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Evaluation of Four Maize Populations Containing Different Proportions of Exotic Germplasm¹

Introgression of exotic maize (Zea mays L.) germplasm into adapted U.S. breeding populations has been proposed as a means for increasing useful genetic variability of quantitatively inherited traits and for enhancing response to selection. However, it is not clear what proportion of exotic germplasms most favorable. The present study was conducted to assess the relative breeding value of four populations with different proportions of exotic germplasm. Two maize populations, Illinois Stiff Stalk Synthetic Composite (RSSSC; adapted to the central Corn Belt) and South African Photoperiod Insensitive Composite II (PIC2;t ropical origin) were crossed and the F₁ was backcrossed once to RSSSCT. he F₁ and BC₁ were randommated to produce the F₂, and BC₁-F₂ generations, respectively. Random sets of at least 80 to 100 S₁ families were derived from the RSSSC, BC₁- F₂, F₂, and PIC2 populations (with 0, 25, 50, and 100% exotic germplasm, respectively). These were grown in 1983 and 1984 in replicates-in-block designs with two replications at one location. Grain yield, pollen-shed date, rind puncture resistance, stalk rot and leaf blight severity, and other agronomic traits were evaluated. Genetic variance and covariances were estimated for all traits and all pairs of traits, respectively.M eagnr ain yields for RSSC, BC₁-F₂, F₂, and PIC2 were 3.31, 3.08, 2.88, and 1.96 t ha^–1, respectively. In both years, highest genetic variance estimates for yield were obtained in BC₁-F₂. Pollen-shed in RSSSCw as 10 days earlier than in PIC2 and percentage grain moisture increased linearly with increasing proportion of exotic germplasm. In trogression of PIC2, as expressed in BC₁-F₂ and F₂, improved stalk rot, leaf blight, and rind puncture resistance of RSSSCP. redictions of gain from selection for grain yield, per se, indicated that the BC₁-F₂ would be the most favorable foundation population. However, A desired gain performance index including grain yield, grain moisture, rind puncture resistance, and stalk rot and leaf blight ratings favored the F₂ if little weight was given to yield, but ranked RSSSC,BC₁-F₂, and F₂ as equally valuable foundation of pulations if considerable weight as assigned to yield. The PIC2 population was markedly less valuable mainly due to its lack of adaptiveness.

Key Words: Zea mays L. • Penetrometer measurements • Desired gain index • Corn • Quantitative genetics • Genetic variance

² Former graduate research assistant [now, corn breeder, Funk's Seeds, Casalmorano (Cremona), Italy], and professor of plant genetics, Agronomy Dep., Univ. of Illinois, 1102S . Goodwin Ave., Urbana, IL 61801.

Received for publication August 25, 1986.

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