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Date of Award


Degree Name

Doctor of Philosophy




S(,1)-line recurrent selection for improved cold tolerance was evaluated after five cycles in BS13(SCT) and BSSS2(SCT). Selection was based on an index of three traits: percentage emergence at 30 days after planting, rate of emergence and seedling dry weight at 42 days after planting with planting as close to 1 April as possible. Thie six cycles (C0 through C5) of BS13(SCT) and BSSS2(SCT), the six possible population crosses among the C0 and C5 cycles of both populations, bulked S(,1)-lines from each C0 and C5 cycle and population crosses and two single cross check hybrids were evaluated in 19 environments in Iowa, Wisconsin, Minnesota, and Nebraska for cold tolerance traits and at 18 environments in Iowa, Nebraska, and Missouri for agronomic traits;Selection caused significant genetic gain measured as twice the change in allelic frequency weighted by the average effect of an allelic substitution for dry weight per plot, seedling dry weight, percentage emergence at 30 and 45 days after planting and visual seedling vigor score but not rate of emergence. Genotypes superior for these traits were generally superior in both stress and nonstress environments. Drift due to restricted population size was significant for all traits except percentage emergence at 30 days after planting so changes in means were different than estimates of 2(DELTA)p(alpha). Alleles with some directional dominance were important for all cold tolerance traits except rate of emergence. Initial allelic frequencies were near 0.5 in BS13(SCT) and BSSS2(SCT) for these traits;Recurrent selection altered genotype x environment interactions in the populations. Improved cycles were more responsive to environmental variation for dry weight per plot and seedling dry weight and less responsive for percentage emergence at 30 and 45 days after planting;Selection for cold tolerance was associated with correlated improvements in both populations due to pleiotropy or gene linkage in grain yield per plot and per plant, early and final stand counts, moisture at harvest and days to 50% shed and silk. The improvements in grain yield per plot, moisture and days to silk and shed were expressed at both early and normal planting dates. Genetic changes in yield per plant were expressed only at early dates. Gains in final stand counts were also influenced by date of planting.



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Kathleen Gehl Hoard



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285 pages