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Comparison of Selection Based on Yield of Topcross Progenies and of S₂ Progenies in Maize (Zea mays L.)¹

Three methods of breeding for higher grain yield in maize (Zea mays L.) were compared: (a) recurrent selection for combining ability with the parental population (Parental Tester method), (b) recurrent selection for combining ability with an unrelated inbred line (Inbred Tester method), and (c) recurrent selection for yield of S₂ progenies per se (S₂ Progeny method).

After three cycles of selection for grain yield had been completed with each of the three methods, the nine resulting selected populations were evaluated for grain yield (a) as random-mated populations (synthetics), (b) as selfed populations (bulked S₁ lines), and (c) in crosses with 11 unrelated testers. Significant differences among the breeding methods were noted for the second and third cycles of selection, but not for the first cycle. The Parental Tester method produced the highest yielding random-mated (syn-3) population; whereas the S₂ Progeny method produced the highest yielding selfed population. Average combining ability with unrelated testers for all three methods was increased significantly (5.2%) by the second and third cycles of selection, but there was no significant difference among methods. Although the Inbred Tester method resulted in significant improvement in yield of selfed populations, it was inferior to the other two methods for population improvement in the over-all evaluation.

Selfing one generation resulted in over 70% reduction in yield of the first cycle populations, which indicates that dominance is of considerable importance in the base population. Inbreeding depression was progressively less drastic in the second and third cycle populations, particularly with the S₂ Progeny method.

Key Words: Recurrent selection • Combinating ability • Additive genetic variance • Dominance variance • Synthetic varieties • Inbreeding depression • Corn

² Agronomist, Department of Agronomy; Agronomist and Head, North Florida Station, Quincy; Associate Soils Chemist, West Florida Station, Jay; and Associate Agronomist and Head, Suwannee Valley Station, Live Oak, Florida.

Received for publication March 17, 1969.