Greenhouse gas emissions of confinement and pasture-based dairy farms: Implications for mitigation

ABSTRACT: Despite extensive research on the environmental effects of dairy farming, comparative GHG emissions from confinement and pasture-based systems remain unclear due to inconsistent findings from prior studies, which were often specific to the local conditions of each system and overlooked carbon sequestration by trees. The present study aimed to compare the GHG emissions of 2 Australian milk production systems (confinement and pasture-based) using a life cycle assessment approach that incorporates C sequestration by trees. The confinement system used a TMR, whereas the grass-based system primarily relied on grazed forage with concentrate supplementation. The Australian Dairy Carbon Calculator, a Tier 3 tool, predicted emission intensity using the National Greenhouse Gas Inventory and Intergovernmental Panel on Climate Change methods, as reported to the United Nations Framework Convention on Climate Change. Emission intensity was calculated as net GHG exchange in CO2 equivalents (CO2eq), allocated to milk and meat. Animal emissions dominated: 85% of total emissions in confinement systems (54% enteric CH4, 31% manure) and 71% in pasture-based systems (58% enteric CH4, 13% manure). The confinement system showed 13% lower enteric CH4 intensity and 88% lower prefarm embedded intensity (kg CO2eq/kg fat- and protein-corrected milk [FPCM]) but 129% higher manure-related GHG intensity than the pasture-based system. Emission intensities for milk (1.02 ± 0.038 vs. 1.07 ± 0.069 kg CO2eq/kg FPCM) and meat (5.51 ± 0.779 vs. 6.76 ± 0.868 kg CO2eq/kg liveweight) were similar between systems. The emission offset by tree C sequestration (kg CO2eq/kg FPCM) was relatively low in both systems, contributing about 1% of total CO2eq emissions in confinement systems and up to 6% in pasture-based systems. Targeted mitigation should address manure emissions in confinement systems, prefarm embedded, and fertilizer emissions in pasture-based systems, and enteric CH4 in both.