A case study of the potential environmental impacts of different dairy production systems in Georgia

The biological and physical processes of an intensively-managed rotational pasture-based dairy and a confinement fed dairy in the southeastern United States were simulated with the Integrated Farm System Model (IFSM) to evaluate management effects on greenhouse gas emissions, soil carbon sequestration, carbon footprint, nitrate leaching, ammonia volatilization, erosion, phosphorus runoff, and phosphorus accumulation in the soil. Edge-of-field erosion and phosphorus runoff were less for the pasture-based dairy per unit of land and per unit of milk produced, but nitrate leaching was greater. Ammonia emissions were greater from the confinement dairy because of the greater handling of manure. Greenhouse gas emissions per cow were greater on the confined dairy, but with greater milk production per cow, the carbon footprint of milk produced was similar to that of the pasture-based dairy. Considering the potential soil carbon sequestration following the conversion of crop land to perennial grassland, the carbon footprint of the milk produced by the pasture-based dairy was slightly less than that of the confinement dairy. The results of this study were generally consistent with similar simulation studies done in the northeastern US with variations due to regional differences in climate, soil type, and agronomic practices. Simulated changes in production practices predicted that increasing milk production through improved animal management or feeding more corn decreased the carbon footprint of milk produced by the pasture-based dairy, while decreasing the inorganic nitrogen fertilizer application rate or raising replacement heifers on the farm had little effect. On the confinement dairy, covering the manure storage and flaring the biogas decreased the carbon footprint, using higher producing, pure-bred Holstein cows or producing less forage on the farm increased the footprint, and eliminating free-stall barns and placing all cattle on pasture had little effect on the footprint. The IFSM was capable of adapting to the climate and production practices of the southeastern US, but further improvements could be made to better represent the cropping practices used in this region.