Greenhouse gas (GHG) emissions and their potential effect on the environment has become an important national and international issue. Dairy production, along with all other types of animal agriculture, is a recognized source of GHG emissions, but little information exists on the net emissions from dairy farms. Component models for predicting all important sources and sinks of CH(4), N(2)O, and CO(2) from primary and secondary sources in dairy production were integrated in a software tool called the Dairy Greenhouse Gas model, or DairyGHG. This tool calculates the carbon footprint of a dairy production system as the net exchange of all GHG in CO(2) equivalent units per unit of energy-corrected milk produced. Primary emission sources include enteric fermentation, manure, cropland used in feed production, and the combustion of fuel in machinery used to produce feed and handle manure. Secondary emissions are those occurring during the production of resources used on the farm, which can include fuel, electricity, machinery, fertilizer, pesticides, plastic, and purchased replacement animals. A long-term C balance is assumed for the production system, which does not account for potential depletion or sequestration of soil carbon. An evaluation of dairy farms of various sizes and production strategies gave carbon footprints of 0.37 to 0.69kg of CO(2) equivalent units/kg of energy-corrected milk, depending upon milk production level and the feeding and manure handling strategies used. In a comparison with previous studies, DairyGHG predicted C footprints similar to those reported when similar assumptions were made for feeding strategy, milk production, allocation method between milk and animal coproducts, and sources of CO(2) and secondary emissions. DairyGHG provides a relatively simple tool for evaluating management effects on net GHG emissions and the overall carbon footprint of dairy production systems.

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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.

Farming practices can have a large impact on the net emission of greenhouse gases, including carbon dioxide (CO 2), methane, and nitrous oxide (N 2 O). The primary sources of N 2 O from dairy farms are nitrification and denitrification processes in soil, with smaller contributions from manure storage and barn floors. Emissions from all greenhouse gas sources are interrelated, so strategies to reduce emissions from one source can affect emissions from another. Therefore, a comprehensive whole-farm evaluation is needed, which can be cost-effectively achieved through computer simulation. The Integrated Farm System Model (IFSM), a process-based whole-farm model, was extended to simulate sources of N 2 O and other greenhouse gas emissions. A module was added to simulate N 2 O emissions from croplands using relationships from the previously established DAYCENT model, and relationships were incorporated to predict emissions from slurry storage and free-stall barn floors. The new module was found to predict N 2 O emissions consistent with reported values from specific experiments and previously estimated whole-farm emissions. The model also predicted sensitivity to soil texture and soil water content similar to experimental data and DAYCENT model predictions, which further verified this most important component of N 2 O emissions. Simulations illustrated the impact of management practices on a representative farm in central Pennsylvania. Reducing the use of inorganic fertilizer by accounting for manure nitrogen (N) reduced N 2 O emission from the farm by 6% and reduced the net farm emission of all greenhouse gases in CO 2-equivalent units by 1%. Adding a mulch cover crop to corn land reduced N 2 O emission by 34% with a 7% reduction of all greenhouse gases. Use of a top-loaded manure storage tank prevented formation of a surface crust, which eliminated storage N 2 O emission with little effect on net farm emission of all greenhouse gases. This extended whole-farm model provides a tool for evaluating proposed N 2 O reduction strategies along with their effects on other greenhouse gas emissions, other N and phosphorus losses, and farm profitability.