General Combining Ability and Its Interaction with Environments in a 7 ✕ 7 Diallel Cross Population of Alfalfa1

The relative importance of general combining ability (GCA) and specific combining ability (SCA) in the year of establishment and in the following year were studied for forage yield, plant height, days to flower, growth habit, vigor, and frost tolerance in a diallel cross population of alfalfa (Medicago sativaL.). For both years GCA was more important than SCA for most of the characters. The ratio of GCA to SCA was generally greater for plants in their second year. There were marked differences in the GCA effects of the seven clones. Clones selected from adapted cultivars produced progenies that performed relatively well; those selected from unadapted cultivars gave progenies that were found to be generally inferior in forage yield, vigor, and frost tolerance. Both days to flower and vigor showed appreciable genotype ✕ environment interaction.