Whole-Farm Greenhouse Gas Emissions: A Review with Application to a Pennsylvania Dairy Farm

Abstract

Greenhouse gas (GHG) emissions to the atmosphere and their potential impact on global climate have become important concerns world-wide. Livestock production systems, such as dairy farms, provide both sinks and sources for GHGs. Typical emissions have been quantified in Europe and synthesized to estimate farm-level GHG emissions. However, fewer data are available in the United States, and little has been done to estimate emissions for our farms. Through an extensive literature review, average GHG flows were quantified for each major farm source and sink including the soil, growing crops, animals, and manure storage. These typical gas exchanges were then combined to estimate farm-level net emissions for a representative, 100-cow dairy farm in Pennsylvania. Emissions from animal facilities primarily consisted of animal respiration (532 Mg CO2 yr-1) and enteric fermentation (16.9 Mg CH4 yr-1) with a total annual emission of 971 Mg CO2 equivalent (CO2e) where each unit of CH4 is equivalent to 25 units of CO2 in global warming potential. Manure storage emissions included CO2, CH4, and N2O for a total annual emission of 216 Mg CO2e with each unit of N2O equivalent to 298 units of CO2. Cropland provided a net flux or sink of -784 to -168 Mg CO2e yr-1 depending upon the amount of manure carbon sequestered in the soil. The estimated whole-farm net annual GHG emission ranged from 2.5 to 5.8 Mg CO2e per 500 kg livestock unit or 0.50 to 1.2 kg CO2e kg-1 of milk produced. This review and farm analysis has helped direct modeling efforts by determining the important processes that drive emissions of CO2, CH4, and N2O in dairy production along with expected ranges for these emissions. Such data expand the knowledge base of researchers, farm planners, and policymakers as they work to develop and maintain sustainable farming systems in the United States.