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Hybrid Performance of Maize Inbred Lines Selected by Testcross Performance in Low and High Plant Densities¹

Nineteen S₃ maize inbred lines (Zea mays L.) selected through three successive generations, S₀, S₁, and S₂, on the basis of testcross performance in a low or a high plant density were evaluated further to determine the importance of plant density in iubred development. Two testers were used in the final study: the double-double cross (DDC) used in the selection of the lines and a single cross (SC). The testcrosses were grown in low, medium, and high plant densities in five environments. With the DDC tester, the S₃ lines as a group showed a significant yield gain when compared with a testcross of the source synthetic variety. Lines selected at low and high densities gave similar group testcross yields at low densities, but at medium and high densities, testcrosses of the highdensity group yielded better than those of the low-density group. The difference in yield at medium and high densities was caused by a greater incidence of barren stalks for the low-density selection group. With the SC tester, there was no gain in yield of the selections as a group over the source variety. Greater relative importance of nonadditive gene action as compared with additive gene action appears the only explanation for the results obtained.

Key Words: Zea mays L. • Early Testing • Combining ability

² Professor, Iowa State University, Ames 50010.

Received for publication August 6, 1968.