Simulation of methane emissions from dairy farms to assess greenhouse gas reduction strategies

As a sector, agriculture is reported to be the second greatest contributor to atmospheric methane (CH ₄) in the U.S., emitting 31% of the total emission. Primary sources of CH ₄ on dairy farms are the animals and manure storage, with smaller contributions from field-applied manure, feces deposited by grazing animals, and manure on barn floors. The Integrated Farm System Model (IFSM) was expanded to include simulation of CH ₄ emissions from all farm sources along with modules predicting other greenhouse gas (GHG) emissions. The new CH ₄ module incorporated previously published relationships and experimental data that were consistent with our modeling objectives and the current structure of IFSM. When used to simulate previously reported experiments, the model was found to predict enteric fermentation and slurry manure storage emissions similar to those measured. In simulating a representative 100-cow dairy farm in Pennsylvania, the model predicted a total average annual emission of 21 Mg CH ₄. This included annual emissions of 142 kg CH ₄ per cow from the Holstein herd and 6.4 kg CH ₄ per m ³ of slurry manure in storage, which were consistent with previously summarized emission data. To illustrate the use of the expanded whole-farm model, potential CH ₄ reduction strategies were evaluated. Farm simulations showed that increasing the production and use of forage (corn silage) in animal diets increased CH ₄ emission by 16% with little impact on the global warming potential of the net farm emission of all GHGs. Use of grazing along with high forage diets reduced net farm GHG emission by 16%. Using an enclosed manure storage and burning the captured biogas reduced farm emission of CH ₄ by 32% with a 24% reduction in the net farm emission of GHG. Incorporation of GHG emission modules in IFSM provides a tool for estimating whole-farm emissions of CH ₄ and evaluating proposed reduction strategies along with their impact on net GHG emission and other environmental and economic measures.