Recurrent selection methods were conducted in corn (Zea mays L.) populations to increase the frequency of favorable alleles for grain yield. Populations under recurrent selection were evaluated to estimate response to selection and to compare the relative effectiveness of the different methods of recurrent selection for grain yield improvement. Ten populations, their improved strains, and the S1 generation of the original and improved strains were evaluated in four field environments. This study was conducted to estimate the direct and indirect responses to selection of the 10 populations and their respective S1 generations for different methods of selection. Positive response to selection for greater grain yield was realized for each selection method except for one population (BSCB1) undergoing reciprocal recurrent selection. Average response (0.249 Mg ha−1 cycle−1) for the intrapopulation selection methods was greater than the average response (0.033 Mg ha−1 cycle−1) for the interpopulation selection methods. Response in the S1 generations was similar to the response of the noninbred populations. Reduction in inbreeding depression averaged 12%. The S1 generations of two selected populations [BS13(S)C4 and BS12(HI)C7] had significantly greater yields than the nonselected, noninbred populations from which the selected populations were derived. Positive response to selection was accomplished without selection for taller, later‐maturity genotypes. No consistent trends were detected for changes in root and stalk lodging with selection for grain yield.
Recurrent selection for improvement of yield of corn (Zea mays L.) germplasm emphasizes early testing of either S0 or S1 progeny testcrosses. On the basis of early testing information, progenies are selected and intermated to form populations for continued selection. This study was conducted to determine if early testing was effective in identifying superior full‐sib families of S0 plants in reciprocal fullsib selection. The effectiveness of early testing was investigated by comparing the relative genetic variability and genetic correlations of 136 full‐sib families for the S0 S1, S2, S3, and S4 generations of in breeding. Full‐sib families and six hybrid checks were evaluated in three environments for each generation for grain yield and five other agronomic traits. The among full‐sib component of variance for grain yield tended to increase with inbreeding of the parents, and there was evidence that variability among full‐sib families was reduced in stress environments. Genetic correlations for yield between generations of inbreeding ranged from 0.87 for S0‐S1 generations to 0.31 for S0‐S4 generations. Genetic correlations decreased as the number of intervening generations increased. Selection of the 20 highest and lowest‐yielding S4 full‐sib families included only three and four, respectively, of the 20 highest‐ and lowest‐yielding S0 full‐sib families. None of the 20 lowest‐yielding S1 full‐sib families, however, was included in the 20 highest‐yield S4 full‐sib progenies. The objective of early testing in reciprocal full‐sib selection was achieved because none of the poorer performing families was included for intermating.
Genetic variability Is essential for effective selection. Hence, the relative amount of genetic variability within and among populations Is Important to determine the relative progress that can be expected from selection. The genetic portion of the variance of tested progenies from full-slb families Is greater than half-slb families. Use of progenies as parent of full-slb families to Increase the variance among full-slb families by the factor of (1 + F) was suggested by Sprague and Eberhart (1977).The correlation coefficients may be considered as a measure of the commonness of the genes governing the determination of two traits and the relationship between parent and offspring. Hence, the correlation data between traits of parent plant and its progeny may be used to assist in the selection of progenies. Correlations between traits Indicate expected correlated responses that occur with selection, whereas correlations between parent and offspring indicate expected effectiveness of selection.Reciprocal full-sib selection has been a productive breeding technique. It is extremely efficient in that the yield trials for popu lation Improvement also serve as early testing for inbred selection and hybrids combinations. Reciprocal full-slb selection was initiated by Hallauer (1967a) to improve two populations and to develop hybrids. Reciprocal full-sib selection Is based on early testing, in its truest sense. There are differences of opinion on the value of early testing; thus, it is important to determine the relationship among generations of testing on successive generations of Inbreeding. The objectives of this study were: (1) to estimate the variability 3 among full-sib crosses for five generations of inbreeding to determine changes in variability among full-slb progenies with generations of inbreeding; and (2) to estimate the genotypic correlations between fullsib progenies of different generations of inbreeding to determine if a trend occurs with Increased generations of Inbreeding of lines used to produce full-slb progenies. 12.2 1.6
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