Management of Forage Sorghum: Nitrogen, Plant Density and Irrigation Effects on Yield and Quality

Abstract

Increased interest in producing forage sorghum (Sorghum bicolor (L.) Moench) for silage has prompted questions on the effects of production inputs on forage sorghum quality. Our objective was to determine how supplemental irrigation, N fertility and plant density affected yield and quality of forage sorghum. Dekalb hybrid FS-25e was overplanted and thinned to three densities (32 300, 50 800 and 66 300 plants acre^-1) in 1987 and four densities (15 500, 31 400, 56 900 and 107 300 plants acre^-1) in 1988 and 1989 (19 000, 29 200, 54 800 and 79 700 plants acre^-1) in rows spaced 3 ft apart. Plots were not irrigated or received supplemental irrigation when water-deficit stress symptoms were apparent. Nitrogen was applied at 0, 160 or 320 lb acre^-1. Plots were harvested when grain was at the soft dough stage. Whole plants, leaf blades and stalks were analyzed for in vitro true digestibility (IVTD), acid detergent fiber (ADF), cellulose and acid detergent lignin (ADL, whole plants and stalks only). Under the dryland treatment 160 lb N acre^-1 increased dry matter yield 2.5 to 3.3 tons acre^-1. Supplemental irrigation (15.4 inches) increased yield in one year. Increasing N from 160 to 320 lb acre^-1 increased dry matter yield about 0.5 to 1 ton acre^-1. Dry matter yield increased with increasing plant density up to about 80 000 plants acre^-1. Nitrogen at 160 or 320 lb acre^-1 reduced leaf proportion an average of 32% and increased panicle proportion. Increased plant density increased ADF by 10%, and cellulose concentrations by 29% and reduced IVTD by 3% in leaf blades. Increasing levels of fertilizer N reduced IVTD in leaf blades, stalks and whole plants compared to no N fertilizer. Supplemental irrigation reduced IVTD by 2 to 3% in whole plants and stalks. This study suggests that when considering plant population recommendations for forage sorghum grown for silage (i) differences in plant density have minor effects on forage quality and (ii) yield responses to applied N override the small reductions in IVTD.