Abstract Greenhouse gas (GHG) emissions from enteric and manure management (MM) activities tend to dominate the carbon footprint of beef and milk production. In this study, a comparative life cycle assessment of dairy milk production (cradle-to-farm gate) was undertaken where the trade-offs of an automated composter-separator technology were contrasted to those of a conventional liquid manure storage system. A simulation of a dairy farm in Ontario, Canada was carried out using the process based manure-DeNitrification DeComposition (Manure-DNDC) model after it was calibrated to a comprehensive set of on-farm MM GHG emission measurements. Results indicated that the active composter system on this farm reduced the carbon footprint (CF) of milk production by 36% (from 1.92 to 1.23 kg CO 2 eq kg −1 fat-protein corrected milk). For the simulated dairy farm system with only liquid manure storage as a treatment option, the GHG emissions (mainly methane) from the manure storage tank contributed 56% of the carbon footprint, which surpassed the contribution from enteric methane emissions. On the other hand, for the dairy farm with the active composter system, GHG emissions from manure (34% of CF) and enteric emissions (31% of CF) were found to be nearly equivalent in terms of carbon footprint contribution. The composter system created other life cycle benefits (no need to import sand for bedding, increased soil carbon sequestration) and burdens (increased on-farm electricity demand for composter operation), however, the relative contributions of these additional trade-offs were small. Given these additional trade-offs were minor, the GHG mitigation potential of this manure composting technology is likely high in any region of application. In addition, the dairy system with the composter created only marginal problem shifting concerns in terms of the non-CF environmental impact categories considered. For the 146 lactating dairy cow system investigated, an estimated 1460 tCO 2 eq in life-cycle GHG emissions reduction can be achieved on an annual basis. The benefits of manure composter solutions should provide an impetus for jurisdictions to explore the development of climate policies that incentivize increased adoption rates of this important climate change mitigation opportunity.

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