Nitrogen and carbon footprints of dairy farm systems in China and New Zealand, as influenced by productivity, feed sources and mitigations

Three dairy farm systems in China and New Zealand (NZ) varying in intensity based on level of use of brought-in crop feeds were selected from surveyed data. Nitrogen (N) emissions were estimated using country-specific N and life cycle assessment models. Milk production per cow increased with increased use of grain-based feeds but there was no whole-system difference in energy and land use resource efficiency. The N footprint (∑reactive-N emissions kg-1 milk for cradle-to-farm-gate) was 1.3-2.3 times higher for the housed-cow Chinese farm systems than the year-round pasture-grazing NZ farm systems, associated with greater emissions of all forms of reactive-N. It decreased with increased feed use in both countries, mainly due to decreased ammonia emissions. In NZ the N loss to water kg-1 milk increased due to the contribution from feed crops. The source of feed was an important determinant of environmental impacts, and changing to low N-footprint feeds decreased the N footprint of milk by up to 10% in both countries. However, manure management was the dominant contributor to the N footprint for all farms, and particularly in China. Mitigation analysis showed the potential to decrease the N footprint of milk by over 30% with improved manure management practices, particularly from utilizing manure that is currently discharged. The largest mitigation potential (up to -25%) in NZ was from ceasing N fertilizer use on pasture and relying on clover N2 fixation. Scenario analysis for late-autumn/winter housing of cows in NZ decreased N loss to water but greatly increased ammonia emissions, resulting in an increase in N footprint of up to 21%. Thus, while feed and manure management are key to reducing the N footprint of milk in both countries, the largest reduction opportunities require focus on system-specific hot-spots of N emissions.