Forages are an important part of crop rotations in many agricultural systems. In Manitoba, Canada, almost 40% of the 7 million ha agricultural area is devoted to forage crops and pastures. Use of legume forages to reduce fertilizer cost to producers can improve soil quality and productivity and may provide more options for diversification of the agricultural production system. We tested that whether a twofold increase in forage production for feeding beef cattle would change greenhouse gas (GHG) emissions for the agriculture and agri-food system and economic returns at the farm in the year 2011. The forage resulting from the increased production was fed to beef cattle. The GHG emissions were calculated using Intergovernmental Panel on Climate Change Tiers 1 and 2 methodologies and economic returns through an optimization algorithm maximizing net financial margin. The results suggest that the economic effects, in terms of farm-level profitability, may be minimal (decrease of 0.6%) but are likely sensitive to the market conditions in different years. Doubling the forage area and increasing the cattle herd increased GHG emissions from agriculture by 7.6%, mostly because of methane from enteric fermentation by cattle. The GHG emissions were mitigated by carbon sequestration in soil, but this is likely ephemeral, suggesting that the longer term emissions would be even greater. However, a twofold increase in forage production implies less energy use, change in water dynamics, and a reduction in the use of nitrogenous fertilizer, which could be beneficial for ecosystem services, and needs to be assessed.