ABSTRACT:

Three trials were conducted during the 2020, 2021 and 2022 rainy season at the Institute for Agricultural Research (IAR) farm, Samaru in the Northern Guinea Savannah zone of Nigeria to evaluate the growth response of two open pollinated and two hybrid varieties to varying nitrogen levels and plant spacing in Kaduna. The treatment consisted of four maize varieties (SAMMAZ 15, 1 SAMMAZ 51, OBASUPER 13 and SC 651) three Nitrogen rates (90, 120 and 150)kg ha- and three plant spacing (75 X 30, 75 X 40 and 75 X 50).The experiment was laid out in a split plot design with a combination of nitrogen and population density in the main plot and variety in the subplot with three replications. Based on the results obtained it was indicated that among the four maize varieties evaluated OBASUPER 13, SC 651, SAMMAZ 15, and SAMMAZ 51thehybrid varieties (OBASUPER 13 and SC 651)generally exhibited superior vegetative growth, characterized by greater plant height and higher leaf numbers, particularly during the early and mid-growing stages. This superiority highlights the inherent heterotic advantage and enhanced resource-use efficiency of hybrid maize compared to open-pollinated varieties (OPVs). However, varietal performance varied across years, indicating strong genotype × environment (G×E) interactions, with OBASUPER 13 and SC 651 performing best in most seasons, while SAMMAZ 15 showed competitive yield potential under favorable conditions. Nitrogen application had a consistent and positive influence on maize growth and yield. The application of 150 kg N ha⁻¹ produced the tallest plants, greatest leaf numbers, and highest grain yield across most seasons and in the combined analysis. This confirms that adequate nitrogen availability is crucial for promoting vigorous vegetative development, optimizing photosynthesis, and enhancing kernel formation. Conversely, the lowest nitrogen rate (90 kg N ha⁻¹) consistently resulted in reduced growth and yield, emphasizing the importance of appropriate N fertilization for maximizing maize productivity. Nevertheless, the absence of significant nitrogen effects in some seasons (notably 2022) suggests that environmental variability, particularly rainfall distribution and soil nutrient status, can moderate nitrogen response. Plant spacing also influenced maize performance, though its effects were less consistent than variety and nitrogen. Wider spacing (50 cm) generally enhanced grain yield per hectare, likely due to reduced inter-plant competition for nutrients, light, and water, whereas closer spacing (30 cm) limited resource availability and suppressed growth. The findings highlight that optimal spacing should balance plant population density and resource accessibility, particularly under rainfed tropical conditions.