Variability in dairy farm greenhouse gas emissions: implications of gross and net GHG estimates for mitigation

Understanding variability in greenhouse gas (GHG) emissions among dairy farms is essential for identifying effective mitigation strategies. This study quantified gross and net GHG emissions in 62 commercial dairy farms in Québec, including a subset of 42 farms for which soil and tree carbon stock changes were assessed. Mean gross GHG emissions were 1.05 ± 0.16 kg CO₂-eq/kg fat- and protein-corrected milk (FPCM), with moderate variability (CV = 14.6%; range: 0.59–1.42). Net GHG emissions, accounting for soil and tree carbon stock changes, averaged 1.06 ± 0.35 kg CO₂-eq/kg FPCM and showed greater variability (CV = 33%; range: 0.28–1.87). Enteric fermentation was the main emission source (48% of gross emissions), followed by manure management (18%), soil emissions (14%), and feed production (13%). Changes in soil and tree carbon stocks strongly influenced net emissions, with combined effects ranging from −0.66 to +0.52 kg CO₂-eq/kg FPCM. Cluster analyses revealed significant differences among farms for gross and net GHG emissions, showing that inclusion of carbon stock changes modified farm performance interpretation. Greater milk yield per cow, dietary starch content, and soil clay content were associated with lower emission intensity. Both emission sources and carbon sequestration contributed to farm-level variability.