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Index Selection for Several Agronomic Traits in the BSSS2 Maize Population¹

Expected genetic responses were used to compare the simultaneous improvement of several agronomic traits for three selection index methods in recurrent selection programs. Observations for various agronomic traits in maize (Zea mays L.) were obtained from 144 S₁ lines of the BSSS2 maize population grown in several locations during the 1970-72 growing seasons. These traits were European corn borer (Ostrinia nubilalis (Hubner)) resistance, cold tolerance (percentage of emergence, emergence index, and seedling dry weight), grain yield, grain moisture, root and stalk lodgings, and corn root worm (Diabrotica spp.) resistance (root damage, root size, and secondary root growth). Seven of them, grain yield, corn borer resistance, percentage emergence, emergence index, seedling dry weight, root size, and secondary root growth, were used to construct three types of index selection for S₁ selection.

The conventional indices constructed from two sets of arbitrarily assigned relative economic weights did not give satisfactory predicted improvement for all traits included in individual indices. Most of the predicted improvement went to grain yield and percentage of emergence. Base indices, in which relative economic weights were used as index coefficients, were 95 and 97% as efficient as conventional selection indices. Hence, the application of the base index would be preferable with S₁ selection when precise relative economic values of the traits included in the indices are known. When modified selection indices (based on desired gains) were used, final goals from recurrent selection of the traits considered (S₁ testing and 10% selection intensity) were predicted to be attained after 14 cycles or 28 years of selection. In another scheme of recurrent selection, corn borer resistance and cold tolerance traits would be evaluated in the S₁ generation, and yield, root size, and secondary root growth would be tested hi the S₂ generation. With 44 and 8% selection intensities in the S₁ and S₂ generations, respectively, improvement of all traits was expected to reach final goals after 10 cycles or 30 years of selection. Assuming that the two sets of the arbitrarily assigned relative economic weights were realistic, modified selection indices (based on their contributions to aggregate genetic advance) were shown to be only 46 and 61% as efficient as conventional indices for the specified desired gains and relative economic weights considered. Modified selection index with the S₁ testing scheme was expected to give 53% of improvement for yield, compared with single trait selection based on yield alone. Use of the modified index selection is recommended for the simultaneous improvement of several agronomic traits in recurrent selection programs when the relative economic values of the traits are difficult to determine.

Results from comparisons of predicted gain from recurrent selection with S₁ testing and a combination of S₁ and S₂ testings suggested that S₁ testing would be preferable, because fewer resources would be required and the construction of selection indices would be less complicated.

Key Words: Conventional selection index • Base index • Modified selection index • Recurrent selection • Corn

² Graduate student (now rice breeder, rice division, Bangkhaen, Bangkok 9, Thailand), research geneticist, ARS-USDA, and professor of agronomy; associate professor of agronomy; assistant professor of entomology, Iowa State Univ.; and entomologist, USDA, and professor, Iowa State Univ., respectively.

Received for publication January 10, 1975.