Traditional pig production often relies on cereal-based feed, which has adverse environmental effects, e.g. nitrogen leaching and greenhouse gas (GHG) emissions. Alternative production systems are therefore sought to improve the sustainability of pig production. A promising alternative is to use proteinaceous feed from grass, produced in a green bio-refinery (GBR), to substitute part of the cereals in the feed. Cultivation of grass on arable land can reduce nitrogen leaching and pesticide application, and increase carbon storage. The GBR using grass as feedstock also produces valuable byproducts, e.g. fibre and biogas. In this study we combine a life-cycle analysis (LCA) and a cost-benefit analysis to compare the economic and environmental effects of producing the pig feed to produce 1 ton of pork using two feeding systems. We apply this approach to the intensive Danish pork production as a case study. The results show that compared with traditional cereal-based feeding system for producing a ton of pork, using proteinaceous concentrate from small-scale GBR will (1) decrease the average feed cost by 5.01%; (2) produce a profit of 96 € before tax in the GBR; and (3) decrease the nitrogen leaching (NO3-N) by 28.2%. However, in most of the scenarios (except for G2), the nitrogen emissions into the air (N2O-N) will also increase because of the increased N fertilizer application compared to a cereal-based system. In most of the scenarios (except for S1 and G1), the energy and land use will also be saved. However, some important factors, e.g. the soil characteristics, pressed juice fraction in fresh biomass and scale of GBR, could subvert the conclusion about energy and land use saving in the alternative feeding system.