GENETIC GAINS IN THREE BREEDING ERAS OF MAIZE HYBRIDSUNDER LOW AND OPTIMUM NITROGEN FERTILIZATION

A comparative study on the response of six maize hybrids: twohybrids each from the 1980s (8321-21 and 8425-8), 1990s (9801-11 and 9803-2)and 2000s (0103-11 and 0103-15) to sub-optimal and optimal nitrogen fertilizationwas conducted in Oke-Oyi, Nigeria. The trials were set up in a split plot with threenitrogen levels (0, 30 and 90 kg N ha-1) as a main plot and six hybrids as sub-plots.Significant interactions were observed between hybrid and N level for allcharacters, with increasing in variation as the level of N decreased. Mean grainyield reductions across eras were 73.8% at no-N and 32.6% at low-N, and those ofoptimal-N fertilization were 34.3% and 15.7% for 1980s and 1990s genotypesrespectively. Depending on N treatment, grain yield varied from 0.67 to 4.89 t ha-1.Kernel number was most severely reduced by N stress, but had positive and highlysignificant (p≤0.01) correlation with grain yield at all N levels. Genetic gains ingrain yield were 42% (between 1980 and 2000 eras) and 9% (between 1990 and2000 eras) under optimal-N fertilization. The two modern hybrids of 2000 era(0103-11 and 0103-15) were outstanding for all the agronomic traits and leafchlorophyll concentration at all N levels. Improving traits associated with fertilizerN response could accelerate rate of genetic gains in maize yields.