Mitigation of greenhouse gas (GHG) emissions of dairy farms may conflict with farmers' needs to adapt to complex, dynamic and interrelated changes in production. We performed a diachronic study that considers change in GHG emissions at the farm level following implementation of adaptations by dairy farmers. We hypothesized (i) that farm adaptations follow three pathways discussed in the scientific literature: specialization and intensification, eco-efficient intensification, agroecological transition, and (ii) that these three adaptation pathways drive the evolution of GHG emissions of dairy farms, particularly by determining farms' reliance on purchased nitrogen (N) fertilizers and N-rich concentrates. We identified 10 dairy farms in the Aveyron department, France, that capture the diversity of local farming systems on a gradient of intensified land use, dairy herd, or both. We considered two situations for each farm: (i) the current situation and (ii) a former situation, at most 10 years before, suggested by the farmer during the survey and different from the current situation. We estimated product-related (per kg of milk) and area-related (per ha) GHG emissions for each farm with a farm model that includes a GHG emission calculator consistent with the International Panel on Climate Change methodology. Two farms followed the trend of specialization and intensification by increasing their stocking rate and maintaining or increasing their milk production per unit of area and livestock while simplifying and standardizing farm management, such as feeding cows with larger amounts of maize silage and N-rich concentrates. Three farms followed a trend of eco-efficient intensification by increasing their milk production per unit of area and livestock while maintaining (or slightly changing) their consumption of N fertilizers and N-rich concentrates. Three farms adopted agroecological practices by maintaining or decreasing their stocking rate and farm production per unit of area and livestock and overall increasing their agricultural diversity and substituting inputs (e.g., nitrogen fertilizers) with ecosystem services (e.g., nitrogen fixation by legumes). Unlike our first hypothesis, there was a fourth trend, i.e., status quo, operated by two farms. Results showed that agroecological transition reduced GHG emissions the most (by -2% to -40% per kg of milk and by -18% to -29% per ha). The dairy farms following this transition had the lowest mean product-related (0.89 kg CO2eq./kg milk) and area-related (5306kg CO2eq./ha) GHG emissions. Eco-efficient intensification reduced mean product-related GHG emissions (by 10%, to 0.93 kg CO2eq./kg milk), but increased mean area-related GHG emissions (by 18%, to 6239kg CO2eq./ha). In contrast, specialization and intensification increased mean GHG emissions (by 6% per kg of milk, to 1.20kg CO2eq./kg milk, and by 28% per ha, to 8741 kg CO2eq./ha), making it appear unsuitable as a pathway towards mitigating climate change at the farm level.
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