As a sector, agriculture is reported to be the third greatest contributor of methane in the U.S., emitting one-quarter of total emissions. The primary sources of methane on dairy farms are animals and manure storages with smaller contributions from field-applied manure, feces deposited by grazing animals, and manure on barn floors. The Integrated Farm System Model was expanded to include simulation of methane emissions from enteric fermentation and the other farm sources. In simulating a representative 100-cow dairy farm in Pennsylvania, the model predicted a total average annual emission of 20 Mg CH4. This included an average annual emission of 135 kg CH4 per cow from the Holstein herd and an average emission of 5.4 kg CH4 per m3 of stored slurry manure, which were very similar to previously reported emissions. This expanded whole-farm model can be effectively used to evaluate proposed methane reduction strategies along with their impact on other environmental and economic issues.