Effect of nitrogen fertilizer on quantitative and qualitative traits of forage sorghum genotypes at different harvesting times

To investigate the effects of nitrogen fertilizer on yield and quality of three forage sorghum cultivars under Guilan climatic conditions, a field trial was carried out as factorial-spilt plot on time based on randomized complete block design with three replications in Rasht, Iran, during 2010-11 cropping season. The studied factors were three sorghum cultivars (Pegah, Speedfeed, KSF2) and four levels of nitrogen fertilizer (0, 50, 100 and 150 kg.ha-1). The results of analysis of variance showed the significant effect of cultivar × harvesting time on forage yield and qualititative traits, so that KFS2 cultivar at first harvest produced maximum forage yield. The interaction effect of nitrogen × harvesting time was significant on forage yield and protein, ash and ADF percentage and the highest forage yield was obtained with 150 kg.ha-1 nitrogen at first harvesting time. Also, dry forage yield, protein percentage, digestible dry matter percentage and ADF were significantly affected by the cultivar × nitrogen × harvesting time interaction effect. The mean comparisons indicated that the highest forage yield was obtained by Speedfeed cultivar with 150 kg.ha-1 nitrogen at first harvesting time. Also, maximum digestible dry matter and protein percentage as well as minimum ADF was obtained from Pegah cultivar at first harvesting time. The results of this research showed that KFS2 cultivar with 50 kg.ha-1 nitrogen or Speedfeed cultivar with 100 kg.ha-1 nitrogen are suitable to achieve higher forage yield in Rasht climatic conditions, but for better forage quality, Pegah cultivar with 150 kg.ha-1 nitrogen is recommended.