Efforts to rehabilitate the forest cover in upper watersheds of northern Thailand have led to frequent land use conflicts between government agencies and smallholders belonging to ethnic minorities. Hmong herders exploiting grasslands and fallows with an extensive cattle rearing systems are increasingly constrained by the regular expansion of tree plantations. To mitigate the looming land use conflict between herders and forest agencies, an iterative and evolving process of collaborative agent-based simulation activities was implemented in Nan Province. The communication describes the adaptation of the methodological tools to the context and the outputs of the three successive sequences of activities carried out with the local stakeholders. The result and discussion section focusses on the exchange and integration of local empirical knowledge on the vegetation dynamics and extensive cattle rearing system to co-design interactive simulation tools used to explore innovative land use options accommodating the dual interests of the parties in conflict. A quantitative analysis of the communication among the participating stakeholders also shows how an active sub-group of Hmong herders became gradually the drivers of this companion modelling process.