When ag-modelling aims at supporting farmers in facing new challenges, is it not relevant to get a better understanding of how farmers integrate different sources of information and knowledge to achieve their purposeful action? We analyse among nine livestock farmers the diversity of the way they manage information to master and develop their productive activity and to develop themselves. We focused on the "informational activity", as defined from ergonomics, to stress on the way farmers make sense of information coming either from the farming system or from external sources. We analysed how they build information as a resource while acquiring and using this information in a given situation. To account for: (i) this process, we propose the notion of informational resource and defined it by three components: its content, its origin and its support; (ii) the combination of external and internal informational resources, we developed a model of an information system composed of two sub-systems. The first one organizes the informational activity via a finalizing and a monitoring unit. The other one implements the informational activity via a unit which acquires and interprets informational resources, and another unit which exploits them. The flow of information represents the way: (i) information circulates between the various units; (ii) in which a farmer makes sense of it. This triggers some relevant issues to carry on integration of information and knowledge in ag-models.