To improve sustainability, farmers may want to redesign their livestock farming systems in depth, e.g. convert to organic farming. Assuming that modelling livestock farming systems can support such redesign processes, we devised and tested a modelling methodology in which farmers participated. Fifteen cattle dairy farmers converting or converted to organic farming took part in our programme. The programme first consisted in a series of meetings and surveys with both the local extensionists and the farmers. This first step made it possible to identify a list of relevant topics linked to the converting processes, and to make the farmers feel more engaged in the programme. The following step was made of participatory workshops during which farmers were invited to express their own views about the previously identified topics. Such views were modelled as causal maps the structure of which was defined by the farmers themselves. The maps aimed at representing the operation of the livestock farming systems. Some difficulties of our methodology are discussed, together with its potential uses and interests for both research and development purposes. In each case, the benefits of participation are highlighted