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Selection-Induced Differences among Strains of Iowa Stiff Stalks Synthetic Maize¹

Different recurrent selection methods have been used for the improvement of traits in maize (Zea mays L.). The relative performance of nine strains of Iowa Stiff Stalk Synthetic (BSSSCO) derived different methods of recurrent selection for different traits were determined by evaluating strains per se, diallel crosses among strains, and S_l generations of strains and strain crosses. Objectives were to determine selection.induced differences among the nine strains and the direct and indirect effects of selection for yield, grain moisture, root and stalk lodging, and dropped ears. Replicated field trials were conducted in six environments. BS13(S)C2 had the highest yield per se (7.83 Mg ha^-1) and the highest yielding $1 generation (5.82 Mg ha^-1). Inbreeding depression for yield was less for the highly selected strains and for crosses involving those strains. Selection for resistance to European corn borer (Ostrinia nubilalis Hübner) did not significantly affect yield, but selection for resistance to corn rootworm (Diabrotica virgifera Le Conte) did have a significant negative correlated effect on grain yield. Changes in frequency of additive effects due to selection for yield improvement were more important for BSI3(S)C2 (seven cycles half-sib and two cycles S₂ selection) than for BSSS(R)C8 (eight cycles reciprocal recurrent selection). Greater changes were observed in frequency of alleles with dominance effects in BSSS(R)C8 than in BS13(S)C2. After removal the effects of drift, rate of realized gain per cycle was 0.279 Mg ha^-1 (4.4%) for BS13(S)C2 and 0.259 Mg ha^-1 (4.1%) for BSSS(R)C8. Yield of the 36 strain crosses ranged from 6.10 Mg ha^-1 for BSSSCO x BSSS(RW)C2 to 9.37 Mg ha^-1 for BSSS(R)C8 x BS13(S)C2. Average midparent heterosis for yield was 11.7%. Yield differences among strains and level of heterosis expressed in crosses suggested selection-induced genetic diversity among strains of BSSSCO.

Key Words: Zea mays L. • Recurrent selection • Population improvement • Correlated response

² Former graduate student (presently, director of research, Asgrow Seed Co., Guadalajara, Mexico); research geneticist, USDA-ARS, and professor of agronomy, Iowa State Univ., Ames, IA 50011.

Received for publication June 27, 1985.