Comparing greenhouse gas emission accounting tools for organic dairy stakeholders

Dairy farms have established net-zero goals to mitigate greenhouse gas (GHG) emissions in the U.S. requiring accounting of GHG predictions to assess progress. Tools such as Cool Farm Tool (CFT) and COMET-Farm are available to estimate farm-level GHGs, but the outputs from these calculators have not been evaluated for organic dairies. This study compares GHG estimates from 14 organic dairy farm scenarios using CFT, COMET-Farm, and a partial life cycle assessment (LCA) model in four regions of the U.S. Multiple data adjustments were required to compare GHGs from these different accounting tools. Intensities of GHGs are greater for CFT while the LCA and COMET-Farm produce similar results (CFT: 0.97–1.77, LCA: 0.84–1.50, COMET-Farm: 0.83–1.60 kg CO 2 -eq/kg of fat and protein corrected milk (FPCM)). Enteric methane (CH 4 ) accounts for 42–69% of GHGs for all tools and scenarios. Manure management CH 4 and crop production nitrous oxide (N 2 O) constitute the next most prominent sources of GHGs after enteric CH 4 contributing 1–23% and 4–18%, respectively. Crop production GHGs are primarily from manure application N 2 O emissions and are higher in scenarios with greater land use. COMET-Farm is not an LCA tool and does not account for GHGs from materials produced off-farm or express results per functional unit, thus limiting applicability to larger reporting needs. Although the three tools generally agree on trends in enteric CH 4 emissions, estimates can differ by up to 26%, and discrepancies in manure-related emission estimates are even larger. As a result, the estimated impact of a mitigation practice can vary considerably across tools, even when the same practice is evaluated.