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Inbreeding Depression, Inbred and Hybrid Grain Yields, and Other Traits of Maize Genotypes Representing Three Eras¹

Improvement in the genetic worth of maize (Zea mays L.) hybrids over the years can be a consequence of increased heterosis and/or improved inbred parents. The objectives of this study were to determine the nature of the genetic improvement of maize hybrids over a period of about 40 years using a representative series of hybrids and to determine any associated changes in morphological and yield component traits. Non parental single crosses of double crosses from the 1930's and the 1950's and single crosses of the 1970's, their inbred parents, and their F, generations were grown at 31 500 and 58 800 plants/ha. The study was conducted at Urbana and Shabbona, 111. However, the inbreds were grown at Urbana only and, because of poor germination of the inbreds of the earlier decades, were grown at 31 500 and 48 800 plants/ha. The majority of the improvement in inbred and hybrid yielding ability occurred between the 1950's and the 1970's. Recent inbreds and hybrids stand better, stay green longer, and have smaller tassels than their earlier counterparts. Grain yield inbreeding depression (and heterosis) increased from the 1950's to the 1970's and a part of it is manifest at loci that condition the responsiveness of the hybrids of the 1970's to increased plant densities. Plant and ear height inbreeding depression seem to have decreased from the 1950's to the 1970's.

Key Words: Heterosis • Genetic Improvement • Morphological traits • Yield component traits

² Graduate research assistant and professor of plant genetics, Dep. of Agronomy, Univ. of Illinois, Urbana.

Received for publication June 3, 1983.

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