Technical Abstract: Grass-based dairy production has been suggested as a healthier alternative for both the animals and the consumer compared to production using more traditional confinement feeding. In addition, pasture-based systems have been shown to decrease soil erosion and downstream nutrient pollution of surface waters. However, little data are available on greenhouse gas emissions from various production options. Emissions of methane, nitrous oxide, and carbon dioxide are thought to be contributing to change in our global climate. Because agriculture is known to be a major contributor of the emission of these compounds, economically-feasible reduction strategies must be identified and implemented. Feeding a higher fiber diet of pasture can increase enteric methane production, and pastures may emit greater amounts of nitrous oxide than cropland. However, reducing the use of manure storage and related manure handling processes can reduce the total emission of all three gases. As a result, a whole farm evaluation is needed to assess the overall impact of production strategies. To conduct this analysis, components were developed and added to the Integrated Farm System Model to simulate whole-farm emissions of the three greenhouse gases as functions of the production practices used. This revised model provides a tool for evaluating and comparing greenhouse gas emissions from farming systems along with other performance measures such as farm profitability and the losses of nitrogen and phosphorus to the environment. Simulations were done comparing three dairy production systems in Pennsylvania: 1) all perennial pasture with seasonal calving and outwintering of animals, 2) seasonal grazing with random calving and winter confinement, and 3) full confinement of cattle fed balanced diets of harvested feed. As the world seeks to reduce greenhouse gas emissions, these results will help define the role of grazing in dairy production.