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positive correlations for yield — but these decreased with increasing relative yield depression under low N. However, low and high N-specific genotypes were found, indicating that the two environments differed for yield. Since studies in intermediate altitude tropical maize under low N are not reported, the present study would be a benchmark for further research to improve maize productivity. Table 5.5 indicated that six out of 10 and eight out of 10 hybrids that involved local germplasm under high and low N conditions, respectively, were the best performers under such environments. For instance, hybrids NG2 x T3, T20 x T15, and T6 x T13 were consistently in the top 10 under both regimes of N. However, the relative yield loss of hybrids C60 x T5, NG18 x C4, NG3 x T5, T17 x C4 were consistent with low N.
Furthermore, 50% of the poor performers in low N conditions involved one or more local inbred lines. This could strongly suggest that searching and breeding for local hybrid adaptation of maize cultivars should continue.
The present study established that in the case of the intermediate maturing maize hybrids (i.e. ≤140 calendar physiological maturity days in Tanzania, according to Lyimo et al. [2006]), the chance was that some hybrids could perform consistently better under both high and low N environments. Since the majority of the experimental hybrids outperformed commercial hybrid checks in this study, it could be suggested that these hybrids would be proposed for release as single crosses or they may enter advanced evaluation trials for eventual release.
The local x local, West Africa x local, southern Africa x local and check SC513 sets performed above the grand mean. The southern Africa x southern Africa and West Africa x West Africa were inferior to the grand mean under both high and low N, however the low N tolerance index for the southern Africa x southern Africa set was the most desirable characteristic for breeding for wide adaptability to N conditions (Table 5.4). The IITA lines were the worst within and between sets for their hybrids’ low values of low N tolerance indices. The results further suggest that the southern Africa germplasm could be suitable for breeding for tolerance to low N since the southern Africa x local, West Africa x southern Africa, and southern Africa x southern Africa sets were superior under low N conditions. And this may comply with Kang (1994) that any foreign germplasm must introduce new genes into the local breeding programme for the case of low-N tolerance indices from southern African materials. Furthermore, the southern Africa x local, West Africa x local and local x local sets had higher means for low N tolerance indices across N regimes. These results may suggest the possibility of improving maize cultivars for wide adaptation across regimes of N, firstly by banking on exploring local germplasm for their potentially better performance
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under LN, and secondly from foreign germplasm that should have been preceded by a screening exercise, as the present study had done. Interestingly, hybrids performing better for HN, LN and under both conditions of N regimes were identified in the present study, which suggested the possibility of improving yield under low N conditions.
5.4.2 Relative effects of low nitrogen on maturity characters in maize hybrids
There was mixed information among entry sets, with some hybrids indicating values for characters related to maturity above and below the grand average (Table 5.5). West Africa x West Africa, southern Africa x local and southern Africa x West Africa sets had mean performance for characters related to maturity of above the grand average, whereas local x local hybrid combinations for all characters were below grand average. This may suggest that local germplasm had no appreciable levels of genes for reduced calendar physiological maturity dates under both conditions of N. Local germplasm may have been bred and adapted to local stresses by fast DMA, which may require the foreign germplasm if breeding for early genotypes is desired. However, the challenge has been, particularly under low N stress, to balance fast dry matter accumulation (DMA) and maintain the accumulated dry matter in the kernels until the mark of calendar physiological maturity. Identification of the prolonged leaf chlorophyll concentration, kernel dry down index and grain fill duration in some hybrid combinations, where the relative effects of the application of low N was above 10% (Table 5.5), would improve the maize ideotype for both yield and early maturity under high and low N conditions. The non-senescence character (stay-green) and grain fill duration may be logically related and Hageman and Lambert (1996) asserted that the SG character was reported to be irrelevant to extra-early-maturing genotypes. As the maturity period of the genotypes decreases, the genotype tends to become source limited, but it becomes sink limited in late-maturing genotypes. Maize ideotypes with reduced vegetative growth stage but with extended grain fill duration have been reported and genetic variation for these traits was reported by Mock and Pearce (1975). Cross and Kabir (1989) reported that the high index of kernel dry-down was negatively associated with yield components and yield per se, while cob and kernel size were reported to be inversely related with the index of kernel dry-down.
However, the challenge remains as to how to integrate the high index of kernel dry-down into a genotype aspired to increase DMA via extended leaf chlorophyll concentration and grain fill duration. To aggravate the challenge further, such traits must be considered under low-N conditions, where tropical maize in SSA is typically produced.