POPULATION MEANS AND GENETIC VARIANCES IN SELECTED AND UNSELECTED IOWA STIFF STALK SYNTHETIC MAIZE POPULATIONS

Recurrent selection is a cyclical breeding procedure that focuses on improving the mean performance of a population by increasing the frequency of favorable alleles while maintaining adequate genetic variability for continued selection response. Iowa Stiff Stalk Synthetic (BSSS) is a maize (Zea mays L.) population that has undergone continuous recurrent selection for more than 50 yr as the base population for two independent selection programs (intra- and inter-population). This study was designed to estimate the mean performance and genetic variances in BSSS after seven cycles of half-sib (HS) progeny selection, six cycles of S-2-progeny selection, and 11 cycles of reciprocal recurrent selection(RRS). A Design II (factorial) mating design was constructed to give direct estimates of additive and dominance variance in the individual populations. Fourteen sets of four male x four female matings for each of the four populations were evaluated in a randomized incomplete block experiment grown in six environments. Half-sib progeny and RRS methods have produced the most effective mean performance responses for grain yield in the populations per se, 0.076 and 0.104 Mg ha(-1) cycle(-1), respectively. The a-progeny selection procedure in BSSS, with a response of 0.052 Mg ha(-1) cycle(-1), has not performed up to theoretical expectations. Genetic variance component estimates for grain yield supported the suggestion that dominance variance is important in BSSS germplasm. Although additive variance decreased slightly, high heritability estimates suggest that further improvement in population means should be achieved by each selection method. Mean performance and estimated genetic variability for other important agronomic traits (grain moisture, root and stalk Lodging, and ear height) generally showed favorable response to selection.