Demand-Driven Technologies for Sustainable Maize ... - IITA

188DiscussionThe observed effects of N on agronomic and yield traits confi rmedthat N stress was a major factor limiting maize productivity. The useof fi elds that have been previously depleted of N resulted in severeN stress as indicated by soil nutrient levels and crop responses.Increasing N fertilization was accompanied by increases in the growthand grain yield of the maize genotypes consistent with the fi ndingsof other studies in the Guinea savannas of Nigeria (Akintoye et al.1998) and elsewhere (Bañziger et al. 1999). Genotypic differenceswere observed for all traits at all N levels. As was expected, genotypesselected for tolerance to N-defi cient environments and the hybrid ObaSuper 2 performed better than the widely adapted and widely growncheck (TZB-SR) at 0 and 30 kg N ha -1 . The good performance of ObaSuper 2 under N-defi cient conditions is contrary to the widely heldview in West and Central Africa that maize hybrids are inherentlysusceptible to low soil fertility. Due to the high yield potential, hybridsmay require more N to attain maximum yields than open-pollinatedvarieties (Akintoye et al. 1998). However, the greatest advantage ofhybrids over open-pollinated varieties may be realized at moderateN levels (Akintoye et al. 1998). Akintoye et al. (1999) attributed thegood performance of maize hybrids under moderate N levels to highN-use effi ciency. They found that, relative to open-pollinated varieties,hybrids were more effi cient in N-use as well as its component traitssuch as N-uptake and N-utilization effi ciencies.The drought-tolerant genotypes were also found to be as goodas or better than genotypes selected for tolerance to sub-optimalN conditions. These fi ndings suggest that the selection of maizegenotypes for drought tolerance may confer tolerance in thesegenotypes to sub-optimal N, consistent with the fi ndings of Bañzigeret al. (1997). Bañziger et al. (1999) showed that drought-tolerantmaize populations perform well under N-defi cient conditions. Theyalso found that gains from selection under well-fertilized, droughtstressedconditions did not diminish as the level of N stress increased.This may be due to the fact that the physiological mechanisms forthe tolerance of maize to drought may not be different from thoseof tolerance to N-defi cient conditions. For example, maize cultivarswith early proliferation of roots in the top soil would allow the plantsto effi ciently use the soil inorganic N while a deep, dense root systemallows the plant to extract nutrients from deeper layers of soils (Jacksonet al. 1986). Similarly, the response of maize to drought is related tothe root system development, which infl uences water-uptake (Aina andFapohunda 1986). Although the root systems of the maize genotypeswere not evaluated in the present study, Kamara et al. (2002) founda positive correlation between the vertical root-pulling resistance