Sunlab, a functional model for the sunflower crop (Helianthus annuus L.) will be presented: it simulates phenological development, biomass accumulation and morphogenesis, taking into account the effects of water stress. It was calibrated on four genotypes, products of more than 30 years of breeding efforts (Albena, Melody, Heliasol and Prodisol), and using two experimental datasets collected on plants grown under standard non limiting conditions and moderate water stress. Extending the use of such a model to the breeding domain will require to estimate the model parameters on large amounts of genotypes: fast, efficient and cheap procedures are therefore needed in order to limit as much as possible the experimental effort and budget. We will present and discuss several methodological aspects of this problem of Experimental Design.