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Reciprocal Recurrent Selection in the BSSS and BSCB1 Maize Populations and Half-Sib Selection in BSSS¹

Evaluation of progress from five cycles of reciprocal recurrent selection (R) in the BSSS(R) and BSCBI(R) maize (Zea mays L.) populations indicated that the improvement in grain yield of the population cross had been linear at the rate of 2.73 q/ha (4.6%) per cycle. significant changes, however, were detected in the parents. BSSS(R) and BSCBI(R) were developed from Stiff Stalk Synthetic and Corn Borer Synthetic #1, respectively. Topcrosses of improved populations to BSBB did not yield significantly more than topcrosses of the original synthetic populations.

Improvement by half-sib selection with the Iowa 13 double-cross tester (HT) in BSSS(HT) also had been linear at 1.65 q/ha (2.6%) per cycle. The population cross BSSS(HT)Cn x BSCBI(R)Cn had been improved 2.31 q/ha (3.8%) per cycle, and BSSS(HT) had been proved 0.74 q/ha (1.4%) per cycle, where Cn is the cycle of selection.

Heterosis increased from 15% in the CO x CO to 37% for BSSS(R)C5 x BSCBI(R)C5 and to for BSSS(HT)C7 x BSCBI(R)CS. Because only 10 lines were recombined each cycle, the estimated inbreeding of the C5 populations (22%) and the C7 population (29%) probably accounted for part of the increased heterosis and lack of progress in the parental populations.

The improvement in yield of the population crosses was obtained with no change in ear height or maturity, and stalk lodging was reduced. Improved population crosses compared favorably with elite single-cross checks; therefore, BSSS(R)C5, BSSS(HT)C7, BSCBI(R)C5 should be excellent sources of additional inbred lines for developing new improved single-cross hybrids. Two new lines, B73 and B78, have already been developed from the C5 and C6 of BSSS(HT), respectively; whereas B14 and B37 were developed from the original Stiff Stalk Synthetic.

Key Words: Corn • Population improvement • General combining ability • Specific combining ability

² Research Geneticist, ARS, USDA, and Professor of Plant Breeding Iowa State University; FAO fellow (now Research Officer in Agronomy, Institute of Agriculture Research, P.O. Box 2003, Addis Ababa, Ethiopia); and Research Geneticist, ARS, USDA, and Professor of Plant Breeding, Iowa State University.

Received for publication December 26, 1972.

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