A new Tier 3 method to calculate methane emission inventory for ruminants.

Livestock is the main source of methane (CH) emissions. It is important to accurately determine emissions from ruminants that meet standardized international guidelines for national greenhouse gas inventories. A new method to improve the accuracy of CH emissions that complies with IPCC rules for a Tier 3 method is described and evaluated. This method, developed by INRA (French Institute for Agricultural Research), was applied to the French inventory of CH emissions by ruminants and compared with the IPCC Tier 2 method. For enteric CH, depending on the animal category, the INRA CH emission estimates lay between 88% and 114% of IPCC's. The INRA/IPCC ratio for enteric emission was close to unity and did not differ between methods (P = 0.43) for adult cows (i.e. most cattle). In France, feedlot manure is stored in aerobic conditions, and so the INRA/IPCC fit for manure emission was poorer (P < 0.05). The INRA/IPCC fit for enteric CH was very close between methods to that for total CH (P = 0.39), enteric CH representing 93% of total emissions. The main improvement is the use of a robust equation (from numerous data and diets), based on digestible organic matter intake (DOMI) corrected for the digestive interactions, to predict CH consistently from enteric and manure sources. It was developed for the French livestock inventory but is customizable for other countries. This new improved CH estimation method, based on equations from a large literature database, complies with IPCC rules for a Tier 3 method.