SYNERGY: A regional bio-economic model analyzing farm-to-farm exchanges and legume production to enhance agricultural sustainability

Agricultural specialization has disconnected crop and livestock production in many farms and regions. As a result, crop farms are deficient in nitrogen to fertilize their crops, while livestock farms are deficient in proteins to feed their animals. The increased consumption of nitrogen rich input raised economics and environmental questions. In this study, we tested two mechanisms to reconnect crop and livestock production: increasing legume production and developing farm-to-farm exchanges of crops and manure. To do so, we developed the SYNERGY bio-economic model, which represents specialized farm types in a region and models exchanges between them. Applied to western France, it analyzes economic, technical and environmental impacts. When the legume share reached 10% of the region's area, legume production increased more on crop farms than on livestock farms and its use in feed was still limited. When farms could export more manure, legume production increased but N losses increased due to an intensification of pig production. This rebound effect offset the environmental benefits linked to the decrease in synthetic N fertilizers use. When local crop exchanges were possible, they remained limited and did not affect indicators. Thus, the two levers studied are insufficient to improve sustainability, mainly because of high livestock production.