Farm level environmental assessment of organic dairy systems in the U.S.

Abstract

A farm scale life cycle assessment (LCA) was conducted to estimate environmental indicators of organic dairy systems in the U.S. and evaluate alternative management practices and methodological decisions. Fourteen farm layouts (including Amish and grass intensive) are evaluated over four U.S. regions. Carbon (C) sequestration from pasture and cropping systems is estimated based on C added to the soil and the crop and grassland management practices. Greenhouse gas (GHG) emissions range from 0.76 to 1.08 kg CO2-eq/kg fat and protein corrected milk (FPCM) after C sequestration. Methane (CH4) from enteric fermentation and liquid-slurry manure storage are major sources of GHGs, with the first related to cow feed to milk conversion efficiency. The production and consumption of fossil energy contribute to GHGs depending on the mix of fuels of regional electricity production. NH3 emissions range from 7.7 to 20.0 g/kg FPCM with differences between regions explained by environmental factors, management practices, and dairy diet composition. Eutrophication potential ranges from 3.4 to 6.6 g PO4/kg FPCM from phosphorus and nitrogen losses after manure application from on-farm and imported feeds. Electricity and diesel are the major contributors to fossil energy depletion (2.1 to3.7 MJ/kg FPCM), with the embedded energy from imported feeds also contributing significantly. Land use ranges from 0.9 to 2.0 m2/kg FPCM, influenced by the composition of the diet, crop yields, and milk production. Water use is increased from a range of 5.3 to 13.6 kg/kg FPCM to a range of 173 to234 kg/kg FPCM in irrigated farms. The analysis of 14 alternative management practices measures GHG reductions of 15% and 30% for individual and combined practices, respectively, where manure storage and renewable energy production have the greatest benefits. Land and animals as alternative functional units change trends in GHGs, but trends are maintained when analyzing different enteric CH4 equations with variations in intensity. The allocation to milk based on nutritional content is greater than an energy-based allocation, while the choice of N2O emission factor from manure deposited on pasture is more significant for farms with long grazing seasons. Combining the alternative methodological choices increases GHGs by 30%.