growth and yield responses of two forage sorghum cultivars to different nitrogen fertilizer rates

in order to evaluate the impact of different amounts of nitrogen fertilizer on growth and yield of two forage sorghum cultivars, a 2-year field experiment was carried out at the college of agriculture, shiraz university, shiraz, iran during 2010 and 2011 growing seasons. two factorial experiments were carried out in randomized block design with three replicates, in which the treatments included nitrogen fertilizer at three levels: 69 (n1), 138 (n2) and 205 (n3) kg n ha-1 and two sorghum cultivars: pegah and kfs2. the results showed that nitrogen fertilizer enhanced plant height, leaf area index, fresh weight, total dry weight and biological yield significantly so that these traits were higher in n3 as 5.71, 22.8, 8.13, 22.5, and 8.7% than n1, respectively. furthermore, increasing nitrogen rate had additive effect on crop growth rate, relative growth rate, biomass duration and leaf area duration. comparing the cultivars, it was found that, plant height, fresh weight, total dry weight and biological yield were higher in pegah than kfs2 cultivar. results also showed that most of the studied traits were superior for pegah under n3 treatment; so, pegah cultivar and application of 205 kg n ha-1 might be offered for similar agro-climatic conditions.