The performance of two sunflower (Helianthus annuus L.) varieties under varying plant density and nitrogen levels was investigated under field condition in 2012 rainy and dry seasons at the Institute for Agricultural Research farm and irrigation research farm in Ahmadu Bello University, Samaru Zaria (11º12`N07º37`E686 above sea level). The treatments evaluated consisted of four rates of nitrogen at (0, 50, 100 and 150 kg per hectare) three plant densities (22,222, 44,444 and 66,666 plants per hectare) and two varieties (SAMSUN 1 and SAMSUN 4). The treatments were factorially combined and laid out in a split-plot design with three replications. The combination of sunflower variety and nitrogen level constituted the main plot while plant density was assigned as sub-plot. Soils of the experimental sites were randomly sampled to depth of 30cm, bulked and analyzed for physicochemical properties before the establishment of the trials. Weather data with respect to rainfall, temperature, relative humidity and sunshine hours for the periods of experimentation was also recorded. The result showed that increase in nitrogen level from 100-150 kgNha-1 significantly increased number of leaves, plant height, relative growth rate, leaf area index, total dry matter, head weight, total seed weight, and 1000 seed weight in both rainy and dry seasons. SAMSUN 4 produced taller plants at 7 WAS (106.24cm) in the rainy season and 13 WAS (170.20cm) in the dry season, it also produced more yield than SAMSUN 1. Plant population density significantly influenced all parameters investigated. It was observed that plants grown at planting density of 66,666 plantsha-1 were taller (170.75cm) when compared to 22,222 plantsha-1 (161.82cm) and has more yields per hectre (1228.05 kgha-1 relative to 741.08 kgha-1) but plots planted at 22,222 plantsha-1 produced plants with significantly thicker stem girth and higher leaf area index in both seasons. Based on this study, the performance of sunflower could be enhanced using nitrogen level of up to 100 kgNha-1 and a plant population density of up to 44,444 plantsha-1 and SAMSUN 4 sunflower variety was superior to SAMSUN 1 variety in both rainy and dry seasons in terms of plant height, number of days to initiation of flowering, lodging percentage, crop growth rate, head diameter, head weight, total seed weight as well as grain yield.
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