Recently adopted in France, conservation agriculture still faces some challenges to its adoption, particularly weed management. To highlight the weed management practices used by farmers in conservation agriculture and the changes induced by its adoption, a large sample of 425 French farmers were invited to complete an online survey. Weed management practices used by farmers were requested for three periods: before adoption, during the first years of conservation agriculture (one to two years after adoption), and when the agricultural system is considered “mastered” by the farmer. The use of each farming practice was firstly studied independently for each period. Then, a multiple correspondence analysis followed by a hierarchical ascendant classification resulted in groups of farmers with different combinations of practices for each period. Finally, the groups of farmers were followed through the periods. Results showed that changes in weed management done according to farmers when adopting and mastering conservation agriculture are multiple and vary according to farmers and their previous weed management. Although some similar choices were identified, some farmers’ trajectories, especially those with a prior combination driven by either a soil disturbance strategy or a crop competitiveness strategy, are difficult to identify when adopting conservation agriculture. Upon mastering the agricultural system, farmers’ choices become more apparent.

Ambrosia trifida L. (giant ragweed, Asteraceae) is native to the North American continent and was introduced into Europe and Asia at the end of the 19^th century. In its native range, this tall annual species is common in riparian and ruderal habitats and is also a major weed in annual cropping systems. For nearly a century, A. trifida has also been of great concern in the U.S. for its highly allergenic pollen, necessitating targeted control measures to reduce its impact on human populations. Based on the distribution of A. trifida in North America and in its introduced range, riparian systems in the rest of the world may be particularly at risk to invasion, with potential negative consequences for their biodiversity. Currently, A. trifida has invaded Asia more widely than Europe, likely due to the more favourable local conditions in Asia. Throughout its introduced range, A. trifida is host to a limited number of invertebrates and pathogens and only a few biological agents are available for its control. The main impacts of A. trifida at a global level are on crop yield and human health, resulting in significant socio-economic impacts. The success of A. trifida invasion in areas in which it has been introduced is still unclear, but climate change may increase climate suitability, increasing the potential for A. trifida to spread. While effective management in cultivated fields seems potentially possible, the development and control of A. trifida in natural riparian habitats is of great concern due to the difficulty of management in these areas.

Au cours de ces 70 dernières années, certaines substances actives herbicides ont acquis le statut de molécules « pilier » dans la gestion des communautés adventices. En France, jusqu’en 2003, l’atrazine a joué un rôle central dans le désherbage du maïs. Depuis 1974, le glyphosate est devenu la molécule la plus utilisée dans la gestion de l’interculture, au point d’être considéré comme indispensable en agriculture de conservation. Une excellente efficacité et un faible coût ont placé ces deux molécules parmi celles les plus utilisées au monde. Mais dans les deux cas, une utilisation trop importante a conduit à la contamination des eaux et à l’émergence de questionnements sur de possibles effets négatifs sur l’environnement. Une comparaison de l’interdiction de l’atrazine et du glyphosate, d’un point de vue agronomique, économique et social, montre des inquiétudes similaires sur la gestion de la flore adventice de la part de la profession agricole. Si le retrait de l’atrazine a pu engendrer une augmentation des coûts de désherbage du maïs, la flore adventice a su être correctement maîtrisée via la mobilisation d’autres leviers, principalement chimiques. L’interdiction potentielle du glyphosate, prévue en France dans un futur proche, pourrait avoir des conséquences fortes sur la gestion de la flore adventice et plus globalement sur les exploitations agricoles. Le principal levier disponible pour se passer de glyphosate est le recours au travail du sol, mais d’autres leviers agronomiques doivent venir en complément pour assurer une gestion efficace. Bien que séparé de 15 années, le retrait de ces deux substances actives est l’occasion de prendre conscience des conséquences que peut avoir l’utilisation prédominante d’une unique substance active sur l’environnement, mais aussi sur la capacité d’adaptation des systèmes agricoles.