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Several strategies are available for mitigating greenhouse gas (GHG) emissions associated with dairy manure management in barns, storage units, and fields. For instance, incorporation of manure into the soil, solid-liquid separation, composting, enclosed manure storage, and anaerobic digestion have been identified as good options. However, these strategies are not widely adopted in Canada because clear information on their effectiveness to abate the whole-farm GHG footprint is lacking. Better information on the most cost-effective options for reducing on-farm GHG emissions would assist decision making for dairy producers and foster adoption of the most promising approaches on Canadian dairies. In this context, whole-farm modeling provides a tool for evaluating different GHG abatement strategies. An Excel-based linear optimization model (N-CyCLES) was used to assess the economics and the nutrient and GHG footprints of two representative dairy farms in Québec, Canada. The farms were located in regions with contrasting climates (southwestern and eastern Québec). The model was developed to optimize feeding, cropping, and manure handling as a single unit of management, considering the aforementioned mitigation options. Greenhouse gas emissions from the different simulated milk production systems reached 1.27 to 1.85 kg CO2e kg-1 of corrected milk, allowing GHG reductions of up to 25% compared to the base system described in Part I. Solid-liquid separation had the greatest GHG mitigation potential, followed by the digester-like strategy involving a tight cover for gas burning. However, both options implied a decrease in farm net income. Manure incorporation into the soil and composting were associated with high investment relative to their GHG abatement potential. The most cost-effective option was using a loose cover on the manure storage unit. This approach lessened the manure volume and ammonia-N volatilization, thereby reducing fertilizer and manure spreading costs, increasing crop sales and profit, and enhancing the whole-farm N and GHG footprints. Consequently, covering the manure tanks appears to be an economically viable practice for Québec dairy farms. Keywords: Anaerobic digestion, Composting, Dairy cow, Farm net income, Greenhouse gas emission, Incorporation, Nutrient footprint, Solid-liquid separation, Storage cover, Whole-farm model.

Tie-stall housing (93%) and solid manure management (44%) are used on many dairy farms in the province of Québec, Canada. However, this could change in the near future because the rise in average herd size and the popularity of milking robots are such that the industry expects an increase in free-stall dairies managing manure with liquid systems. This shift could affect the carbon (C), nitrogen (N), and phosphorus (P) footprints of Québec’s dairy production. In this context, whole-farm modeling (N-CyCLES), considering all the production cycle, provides a tool for evaluating the economics and environmental impacts of standard housing and manure management systems (Part I) in combination with different mitigation approaches (Part II). Two representative dairy farms in southwestern Québec (SWQ; 45.3° N, 73.2° W) and eastern Québec (EQ; 48.45° N, 68.1° W) were simulated considering four scenarios involving combinations of tie-stall or free-stall housing and solid or liquid manure management. Maximum farm net income (FNI) was $0.33 and $0.18 kg-1 of fat- and protein-corrected milk (FPCM) for the SWQ and EQ farms, respectively, with N and P footprints of 12.22 to 16.99 g N kg-1 and 0.52 to 0.79 g P kg-1 of FPCM in SWQ, and 11.48 to 15.39 g N kg-1 and 1.41 to 1.88 g P kg-1 of FPCM in EQ. Greenhouse gas (GHG) emissions reached 1.78 to 1.87 kg CO2e kg-1 and 1.67 to 1.71 kg CO2e kg-1 of FPCM in SWQ and EQ, respectively. The SWQ farm was associated with greater production of cash crops but also greater imports of fertilizers and purchased feeds, which negatively affected the N footprint and GHG emissions. Housing and manure management types did not influence FNI. Free-stall dairies were associated with greater N surpluses. Nevertheless, they emitted slightly less GHG than tie-stall dairies. Dairy farms under liquid manure management imported less fertilizers and produced less GHG despite greater CH4 emissions. As a result, the current transition toward free-stall barns and liquid manure systems in Québec seems advantageous from an environmental standpoint without compromising economic profitability. Keywords: Climate change, Dairy cow, Farm net income, Free stall, Greenhouse gas emission, Manure handling, Mitigation, Nutrient footprint, Tie stall, Whole-farm model.