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

摘要

Three dairy farm systems in China and in 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 (Sigma 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. The N footprint decreased with increased feed use and milk production in both countries, mainly due to decreased ammonia emissions. There were similar trends in carbon (C) footprint (total greenhouse gas emissions) of milk, except within NZ where there was no change with feeding level. 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 of Chinese farm systems 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 N-2 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 and C footprints of up to 21%. Thus, Chinese dairy farms can improve environmental efficiency through sourcing low-impact feeds, improved manure management and integrating manure recycling with feed crops. In contrast, NZ farms can improve environmental efficiency through efficient use of grazed legume-based pastures rather than using crop-feeds or cow housing systems.