Quantitative analysis of Iowa stiff stalk synthetic
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Stiff Stalk Synthetic is a synthetic variety that was deloped in the early 1930's by recombining 16 inbred lines that were considered to be above average for stalk quality. Because of the origin of the lines included in Stiff Stalk Synthetic, it is usually considered a Reid's Yellow Dent type. Stiff Stalk Synthetic (BSSS) has been used extensively in the cooperative federal-state corn breeding project at Ames, Iowa. BSSS has proved to be a good source of lines that have above average combining ability and stalk quality. BSSS was the source population for initiating half-sib recurrent selection in 1939 and one of the populations used for initiating reciprocal recurrent selection in 1949. Both selection programs have been continued to the present time. In addition, BSSS was included in basic research studies to determine the relative importance of different types of genetic effects and to estimate inbreeding depression for several quantitative traits. Results of these studies for BSSS were summarized and compared with data obtained from other corn populations. BSSS per se tended to yield below average, but it was above average for combining ability in crosses with other varieties. BSSS per formed as a Reid's Yellow Dent type because heterosis was greater in variety crosses with Lancaster Sure Crop types than in variety c rosses with Reid's Yellow Dent types. Regardless of the particular variety included in crosses, however, BSSS tended to make a positive contribution to the variety cross. Because nearly all variety cross trials were machine harvested with no gleaning, the above average stalk quality of BSSS may have been a contributing factor in the performance of BSSS in variety crosses. Quantitative genetic studies suggested BSSS has less genetic variability than many of the other corn varieties for yield. Whereas estimates of additive genetic variance in other corn populations were, on the average, 1.6 times greater than the variance due to dominance effects, the estimates of additive genetic variance for BSSS were similar to the variance for dominance effects. Estimates of inbreeding depression for yield, however, tended to be smaller for BSSS than for other varieties. Performance of BSSS per se, less additive genetic variance, and smaller inbreeding effects suggests that BSSS may have a higher frequency of favorable alleles than for other varieties. BSSS may be in the homozygous condition for some important loci. Gene frequencies of favorable alleles greater than 0.5 would reduce the relative proportion of the additive genetic variance to, the variance due to dominance effects and reduce the effects of inbreeding. Also, fixation of favorable alleles would affect variety performance per se and contribute to, improved combining ability. It seems the main features that distinguish BSSS from other corn varieties are better-than average stalk quality, source of lines with above average combining ability that have adaptation over wide areas, and frequencies of favorable alleles greater than 0.5. A wise choice of lines used to form BSSS and continued selection pressure for the past 40 years have developed improved strains of BSSS that have played an important role in continued genetic progress of hybrid corn.