Estimating methane emissions from manure: a suitable case for treatment?

Methane from livestock is a significant source of greenhouse gas emissions. Under the UN Framework Convention on Climate Change (UNFCCC), Annex I countries’ National Inventories report emissions from cattle as enteric or from manure management at ratios of between 3:1 and 9:1 depending on country and cattle type. Field research generally supports the inventories’ assumptions about enteric emissions, but these ratios have focused interest on enteric emissions and diverted attention away from manure management. Official calculations about manure management emissions factors are more varied than those for enteric emissions and evidence from field measurements suggests inventories may be underestimating manure management emissions especially in the dairy sector. This paper has three objectives. First, it reviews the science underpinning the international framework for estimating methane emissions from manure management. Second, it presents data from two dairy farms in south-west England where measured emissions of methane from slurry storage facilities are four to five times greater than the assumptions in the UK’s inventory. If these measurements were representative of the UK, the implication is that total methane emissions from the UK dairy herd would be over 40 per cent greater than the level reported to the UNFCCC and the proportion of total methane emissions from manure management would be almost a half rather than less than a quarter. Finally, the paper assesses the potential value if methane were captured from slurry storage facilities. Its value as a biogas is estimated to be £500 million per year for the UK dairy industry (at forecourt diesel prices). The paper concludes that the scale of emissions and the potential economic value of lost biogas are sufficient to warrant urgent research and action to reduce emissions from manure management with the beneficial prospect that a valuable new income stream for farm businesses could also be realised.