Selection Indices to Improve an Intermating Population of Spring Wheat1

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Abstract
The intermating population of spring wheat (Triticum aestivum L.) NDPl was created to be a source of both cultivars and parental lines for future breeding populations. The objective of this study was to predict the relative merits of four types of selection indices for improving NDPl, and to predict the gain in efficiency of selection from using information on secondary characters in the calculation of index weights. Smith‐Hazel, base, and desired gain indices were constructed to increase yield and percent protein and decrease plant height. Restricted selection indices were constructed to increase yield and eliminate undesirable correlated responses in percent protein and plant height. Comparisons were made based upon predicted responses from S1 family selection. Estimates of the population parameters were derived from 200 S1 families grown in two replications in the field in 1981 and 1982. Broad sense heritabilities for all measured characters were high. Genotypic and phenotypic covariances between yield and percent protein were negative, while covariances between these two characters and the characters plant height and kernel weight were positive. The Smith‐Hazel index maximized predicted gains in the aggregate genotype, with a predicted increase in yield of 9.6%, a decrease in plant height of 0.3%, and an increase in percent protein of 1.1%. Similar responses were predicted for the base and restricted selection indices. A 2.7% increase in yield and percent protein, and a 2.7% decrease in plant height were predicted for selection based upon a desired gain index. The inclusion of information on the secondary characters (kernel weight, spike length, kernels per spike, and spikelets per spike) in indices did not appreciably increase their predicted efficiencies. Indices designed to improve the aggregate genotype should be effective in developing NDPl as a source of high yielding conventional height lines. If high yielding, high protein semidwarf lines are desired, a desired gain index should be considered for improving NDP1.