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Evaluation of Methods for Generation Advance in Bulk Hybrid Soybean Populations¹

We evaluated four methods of generation advance in bulk hybrid soybean (Glycine max (L.) Merrill) populations for their ability to maintain superior genotypes and genetic variability for economic traits and for their efficiency in sampling time. The methods used were single seed descent (SSD), restricted cross-bulk (RCB), maturity-group bulk (MGB), and cross-bulk (CB).

Method means did not differ significantly at either location, and differences in genotypic variance among methods were not consistent in all crosses. SSD, RCB, and MGB maintained a similar number of high yielding lines, about twice as many as CB. SSD and MGB were the most effective methods in maintaining early lines. However, there were no differences among methods for the frequency of late segregates. SSD was about twice as effective for maintaining large-seeded lines as the other methods. The four methods did not differ appreciably for the number of lodging resistant lines. RCB had the highest frequency of tall plants.

Comparison of the harvest time spent in obtaining samples for the next generation showed that CB was the fastest and RCB the second fastest method. SSD was more time-consuming than CB and RCB. MGB was the most time-consuming, since it required pulling plants at several different times to maintain subpopulations restricted in maturity.

SSD was the method least influenced by natural selection; therefore, it may be the most useful method in greenhouse or winter nursery environments where a genotype may perform differently than under field conditions in its area of adaptation.

Key Words: Single seed descent • Bulk breeding • Glycine max (L.) Merrill

² Research Associate, Iowa State University (Present address: Department of Agronomy, University of Nebraska), and Associate Professor of Agronomy and Collaborator, Crops Research Division, ARS, USDA, Iowa State University.