Evaluation of two maize populations after eight cycles of reciprocal recurrent selection

Evaluation of 98 unselected S(,1) lines from each of the BSSSC0, BSSS(R)C8, BSCB1C0, and BSCB1(R)C8 maize (Zea mays L.) populations was conducted to investigate changes in means and genotypic variances that have occurred after eight cycles of reciprocal recurrent selection. Root pulling resistance, measured before and after anthesis, and dates of anthesis were obtained at two locations near Ames in 1978, 1979, and 1980. Yield, number of ears per plant, and root lodging were measured in 1978, 1979, and 1980 at Dayton, where the trapcrop technique was used to enhance the level of infestation of corn rootworms (Diabrotica species). Variance components, heritabilities, means, and phenotypic and genotypic correlations among traits were estimated for individual and combined analyses;Decreases in genotypic variability, as correlated responses to selection for intervarietal hybrid yield, were detected for root strength, yield, and days to anthesis. Comparisons between variance component estimates of BSSS and BSCB1 suggested that inbreeding depression was more severe in BSCB1. Population means for yield and ears per plant increased because of reciprocal recurrent selection, and the observed increases, as affected by stress conditions at Dayton, were discussed. Changes in root strength between C0 and C8 populations, as influenced by changes in days-to-anthesis and by selection for favorable alleles, were discussed;Incorporation of pre-anthesis, S(,1) family root strength evaluation in breeding programs was recommended for BSSSSC0, BSCB1C0, and BSCB1(R)C8 because of the favorable associations between S(,1) line yield and root strength, between pre- and post-anthesis root strength, and between root lodging resistance and root strength. Truncation selection for root strength among S(,1) lines of BSSS(R)C8 was not recommended because lines with superior root strength tended to be lower yielding