High early vigour (plant dry matter in the early development stages) in sunflower may be a desirable character under water-limited environments as it may contribute to higher transpiration efficiency and reduced soil evaporation. However a high early vigour causes a more rapid use of soil water, which may threaten crop water supply during seed filling. This is also influenced by the seasonal pattern of rainfall and genotype season length. The objective of this research was to simulate the performance of cultivars of sunflower differing in season length and early vigour under a Mediterranean climate. A simulation model of the sunflower crop, OILCROP-SUN, was modified and used to find the optimum combination of early vigour and season length of the genotypes for different environments. Field experiments were carried out during 1992, 1993 and 1994 in Cordoba, Spain (38°N). Sunflower populations with similar genetic background but with differences in early vigour were used to study the association of this trait with other characters and with the genetic parameters required to run the model. Changes in early vigour were simulated by modifying leaf growth rate in the model. The field experiments showed that high early vigour, as measured by stem volume 425°Cd after emergence, is not related to reduced root growth and is not associated with season length. Simulations showed that the highest seed yield is obtained using genotypes with high early vigour, provided that their season length is long enough for the growing conditions

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