Greenhouse Gas Emissions and Soil Carbon Stocks in Dairy Farming Systems

Abstract

Dairy farming is essential for global food security but contributes to GHG emissions and nutrient losses driving climate change. In Australia, dairy farms range from pasture-based to confinement systems, each with distinct environmental impacts and management complexities. The present thesis evaluated the environmental performance of these systems and associated monitoring technologies through five interconnected studies. The GHG from pasture-based and confinement dairy systems, assessed using a LCA approach including tree C sequestration, showed similar emission intensities (1.02 vs 1.07 kg CO₂-eq/kg FPCM) but differed in emission sources. Enteric CH₄ dominated emissions in both systems; manure emissions were higher in confinement systems, fertiliser-related emissions were higher in pasture-based systems, and tree C sequestration offset up to 6% of emissions in pasture-based systems. These findings highlight key mitigation priorities: manure management in confinement systems, fertiliser and pre-farm emissions in pasture-based systems, and enteric CH₄ reduction in both systems. Across 120 dairy farms, moderate concentrate supplementation (2–3 t DM/cow/year) increased milk yield (P < 0.001) and reduced emission intensity by 14%, although economic outcomes varied among farms. Low-cost CH₄ sensors compared with GreenFeed showed moderate correlations but low repeatability, indicating they are not yet interchangeable. Pasture-based systems had higher SOC and total nitrogen (TN), especially under improved pasture and trees, while total phosphorus (TP) was higher in confinement systems. Soil and Landscape Grid of Australia showed moderate accuracy for SOC and TN at farm scale but poor performance for TP and sub-farm predictions. Overall, both systems had similar emission intensities but different emission sources and soil outcomes. Sustainable improvement requires integrated whole-farm strategies linking nutrition, manure management, fertiliser use, and soil C stewardship.