Evaluation of a Sorghum-Sudangrass Hybrid at Varying Stages of Maturity on the Basis of Intake, Digestibility and Chemical Composition

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Abstract
Plots of a hybrid from sudangrass (Sorghum vulgare var. Sudanensis) and sorghum (Sorghum vulgare) were harvested at three-day intervals beginning 52 days after planting. Twelve stages of growth were sampled beginning with a vegetative stage 86 cm. in height and extending to a more than 40% headed stage 228 cm. in height at 85 days post planting. The forage was preserved as chopped, artificially-dried hay and was fed to castrated male goats of the Saanen breed in a continuous intake-digestion trial. Crude protein showed the most variation with maturity, declining from 20% to about 10%. Acid-detergent fiber (F), cellulose and cell-wall constituents (W) increased only slightly on a percentage basis through Cut 9 after which they increased sharply. Lignin (L) increased steadily until Cut 9 and then increased more sharply. The ratios L/F and L/W followed maturity more closely than did the simple concentrations of F and W. There was little change in in vivo digestibility during the first four cuts covering a period of 61 days from planting, a vegetative stage of growth and a plant height of 132 cm. Thereafter, there was a marked decline in digestibility. The rates of decline in digestibility between the fourth and twelfth cuts were between 0.50 and 0.65 percentage units per day for dry matter, energy, protein, F, W, and cellulose. Although the digestibility of this forage compared favorably with perennial grasses and legumes, its nutritive value was limited by low voluntary intake. The best single chemical method for the prediction of energy digestibility and relative intake was lignin. Multiple regression-correlation analyses indicated that digestibility and NVI could be best predicted from combinations of F, L and protein, while intake was most accurately estimated by combinations of L/F, L/W and protein. The inclusion of in vitro DDM in the multiple regression equation did not improve the correlations when a single forage species was involved. Relative merits of the various laboratory analyses for estimating nutritive value were discussed.