This study determines the feeding practices that can reduce greenhouse gas (GHG) emissions from milk production in Ontario at least cost, and estimates the associated impact on farm gross margin. A linear programming model is developed to account for emissions both from the farm as well as from the production of feed and other inputs upstream. The results indicate that changing rations can reduce GHG emissions from milk production by up to 35% from original levels, with a corresponding decline in farm gross margin of 23.4%. The cause of the declines in GHG emissions is because corn silage is replaced by high quality perennial forage (alfalfa hay). This allows for an increase in carbon storage in land, due to the enhanced carbon storage capacity of perennial forages as opposed to annual crops (i.e. corn), and due to lower capital and chemical inputs. The implications are that there is large potential for reducing GHG emissions from milk production in Ontario due to the potential for perennial forages to capture and store carbon in soil. This necessitates that perennial forages replace corn silage as the primary source of roughage in dairy rations. Current rations have corn silage as about 20% of dry matter intake.