Response of a New and Commonly Grown Forage Sorghum Cultivars to Limited Irrigation and Plant Density.

Sorghum (Sorghum Spp. L) is considered to be an alternative forage crop to corn in semi-arid areas where less water is available for irrigation.  A two-year experiment was conducted at the Experimental Farm of  University of Tehran during 2009 and 2010 growing seasons to determine if limited irrigation technique may produce high-yielding forage sorghum with acceptable forage quality. The experiment was arranged as a split-plot factorial design with three replications.  Main plots were consisted of three irrigation regimes including optimum irrigation (when evaporation reached 70 mm, using evaporation pan class A), moderate drought stress (100 mm), and severe drought stress (130 mm). Two sorghum cultivars (Speedfeed and Pegah) were factorially combined with three population densities (150000, 200000, and 250000 plants ha^-1), as sub-plots. The results of this study indicated that forage dry matter and forage quality parameters of both cultivars were similar in both years and both were significantly affected by irrigation regimes, plant population, and cultivars. Plants under moderate and severe stress produced  20% and 34% less forage respectively, compared with optimum irrigation. Leaf and stem weight, leaf/stem ratio, and protein yield ha^-1 were also lower in moderate and severe drought stress than that of optimum irrigation regime. Some forage quality parameters including crude protein, dry matter digestibility, water soluble carbohydrates, ash, crude fiber, dry matter intake, relative feed value, and net energy for lactation improved when limited irrigation was imposed. Forage yield did not respond to manipulation of plant population whereas forage quality was significantly changed with changes in plant number. The highest protein yield (1688 kg ha^-1) obtained from optimum irrigation regime and lowest plant density (150000 plants ha^-1) whereas forage produced in moderate stress and low plant density was richer in relative feed value. Speedfeed outyielded Pegah cultivar (11453 kg ha^-1 vs. 8611 kg ha^-1) and produced higher protein yield (1404 kg ha^-1 vs. 1316 kg ha^-1).