The seed yield of 12 varieties was analysed in 12 environments. The main source of variation in seed yield among environments was the biomass. The genetic variance for seed yield was large and the cultivar x environment variance was small. Thus, its broad-sense heritability was high. The genetic correlation between seed yield and lodging was also high with the lodging-tolerant varieties showing a higher seed yield. Seed yield was genetically correlated with harvest index. A higher harvest index was achieved through a genetic increase in seed weight per inflorescence. There was no genetic correlation between seed yield and the vegetative biomass suggesting that breeding for a higher seed yield should not be deleterious to the vegetative growth and forage production potential. Seed weight per inflorescence could be a valuable selection criterion to be used under a spaced plant design.