Genetic variation and heritability of forage yield in Mediterranean tall fescue

The genetic control of tall fescue forage yield has been poorly investigated. Full-sib families from diallel crosses of Mediterranean germplasm were evaluated for forage yield over 34 months in a Mediterranean environment with severe drought stress (diallel 1, with 20 parents) and over 16 months under irrigation in a heated greenhouse simulating the Mediterranean temperature pattern (diallel 2, with 15 parents). Genetic parameters were estimated for fresh biomass in diallel 1 and dry-matter yield in diallel 2. Additive genetic variance was always larger than non-additive (dominance) variance. Narrowsense heritability was fairly high ( h(2) = 0.61) in diallel 1 and moderate (h(2) = 0.45) in diallel 2. Predicted yield gains from one selection cycle were larger in the former diallel (23.9%) than in the latter (10.5%), suggesting that gains can be enhanced by selection under severe drought stress and over a time span sufficient to allow the variation in persistence to fully emerge. General combining ability effects of eight parents that were common to both diallel crosses were highly correlated (r = 0.94) across the contrasting evaluation environments. The extent and consistency of additive genetic effects across environments suggest that rapid improvement of forage yield is possible.