Management practices of cattle farming systems must be improved, particularly to increase the systems' feed selfsufficiency, food production and environmental performances. In mountain areas of the Massif Central (central France), mixed dairy/suckler cattle systems enable farmers to use grassland resources better and cope with economic fluctuations. Our objective was to estimate levels of ecosystem services provided by mixed dairy/ suckler cattle systems as a function of the degree of mixing, along with their greenhouse gas emissions and energy use when their operation is optimized on an economic basis. The hypothesis was that mixed dairy/suckler cattle systems allow for controlled use of biomass, with better environmental performances than specialized systems (pure dairy or suckler herd) by maintaining grassland ecosystem services. Five herd-distribution scenarios were simulated using the Orfee bioeconomic optimization model. Environmental performances of the five systems were assessed according to three functional units (i.e., per farm, ha and kg protein produced). Mixed dairy/suckler cattle systems, which enabled larger herds, had higher greenhouse gas emissions per ha than specialized systems. However, because dairy cows produce more protein (milk and beef) than suckler cows, specialized dairy systems had the lowest greenhouse gas emissions and energy use per kg of protein. Specialized dairy systems had less advantage when dairy cows had less access to grassland. For the production of both milk and beef, mixed dairy/suckler cattle systems favour more sustainable use of biomass and tend to maintain a better combination of levels of ecosystem services for livestock production than specialized cattle farming systems.