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S2 and S4 progenies were obtained from 10 S0 parents each from ‘MSA-W4’ and ‘MSB-W4’ alfalfa (Medicago sativa L.). Two five-parent diallel sets were attempted within S0, S2, and S4 parental generations in each cultivar. All but one of the sets with S2 and S4 parents gave incomplete diallels. First harvest dry matter yields were measured in small row plots during each of two growing seasons.
Average yields of the S2–crosses were less than those of the S0–erosses in all sets. Individual exceptions to the average trend occurred in both populations, but more frequently in MSA-W4 than in MSB-W4. The number of S4–crosses was small, and the results with them were variable. Yields of all S4crosses in one set, which had one parent common to all crosses, were greater than yields of the corresponding S0 or S2–crosses. Other S4 crosses generally yielded less than the corresponding S0 or S2–crosses.
Mean squares for both general (GCA) and specific combinnig ability (SCA) pooled over sets in MSA-W4 increased from the S0 to the S2 parental generation. This increase was greater for SCA than for GCA. GCA was significant in the S2–crosses from MSB-W4, but no combining ability mean square was significant in the S2 or S4–crosses from MSB-W4.
Key Words: Heterosis • General combining ability • Specific combining ability • Inbreeding • Medicago sativa L.
2 Research agronomist, ARS, USDA, University Park, Pa.
Received for publication October 7, 1974.
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