Simulation of Carbon Dioxide Emissions from Dairy Farms to Assess Greenhouse Gas Reduction Strategies
Farming practices can have a large impact on the soil carbon cycle and the resulting net emission of greenhouse gases including carbon dioxide (CO2), methane, and nitrous oxide. Primary sources of CO2 emission on dairy farms are soil, plant, and animal respiration, with smaller contributions from microbial respiration in manure. Strategies designed to reduce emissions from one source can cause an increase in emissions from another source. Therefore, a comprehensive whole-farm evaluation is needed, which can be cost-effectively met through computer simulation. The Integrated Farm System Model (IFSM), a process-based whole-farm model, was extended to simulate the carbon cycle. Relationships were added to represent photosynthetic fixation, soil and plant respiration, animal respiration, and emissions from manure storage and barn floors. The new module was verified to predict the mass of carbon present in soil pools at the end of annual simulations and to predict CO2 emissions within expected emission ranges for both specific sources and overall farm emissions. A farm-level carbon balance was used to further verify that predicted emissions were reasonable across a variety of production strategies. Farm simulations illustrated that changes in cropping practices affected emissions from all farm sources, with a primary effect on the assimilation of CO2 in feed production. For a representative farm in central Pennsylvania, use of more alfalfa in place of corn production caused a 6% increase in net farm greenhouse gas emission in CO2-equivalent units, while replacing non-permanent grassland with corn production reduced the net emission by 16%. Changing from a Holstein herd to Jersey animals with animal numbers increased to produce the same amount of milk affected most emission sources, with a net impact of increasing the net greenhouse gas emission by 20%. Incorporation of greenhouse gas emission modules in IFSM provides a more comprehensive tool for evaluating the overall farm-level environmental and economic impacts of management scenarios used to reduce emissions.