Mitigating GHG emissions from ruminant livestock systems

Improving the net GHG budget of ruminant livestock systems without a reduction in productivity and economic sustainability, requires effective mitigation options in terms of abatement potential and costs. Grasslands and grassland management have a large potential to mitigate livestock GHG emissions at a low (or even negative) cost. A synthesis of eddy flux covariance data (i.e. 189 site years) shows on a mean net carbon storage equal to 0.76 ±0.1 MgC m-2yr-1, indicating a significant carbon sequestration of grasslands in Europe. Simple calculations showed that C sequestration of grasslands can compensate for other GHG emissions on site (i.e. enteric CH4 and soil N2O in CO2e) by staying close/below a critical stocking rate and moderate fertilizer application. However, to implement these findings into effective mitigation options, in terms of technical feasibility, abatement potential and costs of implementation, Life Cycle Assessments are required. For France, a national abatement study has been dedicated to identify and assess mitigation options without production losses at farm level. In addition to grassland management, two other actions have been identified for livestock farms (i.e. ruminants): reduction of enteric CH4 through addition of oilseeds or nitrates in the diet and reduction of nitrogen losses by decreasing dietary protein in dairy cows. The selected measures were quantified concerning their abatement potential and costs using a marginal abatement cost curve approach. In terms of unitary abatement potential (Mg CO2e avoided per ha or head) the most interesting measures are: increase in %legume, reduction dietary protein and nitrate addition to diet. When implemented on national basis, the addition of oilseeds and the increase of legume fraction had the highest abatement potential. In terms of abatement costs (€ per Mg CO2e) the most attractive measures are the extension of grazing season, increase in legumes content and a 10% reduction in fertilisation.