Evaluation of Four Maize Populations Containing Different Proportions of Exotic Germplasm1

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Abstract
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.