Predicted Efficiency of Indirect Selection to Improve Yield in Tall Fescue-Alfalfa Mixtures.

High fertilizer prices have increased interest in grass-legume mixed pastures. It has been hypothesized that compatibility of grasses and legumes, or their ‘ecological combining ability’ can be improved by breeding specifically for mixture performance. However, this is in the proof of concept stage and it is not known if there is variation for ecological combining ability, or if it is under different genetic control than performance in monoculture. This experiment studied the efficiency of using a competitive grass monoculture environment to indirectly improve tall fescue (Festuca arundinacea Schreb.) yield in a grass-legume mixture environment. Narrow-sense heritabilities, genetic and rank correlations, and indirect selection efficiencies were estimated for a tall fescue population (45 half-sib families) grown as spaced plants overseeded with either turf-type tall fescue (monoculture) or grazing-type alfalfa (mixture). Heritability in monoculture varied depending upon harvest, with values of 0.41, 0.37, 0.17, 0.16, and 0.37 for successive harvests in June, July, August, October, and total seasonal yield, respectively. Heritability in mixture was generally lower, with values of 0.02, 0.24, 0.39, 0.09, and 0.30 for harvests in June, July, August, October, and total seasonal yield, respectively. Genetic correlation between yield in monoculture and mixture also varied, with values of 0.43, 1.00, -0.26, 0.53, and 0.24 for harvests in June, July, August, October, and total seasonal yield, respectively. The low heritability of grass yield in mixtures, combined with moderate genetic correlations, resulted in indirect selection (monoculture) predicted efficiencies exceeding that of direct selection for yield in mixtures in June, July, and October harvests. Indirect selection efficiency was negative or low for August and total seasonal forage yields. These results, and low Spearman’s rank correlations, predict that direct selection for mixture performance will be more effective to improve total and stability of seasonal tall fescue forage yield.