Selection Indices and Use of Secondary Traits



Yield is controlled by many genes, and it is highly influenced by the environment along with often high genotype × environment interaction (GEI), resulting in low heritability estimates. Secondary traits are under the control of fewer genes, have lower GEI, and are characterized by higher heritability estimates. Therefore, traits that show consistent genotypic and phenotypic relationships with yield may be more effective as selection criteria than direct selection for grain yield alone. Breeders commonly use three methods to select for several traits, including tandem selection, independent culling, and index selection. In tandem selection, individual traits are improved successively. In independent culling, individuals must surpass a certain minimum value for each trait to be selected. The plants that are outstanding in certain traits may not be selected if they do not meet the minimum standards for other traits, whereas individuals that are relatively mediocre in some traits are selected as long as they meet the cutoff value in the others. In index selection, each trait is weighted by a score, and the individual scores are summed to give a total score, which is referred to as the index value (I) for the genotype. This is an attempt to correct the weaknesses in tandem selection and independent culling methods. For example, selection for maize grain yield under severe drought stress or low-N has often been considered inefficient because the estimates of heritability of grain yield has been observed to decline with reduced yield levels characteristic of stressed environments. Under these conditions, secondary traits may increase selection efficiency provided they have adaptive value, relatively high heritability, significant genetic correlation with grain yield, and are easy to measure. Based on this, base indices for selecting for Strigaresistance, drought, low-N, and multiple stress tolerance have been developed by IITA and CIMMYT scientists for selecting for tolerance to the individual stresses and/or multiple stress tolerance. Ensuring that greater response to selection is achieved using secondary traits in combination with grain yield in a selection index rather than the primary trait per se is an important strategy in the enhancement of early and extra-early maize in SSA.


Secondary Traits Extra-early Maize Independent Culling Tandem Selection Emerged Striga Plants 
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© Springer International Publishing AG 2017

Authors and Affiliations

  1. 1.International Institute of Tropical AgricultureIbadanNigeria
  2. 2.Obafemi Awolowo UniversityIle-IfeNigeria

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