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Correlated Changes in Agronomic Traits from S₁-line Recurrent Selection for Cold Tolerance in Two Maize Populations¹

Cold tolerance is an aggregate trait important to dependable stand establishment in many environments. Selection for improved cold tolerant maize (Zea mays L.) germplasm, however, must not deter improvement of traits that are important later in the growing season. Correlated changes in agronomic traits due to five cycles of S₁-line recurrent selection for cold tolerance were evaluated in two maize populations. Correlated changes in means were expressed in terms of changes in allelic frequency due to pleiotropy or linkage and drift. Selection was associated with correlated genetic changes (2p) grain yield [0.22 Mg ha^-1 cycle^-1 when averaged over both BS13(SCT) and BSSS2(SCT)], grain moisture (-0.3% cycle^-1), early stand count (0.7 plants plot^-1 cycle^-1), and final stand (387.4 plants ha^-1 cycle^-1). Root lodging resistance was increased 2.5% cycle^-1 in BS13(SCT) and decreased 2.0% cycle^-1 in BSSS2(SCT). Stalk lodging resistance decreased 0.40% cycle^-1 in BS13(SCT). The increased frequencies of pleiotropic or linked genes controlling grain yield and grain moisture were expressed at both early and normal planting dates. The correlated genetic changes in early stand count, final stand, and root and stalk lodging resistance were influenced by date of planting. Genetic drift (2p²d) was significant for grain yield (-0.23 Mg ha^-1 cycle^-1). Genetic drift overwhelmed the increase in allelic frequency due to selection for grain yield; hence, the populations per se were unchanged. The data suggesthat selection for cold tolerance with larger effective population sizes may result in correlated improvements in yield of the populations per se.

Key Words: Corn • Population improvement • Population size • Genetic gain • Correlated response • Zea mays L.

² Former graduate assistant, Iowa State Univ., Ames, IA 50011; director of research, Garst Seed Co., Ames, IA 50010 (former assistant professor of agronomy, Iowa State Univ., Ames, IA 50011).

Received for publication March 19, 1985.