Combining Ability in Alfalfa 1

AMAJOR problem in forage crop breeding is ascertaining the breeding potential of selected plants. Various methods have been suggested. Tysdal et al. (7) suggested the polycross test to measure general combining ability of cross-pollinated plants where the pollen could not be controlled. Tysdal and Crandall (6) obtained similar rankings of combining ability of six clones using polycross, single cross, and top cross yields. Knowles (4) observed significant correlations for yields between top crosses and open-pollinated progenies produced in a nonreplicated clonal nursery. However, he obtained nonsignificant correlations between the yields of controlled crosses and open-pollinated progenies (3). Davis (1) pointed out the limitation of the polycross test since theoretically the range of the polycross progenies could be only one-half that of the clones and the Sis, and he suggested that Sl’S could be used to isolate superior clones. Timothy et al. (5) compared the average of the two-clone combinations, the polycross progenies, and the clones per .re for yield. The range of the yield of the clones was 81 g. per plant, the range of the polycross progenies was 53 g., and the range of the average of the single crosses was 37 g. It is the purpose of this paper to present data on the relation between two-clone crosses, Sl’s , and polycrosses, and the limitations of the polycross progeny test to isolate superior clones.