Methane emissions from enteric fermentation in dairy cows, 1990-2008

The Dutch protocol for the national inventory estimates the methane emission of the average Dutch dairy cow based on a Tier 3 approach. A dynamic, mechanistic model is used to represent the enteric fermentation processes, using annual national statistics on feed intake and feed composition as model inputs. Dutch dairy rations are based mainly on roughage (3/4 of dry matter ingested) with a high proportion of grass products (2/3 of roughage dry matter). Between 1990 and 2008, there were continuous increases in dry matter intake, milk production and enteric methane emission. Methane emission ranged from 111 to 129 kg/cow/year, and from 17.6 to 15.4 g/kg fat- and protein-corrected milk. The present study indicates that uncertainties in the feed intake level and the proportion and composition of grass products contribute to the variation in predicted methane emission. In addition, internal model equations also greatly contribute to the uncertainty (representation of rumen acidity and yield of volatile fatty acids). The greatest part of the uncertainty in the methane emission factor (kg methane/cow/year) is determined by the uncertainty in the feed intake and stoichiometry of volatile fatty acid production, while the greatest part of the uncertainty in the methane conversion factor (methane energy as % of gross energy intake) is determined by the stoichiometry of volatile fatty acid production in combination with the acidity of rumen digesta. Although the applicability of national statistics as model inputs can be investigated relatively easily in follow-up studies, physiological research with dairy cattle will be required to fully validate the current internal model equations.