Breed and Season-Specific Methane Conversion Factors Influence Methane Emission Factor for Enteric Methane of Dairy Steers

This study determined the breed and the season-specific methane (CH4) conversion factor (Ym) and the emission factor (EF) for the enteric CH4 of dairy steers. The Ym values for Holstein and Jersey steers at different seasons were calculated using the IPCC 2006 equations by incorporating the input and/or output value of the chemical composition of feed, methane production, methane yield, dry matter intake, and methane energy emission. EFs were categorized into five types depending on the 2019 refinement to the IPCC 2006 Tier 2 equations used. EFA was calculated from Equation 10.21A (New), while other EFs were estimated from the Equation 10.21 which were designated according to the gross energy intake (GEI) and Ym as EFB (GEIi and Ym), EFC (GEIii and Ym), EFD (GEIii and Ym (6.3)), and EFE (GEIii and Ym (4.0)). The calculated overall Ym for Holstein and Jersey steers were 4.90 and 7.49, while the recorded EF of group EFA were 56.44 and 67.42 kg CH4/head/year for Holstein and Jersey steers, respectively. For Holstein steers, EFD was overestimated (75.91 vs. 48.20~58.15), while in Jersey steers, the EFF underestimated the EF (kg CH4/head/year) compared to others (40.15 vs. 63.24~73.28) (p < 0.05). Mixed analysis revealed that the breed influenced EFs of all the EF groups, while the season, and the breed × the season influenced EFs of group EFC, EFD, and EFF. The overall results recommended using the breed-specific Ym for the estimation of the EF for enteric methane in dairy steers.