Striga hermonthica (Del.) and low soil nitrogen (low N) are among the most important constraints to maize production and productivity in West Africa (WA). Knowledge and understanding of the inheritance of Striga resistance and low N tolerance in early maturing maize inbreds are invaluable in developing hybrids adapted to both low N and Striga-infested environments in WA.The objectives of the present study were to (i) assess the genetic diversity among the inbred lines using simple sequence repeats (SSR) and single nucleotide polymorphisms (SNPs), (ii) assess the levels of resistance/tolerance of inbred lines to Striga and Low N (iii) examine the combining ability of early maturing maize inbreds across low-N, Striga-infested and optimum growing environments, and (iv) assess the performance and stability of the hybrids across the stress and non-stress environments.

Genetic diversity among nine CIMMYT and 87 IITA early maturing inbred lines was assessed using 31 polymorphic SSR and 261 SNP markers. SSR and SNP analyses revealed a relatively high level of variability among the lines. One hundred inbred lines (11 CIMMYT and 89 IITA lines) were evaluated under low N and Striga-infested environments in 2010 and 2011 in Nigeria. Using the base indices for selection, 27 % of the lines combined resistance to Striga and tolerance to low N. Thirty lines were selected based on their performance under low N and Striga. The lines were used to generate 150 early single-cross hybrids using the North Carolina design II. The hybrids plus six hybrid checks were evaluated under Striga-infested, low-N and optimum growing environments at two locations in Nigeria in 2011 and 2012.
General combining ability (GCA) and specific combining ability (SCA) mean squares were significant for grain yield and other traits indicating that additive and non-additive gene effects were important in the control of the inheritance of grain yield and other traits across the contrasting environments. GCA mean squares of grain yield, Striga damage and number of emerged Striga plants were larger than those of SCA, indicating that additive gene action was more important in the inheritance of these traits. Similarly, the contribution of GCA to the total sum of squares was higher than that of SCA for grain yield (52.4%) under low N indicating that additive gene action was more important in the inheritance of low N tolerance. The superior GCA-female effects (GCAf) for days to silking and GCA-male effects (GCAm) for ears per plant under Striga infestation indicated that maternal and paternal effects modified the expression of the traits under Striga infestation. In contrast, there were no maternal or paternal effects on the traits under low-N environments. Inbreds TZEI 173, TZEI 175 and ENT 11 had superior positive GCAm and GCAf effects for grain yield under Striga infestation. These can be used to improve germplasm for Striga resistance. The lines ENT 11, ENT 12, ENT 16 and TZEI 32 with outstanding positive GCA effects for grain yield under low N environments can be used to improve germplasm for low N tolerance. Grain yield of the hybrids ranged from 1254 kg ha-1 for TZEI 173 X TZEI 175 to 5541 kg ha-1 for ENT 16 x TZEI 32 under low N, 775 kg ha-1 for TZEI 168 x TZEI 54 to 4735 kg ha-1 for ENT 11 x ENT 12 under Striga infestation. The polymorphic information content (PIC) from the SSR marker data ranged from 0.10 to 0.87 with an average of 0.58. The PIC values for SNP ranged from 0.01 to 0.38 with an average of 0.25.The number of alleles per locus identified by SSR markers ranged from 2 to 8 with an average of 3.7. Cluster analysis based on genetic distance (GD) from SSR and SNP classified the lines into five and three groups respectively. The lines clustered predominantly according to their pedigree. ENT 11 x TZEI 4 and TZEI 65 x ENT 11 identified as the most stable and high yielding hybrids should be extensively tested and promoted for adoption and commercialization. This would contribute to improved maize productivity and food security in the sub-region.