There are different strategies for producing fodder crops without irrigation under Mediterranean climate conditions. One strategy is to avoid the drought period by using annual plants such as cereals or Italian ryegrass. Another strategy is to use perennial forage species highly resistance to drought such as Dactylis glomerata or Festuca arundinaceae. The advantage of the second strategy is a rapid growth when favourable conditions come back after summer and good ground cover, which limits erosion. Summer dormancy, defined as an absence of growth in summer despite irrigation, is a very effective adaptation to drought which has been observed in cocksfoot. Nevertheless, summer dormancy in cocksfoot is associated with low vegetative productivity. The objective of this study is to analyse the genetic and eco-physiological determinisms of summer dormancy and vegetative growth in cocksfoot. For this purpose, the progeny between a summer dormant genotype from the variety Kasbah and a summer active genotype from the variety Medly is studied. The first results show that despite an expected negative correlation (r = −0.40) between summer dormancy and vegetative plant growth, genotypes exist with the same summer dormancy level as the genotype of Kasbah and a higher vegetative productivity than the genotype of Medly. These results are promising for the development of new cocksfoot varieties combining both summer dormancy and high vegetative productivity. The genomic regions implied in the variation of these traits are under study and should be useful for breeding.