Improving the management of coexistence between GM and non-GM maize with a spatially explicit model of cross-pollination

Summary

The European Commission have established the concept of coexistence, according to which, farmersshould be able to grow whatever type of agricultural crops they wish (genetically modified (GM), con-ventional or organic), provided that they comply with the legal obligations for labeling and/or puritystandards. In the case of maize, the main factor conditioning the feasibility of coexistence is gene flowfrom GM fields to other types of production. The distance between fields has been identified as a key fac-tor governing this gene flow. As a consequence, the existing regulations mostly concern the maintenanceof a fixed isolation distance between GM fields and the closest non-GM field. However, other factors, suchas temporal dynamics of pollen shedding, wind, relative field sizes and shapes and the spatial distributionof the different types of fields, may greatly modulate the effect of distance. Moreover, uniform distance-based rules create a “domino effect”, in which it is difficult for GM crops and non-GM crops to co-exist atthe landscape scale. In this study, we hypothesized that the use of a spatially explicit gene-flow model,MAPOD®, would result in a significant gain in proportionality and freedom of choice for the farmer overuniform distance-based rules. To test this hypothesis, we performed a global sensitivity analysis on thisprocess-based model but, instead of exploring a random set of situations, the sensitivity analysis wascarried out on a subset of realistic scenarios based on farmers’ strategies. To select those scenarios, weconstructed a multicriteria decision-making model describing the decision process used by farmers whendeciding whether or not to grow GM maize, and used this model to generate realistic allocation scenariosfor GM, non-GM conventional and organic maize cultivation. We showed that the coexistence methodbased on the MAPOD®model allowed the presence of a higher percentage of GM maize in the landscapethan the distance-based method. This made it possible to follow the farmer’s field intended allocationsmore closely, whilst complying with the legal threshold requirements. This gain in proportionality wasgreater at high maize densities, for which the distance-based method allowed almost no cultivation ofGM crops. However, in case of high proportions of organic fields, our study indicated that coexistencebetween GM maize and organic maize at the landscape level is difficult, if not impossible in case of farm-saved seeds, without a spatial aggregation of fields, leading de facto to separate non-GM and GM zones.Finally, the use of MAPOD®resulted in better discrimination between acceptable and risky situations,and greater flexibility, which is crucial for the implementation of an efficient coexistence strategy.