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.

Introduction

     Agricultural systems are challenged to balance feeding a growing population while addressing environmental concerns and providing a high quality of life for farmers and rural communities. Agricultural emissions to air and water contribute to climate change, ecosystem deterioration, and human health issues (Poore and Nemecek, 2018). In the U.S., agriculture is responsible for 10% of total greenhouse gas (GHG) emissions, with soil management, enteric fermentation, and manure accounting for 55%, 28% and 10%, respectively (U.S. EPA, 2021a). Up to 70% of the excreted nitrogen (N) in manure can be emitted as ammonia (NH3) in livestock operations (Hristov et al., 2002). These losses represent 80–۹۰% of the global anthropogenic NH3 emissions (Xu et al., 2019) that can redeposit and lead to impaired waterways (U.S. EPA, 2004), or further transform to particulate matter or N2O. Nutrient losses from manure have the potential to reach ground and surface water contributing to eutrophication.

Conclusions

     GHG emissions (including C sequestration), NH3 emissions, resource depletion (energy, land, and water use), and EP at organic dairy farms in the U.S. were evaluated. Despite that milk production levels are generally lower than conventional systems, environmental impacts are comparable. Carbon sequestration is largely absent from LCA studies given its complexity but can be important for dairies that rely on pasture and forages. This study presents a procedure that can be implemented in other dairy or agricultural related LCA studies and promote discussion around this topic. Enteric CH4 remains the most prominent source of GHGs from cradle-to-farm gate, but results show that farms can still improve feeding efficiencies. Impacts from energy consumption are directly related to the regional electricity mix, with the implementation of renewables having the greatest impact on Amish farms that use diesel generators. As NH3 emissions continue to be an important concern for animal agriculture, baseline estimations will help establish mitigation targets and strategies. Manure management remains an important source of environmental impacts, especially during storage and after land application. Water use is significantly higher on farms that use irrigation and import feed. Alternative management practices can reduce GHGs and can guide on-farm implementation. Variations in results by different methodological decisions show these choices remain an unresolved topic in LCA worthy of discussion.