The influence of increased plant densities and nitrogen fertilizer levels on forage yield, seed yield and qualitative characteristics of forage sorghum (Sorghum bicolor L. Moench)

Nitrogen fertilization is a major agronomic practice that affects the yield and quality of crop. A combined split-plot factorial design based on randomized completely block design (RCBD) with four replications was used to determine the influence of plant density and nitrogen fertilizer on forage and seed yield of sorghum. The main plots were years, namely 2009 and 2010. The subplots were 250,000, 300,000, 350,000 and 400,000 plants/ha, and sub-subplots were four levels of nitrogen fertilizers, namely 0, 80, 160 and 240 kg N/ha. The results obtained from statistical analysis showed that the higher stem diameter and dry forage yield were obtained for 2010 than 2009. The maximum plant height, number of leaves per plant, leaf area index (LAI), fresh forage yield and dry forage yield was achieved in 400,000 plants/ha. At 250,000 plants/ha, the highest number of seeds per panicle (2,484), a thousand seed weight (7.72 g) and seed yield (5.58 t/ha) were obtained. There is no difference in prussic acid among treatments. The highest protein and ash percentages of the first cut were related to 350,000 (11.19%) and 250,000 plants/ha (12.16%), respectively. There were not any significant differences in terms of plant height, number of tillers per plant, stem diameter, prussic acid and ash percentage between the levels of nitrogen fertilizers. The highest fresh forage yield and dry forage yield was achieved in 240 kg N/ha and 160 kg N/ha, respectively. An adequate supply of N to crops is fundamental to optimize crop yields and crop production. The higher a thousand seed weight and seed yield obtained for 160 kg N/ ha than those of other treatments, which were 8.04 g and 7.18 t/ha. Grain yield is probably the most important characteristic on which to base nitrogen fertilizer application and selection of appropriate plant density. Moreover, the application rate of 160 kg N/ha had obtained the highest amount of protein percentage of the first cut (11.17%). With appropriate usage of nitrogen fertilizer and improving grain yield accompanied with qualitative traits, sustainability in sorghum production will appear.