Breeding progress for drought tolerance in maize has been slow since drought tolerance is a complex trait controlled by many genes. Breeders improving maize for drought tolerance have therefore been using secondary traits and selection indices for selecting the best genotypes under drought stress. The objectives of this study were to determine the combining ability of the inbred lines in stress and non stress environments as well as compare the use and efficacy of secondary traits and selection indices in selecting for drought tolerant genotypes. Fifty hybrids formed through a North Carolina Design II and four checks were evaluated using the 0.1 alpha lattice planting design under optimum and drought environments. General combining ability (GCA), specific combining ability (SCA) and seven drought indices (SDI), stress susceptibility index (SSI), tolerance (TOL), yield index (YI), yield stability index (YSI), mean productivity index (MPI), stress tolerance index (STI) and geometric productivity index (GMP) were used in the computation of results. Results showed that under drought conditions, GCA was highly significant (P<0.001) for grain yield, anthesis-silking interval (ASI) and ears per plant (EPP) while SCA was significant for grain yield and EPP. Selection indices STI and GMP had positive and significant correlation (P<0.01) with grain yield under both drought stress (Ys) and optimum (Yp) conditions. EPP also had significant (P<0.01) and positive correlation with Ys and Yp. ASI had significant and negative correlation with Ys (P<0.01) and TOL (P <0.05). In addition EPP had a positive and significant correlation with STI (P<0.05) and GMP (P<0.01). The results indicated that ASI, EPP, STI and GMP are effective in identifying high yielding genotypes under different moisture regimes. Narrow sense heritability showed that phenotypic variation attributed to genetic effects increased under stress conditions for EPP and ASI and making them more reliable parameters for use in selecting for genotypes under stress conditions. The two secondary traits (ASI and EPP) together with two drought indices; (STI and GMP) can therefore be used in tandem for increased efficiency in selecting for genotypes under stress environments.
Key words: Combining ability drought stress, drought index, heritability.
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