The forage systems based on grass are particularly complex to manage. Tools for diagnosing farmers’ practices lead to recommendations for the management of the pastures, but whether they may be applied on the scale of the whole system is questionable. The strategy presented here, which concerns 2 suckling cattle farms in the Pyrenees, gives a systemic evaluation by simulating the proposed improvements. It combines the functional analysis of the forage systems, diagnoses of the forage practices, and the modelisation of the 2 systems, making simulations possible. The diagnosis on the field scale gives an inkling of possible large improvements in these systems (for the organisation of harvesting operations), but the simulation shows these proposals to be limited by structural and climatic constraints. This is an original example of methods that may add to the expertise of the scientists, the agricultural advisors and the farmers.