|dc.description.abstract||Maize production in Africa is constrained by abiotic and biotic stresses. Breeders need to
have information on the nature of combining ability of parents, their traits and performance
in hybrid combination. This requires careful determination of genetic variability of parents,
and studying associations between grain yield and adaptive traits to breed superior cultivars
which are better able to withstand such stresses. Therefore, this study was aimed at
selecting parental testers with best combining ability in hybrid combination with
recombinant inbred lines (RILs); and studying the correlation between grain yield and its
components in eastern and western South Africa. It was also aimed at determining genetic
variation and associations among adaptive traits in hybrids involving RILs. The final
objectives of the study were to determine cultivar superiority of testcrosses involving RILs,
and to select the best cultivars within and across four different environments.
The 42 RILs were crossed to 9 Zimbabwean tropical testers resulting in 1009 hybrids with
sufficient seed for planting in trials. From these a sample of 87 hybrids with adequate seed
were selected and planted at four sites for combining ability analysis. The hybrids were
evaluated at four sites in two regions; western region (Potchefstroom research station) and
eastern region (Cedara, Ukulinga and Dundee research stations), during 2011/12 season.
The experiments were laid out as augmented alpha lattice design. Trials were managed in
accordance with production culture for each region. All quantitative data was subjected to
GenStat and SAS statistical softwares.
The results from combining ability study indicated that the line general combining ability
(GCA) effects played a non-significant role (p > 0.05) in determining grain yield, grain
moisture and anthesis date, while they were significant (p ≤ 0.05) for the other traits such as
ear prolificacy. The tester main effects were significant for all the traits except ear
prolificacy and plant height. Results also revealed that all the traits were controlled by both
additive and non-additive genes, where additive gene action had the most contribution to
the traits. The non-additive gene action played a minor role suggesting the total GCA effects
attributed to both lines and testers predominantly higher over the specific combining ability
(SCA) for all traits. In general the additive effects were preponderant over the non-additive
gene effects. One cross (L114 x T12) had a significant and positive SCA effect for grain yield.
The correlation between grain yield and secondary traits (number of ears per plant, grain
moisture content, ear height, plant height, ear position and anthesis date) suggested that
indirect selection can be employed to enhance grain yield by breeding for these particular
adaptive traits. Path analysis showed that plant height had the highest direct and indirect
effect on grain yield indicating its importance among other secondary traits for grain yield
enhancement. Phenotypic coefficient of variation (PCV) was higher than genotypic
coefficient of variation (GCV) for all the studied traits across all the four environments. All
the traits displayed high heritability at Potchefstroom except anthesis date which was highly
heritable at Ukulinga. Cedara was the second best site for heritability of all the traits except
for the number of ears per plant. The genetic advance for grain yield was the highest at
Cedara followed by Potchefstroom, Dundee and Ukulinga. The hybrids exhibited different
patterns of variation and distribution for all the traits. This indicated that selection
strategies to exploit GCA should be emphasised.
Association studies among grain yield and secondary traits such as ear length, number of
ears per plant, plant height, anthesis date, silking date and ear leaf area revealed that there
were significant phenotypic correlations between grain yield and secondary traits, and
among the secondary traits. Ear length had the highest direct effect on grain yield at
Ukulinga; number of ears per plant had the highest direct effect on grain yield at Cedara and
Potchefstroom; whereas plant height had the highest direct effect on grain yield at Dundee.
Grain yield was least affected by indirect factors at all the sites except Ukulinga, where
anthesis date had the highest indirect effect on grain yield through silking date followed by
plant height through leaf area. The study reveals that there is significant variation among
the hybrids for mean performance, indicating that there is opportunity for selection. Overall
the findings suggest that direct selection would be appropriate to enhance grain yield. Path
analysis revealed that plant height had the highest direct and indirect effects on grain yield,
indicating that plant height can be further exploited as the main trait in future breeding programmes for grain yield increment.
Hybrid 10MAK10-1/N3 was the best hybrid at Ukulinga in terms of grain yield, relative yield
and economic traits. Whereas hybrid T17/L83 was the best hybrid at Cedara in terms of
grain yield and relative yield; however, T11/L102 was selected as the most elite hybrid with
respect to grain yield, relative yield and economic traits. Hybrid T3/L48 was identified as thebest hybrid at Dundee with respect to grain yield, relative yield and prolificacy. At Potchefstroom the standard check PAN6611 was identified as the best hybrid in terms of grain yield and relative yield followed by developmental hybrid T1/L28; however,
developmental hybrid T1/L28 was the best in terms of earliness, prolificacy and ear aspect.
Stability coefficients and cultivar superiority index across the sites revealed that four
developmental hybrids were identified as best hybrids and they performed better than the
standard check. These hybrids will be recommended for further testing in advanced trials.
With respect to cultivar superiority, the desired hybrids are required to combine high grain
yield with economic and adaptive traits such as high ear prolificacy, low grain moisture, and
low ear aspect score (desired) for them to adapt to production environments in South
Africa. There was significant variation among the top 25 yielding hybrids. At least 5 hybrids
combined high grain yield with the desired complimentary adaptive traits such as quick
moisture dry down, prolificacy and ear aspect. The results showed that there is variation in
the performance of high yielding genotypes within all the sites, and that agronomically superior cultivars can be identified.
The study shows that there is significant variation among the RILs since they interacted
differently with the 9 tropical testers. Even among the top 25 selections of RILs in each
environment there was still variation for combinations of the desired traits. Significant
associations among grain yield and other economic and adaptive traits were observed with
implications for breeding strategy. Above all the significant variation gives large score for future breeding of new unique lines.||en