Response of a new and a commonly grown forage sorghum cultivar to limited irrigation and planting density

Increasing scarcity of water for irrigation is one of the major challenges for forage producers in all arid and semi-arid regions. Selecting drought tolerant forage species such as sorghum (Sorghum spp. L.) over corn, as a common forage crop, could be a viable option to cope with the limited available water for irrigation and increasing the productivity in such climates. A two-year experiment was conducted during the 2009 and 2010 growing seasons to determine if limited irrigation regimes and different plant densities may produce high-yielding forage sorghum with acceptable forage quality. The experiment was arranged in a three-replicated split-plot factorial design with 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) as main plots. A common sorghum cultivar (Speedfeed) and a newly released cultivar (Pegah) were factorially combined with three plant densities (150,000, 200,000, and 250,000 plants ha−1), as sub-plots. Results of this study indicated that forage dry matter and forage quality parameters were significantly influenced by irrigation regimes, plant densities, and cultivars. Increasing water stress from optimum irrigation (Ir70) to moderate (Ir100) and low irrigation (Ir130) resulted in 20 and 34% less forage dry matter yield. Protein yield was also lower when applying moderate and severe drought stress than with the optimum irrigation regime, whereas some forage quality parameters including crude protein, dry matter digestibility, water soluble carbohydrates, dry matter intake, relative feed value, and net energy for lactation improved when limited irrigation was imposed. Highest protein yield (1688 kg ha−1) was obtained from the combination of optimum irrigation regime and lowest plant density, whereas forage produced in moderate stress and low plant density was richer in relative feed value. Speedfeed outyielded Pegah cultivar and produced higher protein yield.