A multi-modelling approach to help agricultural stakeholders design animal wastes management strategies in the Reunion Island

Recent concentration of indoor livestock farming in the Highlands of the Reunion Island generates environmental risks. Livestock effluent management is often difficult due to the lack of suitable spreading areas close to the farms. However, crop farms in the Lowlands are demanding organic matter to maintain soil fertility. This is the case of the locality of Grand-Het where pig and poultry effluents are intensively produced that should be exported towards the coastal zone where large areas of sugar-cane are available. This paper presents how we are supporting Grand-Ilet stakeholders to devise territorial management strategies of their wastes. To this end, a stepwise approach is being tested. First, the supply of animal wastes and the manage ment by farmers are characterized through farm surveys and agronomic expertise. Then, the crop demand is determined by using a GIS considering a large zone surrounding Grand-Ilet onto which several constraints are applied to eliminate unsuitable spreading areas. Comparing supply and demand enables one to draw gen-eral conclusions about the strategic choices to be considered. In the case of Grand-Ilet, distances between livestock farms and crop locations, and amounts of liquid manure to be processed, led one to consider implementing a treatment plant collectively managed by farmers. To devise sustainable management strategies for such a trade-off, we are developing a participatory approach based on several simulation models (spread-sheet, hybrid dynamical system, agent-based). These models, developed in our team, aim at tackling several decision-making issues: (i) Which treatment process should be chosen? (ii) How to supply it with raw wastes? (iii) How to best organise organic material fluxes amongst farms at the district level? Together with the description of those tools, each step of the approach is exemplified and the way of integrating the use of simulation models within the stakeholders' decision process is outlined. (Résumé d'auteur