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Simultaneous Evaluation of Grain Sorghum A-Lines and Random-Mating Populations with Topcrosses¹

Grain sorghum [Sorghum bicolor (L.) Moench] topcrosses from 19 A-lines crossed to six random-mating populations were evaluated for general combining ability (GCA) and specific combining ability (SCA) effects. Topcrosses were produced by interplanting short rows of the A-lines within each population in an isolation block. Six replications of a split-plot experimental design with populations as main plots and A-lines as subplots were grown at Mead, Nebr., in 1979 and 1980. Data were recorded on seven traits. Significant differences were detected among A-lines and among populations for grain yield, plant height, days to flowering, tillering, and percent oil and among A-lines for 100-seed weight and percent protein. No genotype x year yield interaction was significant for either parental group. Significant A-line x population interactions existed for yield, height, and flowering although individual analyses by population often ranked the A-lines similarly. There were no significant A-line x population x year interactions. Based on GCA effects, A-line N37 and population RPIR were superior parents for producing high yielding topcrosses. The SCA yield effects were significant in only eight topcrosses. N37 x RPIR had the highest yield, but its SCA effect was nonsignificant. Topcrosses appear to be suitable for preliminary evaluation of grain sorghum germplasm. They identify superior A-lines and identify populations in which to initiate inbreeding to develop R-lines.

Key Words: Sorghum bicolor (L.) Moench • Testcross • General combining ability effects • Specific combining ability effects • Line development • Hybrids

² Former graduate student and research technician, Dep. of Agronomy, Univ. of Nebraska (now research director, ARCO Seed Co., P.O. Box 181, E1 Centro, CA 92244); research geneticist, USDA-ARS; and professor, Biometrics and Information Systems Ctr., Univ. of Nebraska, Lincoln, NE 68583.

Received for publication December 16, 1982.