Variation in agronomic characters among high and low nitrogen S2 maize (Zea mays L) lines grown in high and low nitrogen environments

Poor soil fertility, especially with low levels nitrogen (N), is a major constraint to productivity and the production of maize in Sub-Saharan Africa (SSA). This phenomenon has raised the need to combine the breeding goals of yield improvement for conditions with low levels of N and yield improvement for conditions with high input of N fertilizer. The objective of this study was to evaluate variation in agronomic characters, heritability, genotype x environment (GxE), and predicted gains from selection of high and low N S2 maize breeding lines in both high and low N envi¬ronments. Fertilizer was not applied in low N environments, while the high N environments were fertilized at the rate of 150 kg N, 75 kg P2O5 and 75 kg K2O per hectare. The high and low N S2 progenies were evaluated under high and low N environments respectively in the research farms of the Nigerian Institute for Oil Palm Research (NIFOR), near Benin City (Latitude 6o 33’ N and Longitude 5°33’E), Edo State and Delta State College of Agricul¬ture, Ozoro (latitude 6°13’E and longitude 5°33’N). Both locations are situated in the rainforest ecological zone of Nigeria with average rainfall of 2,500 mm. The experimental designs used for both high and low N environments were 10 x 10 and 8 x 8 lattices respectively. The mean squares for the two high N environments were significant for all agronomic traits evaluated. Moreover, the mean squares for the high N S2 progenies were significant for all the observed agronomic characters except ear height and leaf senescence. The high N progenies x environ¬ments interaction was significant in most of the observed agronomic characters except leaf area, ear height and leaf senescence. On the contrary, the mean squares for the two low N environments were significant for all the observed agronomic characters except leaf senescence. Significant mean squares were observed among low N S2 progenies in leaf area, ear height, leaf senescence and grain yield. However, there was no low N x environment interaction for all the observed agronomic characters. Grain yield of low N S2 progenies was only 5.46% of the yield obtained among high N S2 progenies. Nitrogen stress resulted in reduction of growth (height) and yield of low N S2 progenies by 52.3% and 94.5%, respectively. Low N environments resulted in lower broadsense heritability than high N in most of the observed agronomic characters except plant height, leaf senescence and ear height. Heritability estimates in low N and high N ranged from -9.8 to 53.9% and -7.8 to 59.5%, respectively. The results indicated that the magnitude of genetic variance, heritability, and gains from selection are dependent on the nature of the crop environment. In spite of lower heritability under low N stress conditions, selection under low N would be more efficient than selection under high N for targeting low N stress environment