The use of exotic maize (Zea mays L.) germplasm, particularly of tropical and subtropical adaptation, has been suggested to increase genetic variability and grain yield of maize breeding material in temperate environments. Lack of adaptation to temperate areas and photoperiod sensitivity may mask desirable traits and reduce breeding progress. The objectives of this study included two-stage evaluation and selection of segregating families in the central U.S. Corn Belt derived from the backcross introgression of previously improved tropical CIMMYT germplasm into adapted, elite U.S. Corn Belt germplasm. The U.S. Corn Belt heterotic patterns of Stiff Stalk and non-Stiff Stalk were combined with germplasm of primarily Tuxpeño and non-Tuxpeño racial origin, respectively. Per se evaluation of 891 backcross (BC1F1) families from 33 different backcrosses, component F1 crosses, and adapted recurrent populations were conducted in Iowa in 1997. Backcross populations displayed grain yields either similar to or greater than the respective F1 crosses but with significantly less grain moisture and days to mid-pollen, suggesting 25% tropical germplasm to be the preferred starting point to initiate selection. Stalk lodging of backcross populations was similar to and sometimes less than the respective adapted recurrent population check while yield was greater in more than 50% of the crosses. BC1F1 families were selected from within Stiff Stalk and non-Stiff Stalk oriented materials and testcrossed to an elite inbred of the opposite heterotic pool. Testcrosses of backcross families and testcrosses of adapted parental populations were evaluated at seven locations in Iowa and Nebraska in 1999. Grain yield, percent root and stalk lodging, and plant and ear heights of selected testcrosses were generally similar (p ≤ 0.05) to their respective checks. Harvest grain moisture and days to mid-silk and mid-pollen were either similar to or greater than the checks, averaging early zone 8 for maturity.; Results support backcrossing to introduce previously improved tropical germplasm to the central U.S. Corn Belt. The alignment of established U.S. Corn Belt heterotic pools with tropical racial or heterotic pools during introgression will serve to enhance genetic variation within and among pools. Previously selected tropical germplasm of known heterotic affinity may be considered a significant resource for introduction of tropical germplasm into temperate breeding programs.
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