Nitrogen Flows, Nitrogen Use Efficiency, Nitrogen Footprint and Mitigation Strategies in a Large-Scale Dairy Farm: A High-Resolution Case Study in China

Large-scale dairy farms with anaerobic digestion (AD) are expanding rapidly in China, yet high-resolution assessments of N flows, N use efficiency (NUE), and N footprint remain limited. This study conducted a process-based N accounting and N footprint assessment for a large-scale commercial dairy farm with AD in China using farm-specific data on herd structure, feed intake, milk production, manure management, feed production, transportation, and on-farm energy use. Whole-farm N flows were traced from feed inputs to animal products, manure excretion, manure treatment, field application, soil deposition, and environmental losses. Total herd N intake was 3.12 × 106 kg N yr−1, of which 5.83 × 105 kg N was exported in milk, and 2.45 × 106 kg N was excreted. Lactating cow milk NUE was 28.11%, while whole-herd milk-only and product NUE were 18.70% and 19.90%, respectively. The milk-allocated N footprint was 7.21 g N kg−1 fat- and protein-corrected milk (FPCM) under the cradle-to-farm-gate boundary and increased to 13.56 g N kg−1 FPCM when manure land application was included. Feed production was the largest contributor under the cradle-to-farm-gate boundary, whereas manure management dominated under the expanded boundary. Scenario analysis showed that the largest simulated reductions were 30.52% for dietary mitigation under the cradle-to-farm-gate boundary and 18.77% for combined manure-management mitigation under the expanded boundary. These results highlight the need for coordinated improvements in feeding, herd management, and manure management.