Climate change progressively makes obsolete the current agricultural production strategies built with respect to previous landmarks of average and extreme climatic situations. In order to remain viable in the expected climatic context farming systems will need to evolve both in their structure and management. Because farmers and extension services are the primary actors in agricultural production they have to be involved in the process of designing adequate renewed systems. However, the complexity of most design approaches produced by research (e.g. based on computer models) prevents them from deep involvement in such design projects. This paper presents a game-based design approach developed to foster the involvement of farmers and extension services in the design and evaluation of livestock systems. The game, called “forage rummy”, relies on a number of “boundary objects” that put relevant pieces of scientific knowledge in a more understandable and tractable form for the design task. These are a player-friendly game board in which sticks representing year-round forage production and animal feeding requirements have to be assembled. Playing the game consists in iterating tentative configurations of a livestock herd (production goal, size), the grassland resources (types of grass and associated area and usage), and feeding diet along a 12 month cycle in a given economic and climatic scenarios. The generation and evaluation of each configuration are done collectively in a participatory workshop and exploit a simple balance model. A diversity of use is permitted by this game depending on the matter being discussed. The paper describes its use in the design of dairy systems adapted to climate variability and change expected by 2050. From our experience with the forage rummy, we synthesise lessons about key factors enhancing participation, focus and depth of discussion, and learning about the relevance of potential farming systems adaptations.