This study explores the meanings that people attributed to their involvement in a participatory on-farm practice change project. Three techniques of discourse analysis were used. The first two identify the diversity among narratives of the participants and explores the origin of these differences. The third technique examined differences and tensions within and across the narratives to identify the discourses that were operating. Participation was found to be mediated through discourse as agents created and reproduced some discourses through their many social acts. For example, some participants recalled incidences of feeling excluded when they presented an alternative understanding of the project. As a result, these people tended to reduced their involvement rather than explore the differences. The project's discourses therefore routinised the participatory experience and tended to lock the narrative in time despite over two decades of rapid social change. Thi s meant the project discourse mediated a favoured type of participation, one that met a symbolic character rather than the particular farming practices it promoted. The discourses reveal different patterns of sense making among participants involved in the same event. Uneven participation is comprehended from the multiple patterns as a consequence of the participants' discursive practices. Articulating differences in discursive design will assist to create conditions useful for an authentic communication among participants engaged in change programs.

Afin d’expliquer la diversité des pratiques des agriculteurs, cette thèse explore le rôle des caractéristiques du processus de décision dans les choix opérationnels réalisés. Sur la base d’une méthode à deux volets (modélisation et enquêtes), nous avons proposé un modèle conceptuel du processus de prise de décision des agriculteurs, caractérisé les différences possibles au sein de ce processus (appelées modes de décisions), mis en évidence le lien entre ces modes et les pratiques, précisé les modalités de simplification du processus de décision et identifié de nouveaux leviers d’action pour l’accompagnement des agriculteurs. De nature générique, les modèles et les méthodes d’enquêtes proposés ont été appliqués à un cas d’étude qui à permis d’expliquer l’hétérogénéité des pratiques antifongiques des agriculteurs sur le blé tendre en MidiPyrénées par leurs caractéristiques personnelles, par les caractéristiques de leurs exploitations et par leurs modes de décision / In order to explain the diversity of farmers’ practices, this these explores the decision-making process features involved in operational choices. Using a modeling and survey-based approach, we built a conceptual model of the farmer’s decision-making process, distinguished different use of the decision-making process (referred to as decision modes), highlighted the link between thesedecision modes and farmers’ practices, showed some simplification made by farmers in their decision-making process and offered suggestions for improving the decision support means to farmers. Both the models and the survey design are generic. They have been applied in a case study that makes it possible to explain the heterogeneity of fungicide use on soft wheat between farmers in the Midi-Pyrénées region by their personal characteristics, the characteristics of their production situation and their decision modes.