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Yields of S₁ Lines from Original and Advanced Synthetic Varieties of Maize¹

Four maize (Zea mays L.) synthetic varieties were selfed and approximately 200 S₁ lines of each were grown in separate replicated yield trials with the parental populations to compare inbreeding depressions and yield distributions among the S₁ lines. The four parental populations were (i) ‘iowa Stiff Stalk Synthetic,’ SSS, (ii) an advanced synthetic derived from SSS by seven cycles of recurrent selection based on half-sib yield, SSS(HT)C7, (iii) ‘Virginia Corn Belt-Southern Synthetic,’ CBS, and (iv) an advance synthetic derived from CBS by four cycles of recurrent selection for S₁ yield, CBS(S)C4.

Inbreeding depression was lower for S₁ lines from each of the advanced synthetics than for those of the corresponding original and was lowest for the S₁ lines from SSS(HT)C7. Inbreeding depressions were 39.1 and 27.8%, respectively, for S₁ lines from SSS and SSS(HT)C7, and 38.3 and 32.5%, respectively, for S₁ lines from CBS and CBS(S)C4. S₁ lines from SSS(HT)C7 averaged 9.3% higher in yield than those from SSS, although the range of yields was less at both extremes for lines from the advanced population. S₁ lines from CBS(S)C4 averaged 27.2% higher than those from CBS with a comparable range of yields.

SSS(HT)C7 yielded slightly less than SSS. CBS(S)C4 yielded approximately 19% more than CBS. In all populations the maximum S₁ yield approximated that of the parental source. Two S₁ lines from CBS(S)C4 were comparable in yield to that of a double-cross hybrid check variety. One effective means of increasing inbred line yields appears to be increasing the yields of parental source populations.

Key Words: Inbreeding depression • Inbred yield distribution • Productive inbred lines • Recurrent selection methods • Population improvement

² Professor of Agronomy.

Received for publication February 27, 1971.