24%. Assuming a global warming potential of 25 for methane, this represents a 12.6 Mt CO 2 -e reduction in calculated annual emissions from Australian cattle.

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Human activities have increased the atmospheric concentration of methane by about 140% since pre-industrial times. The accumulation of methane and other ‘greenhouse’ gases is anticipated to cause significant climate changes in the future. Ruminant livestock are the largest producers of methane in Australia and this source constitutes about 12% of the national net emissions. Australia is a signatory to the Kyoto Protocol, which, if it comes into force, requires limiting annual emissions during the period 2008–2012 to 8% over the 1990 value. Australian livestock emissions are projected to increase by 7% by 2010 with total Australian emissions expected to increase by 28–43%. Emissions per unit GDP are higher for the livestock sector than for most other sectors and this may negatively affect the sector if free market emission trading is implemented and no new technologies to reduce emissions cost-effectively are introduced. Using information from the National Greenhouse Gas Inventory, we demonstrate that reductions in emissions per unit product are already occurring in at least one Australian livestock industry and discuss ways to ensure that similar future changes will be recorded. Cautionary notes are made regarding options of grain feeding and more intensive production, which appear to be attractive but may lead to increasing emissions when viewed on a broader basis. The potential for increased animal production with new technologies developed to reduce methane emissions suggests that there may be significant opportunities for the Australian livestock industries arising from the issue of greenhouse gas reductions. Opportunities to establish carbon sinks are also discussed. We suggest that addressing reduction of emissions per hectare rather than per head or per kilo of product results in a strong alignment with the development of more sustainable livestock industries.