Chemical composition and in vitro dry matter digestibility of various utilization forms of grain sorghum residues

The chemical composition and in vitro dry matter digestibility (IVDMD) of various utilization forms of grain sorghum residues were investigated. Two grain sorghum cultivars, SNK 3640 (bitter tasting) and Pioneer 8564 (sweet tasting), were used. Each taste type was grown on two soil types, viz. Rensburg and Bainslvei. The results indicate that energy availability as reflected by the IVDMD, acid detergent fibre (ADF) content, neutral detergent fibre (NDF) content, and water soluble carbohydrate (WSC) content, seemed to be highest for fresh total plant residues (IVDMD = 53.9%) and for leaves of fresh plant residues (IVDMD = 52.3%). Silage made from fresh total plant residues seemed to be somewhat lower in energy availability (IVDMD = 49.4%), while leaves of frosted plant residues (IVDMD = 44.8%), frosted total plant residues (foggage) (IVDMD = 44.0%), and hay made from fresh total plant residues (IVDMD = 42.4%), seemed to be lowest in energy availability. A high WSC content (18.9%) accompanied with a relatively low NDF content (68.8%) was measured for fresh total plant residues while a relatively low NDF content (67.5%) was also measured for silage. Variation in the IVDMD of grain sorghum residues occurred among taste and soil types. The crude protein (CP) content of leaf material (12.6% for fresh, and 10.6% for frosted, leaves) was higher than the CP content of the various utilization forms of total plant residues which varied between 5.8 and 6.5%. Crude protein quality, as measured by the percentage of nitrogen appearing as non-protein nitrogen (NPN) and the percentage of nitrogen associated with ADF (ADF-N), was most favourable for fresh (NPN = 19.1% and ADF-N = 13.4% and frosted (NPN = 22.5% and ADF-N = 12.5%) leaf material. The NPN and ADF-N values, respectively, varied between 28.4 and 53.7% and between 17.6 and 21.2% for the various utilization forms of total plant residues. Frost had little detrimental influence on the content and quality characteristics of CP. Phosphorus content was highest for fresh (0.20%) and frosted (0.18%) leaf material. Phosphorus values for the various utilization forms of total plant residues varied between 0.10 and 0.12%. Soil type significantly (P < 0.05) affected the chemical composition and IVDMD of grain sorghum residues, with Rensburg the more favourable soil type. Fermentation properties of silage reflected a lactic acid type of fermentation, showing grain sorghum residues to be suitable for successful ensiling. The chemical composition and IVDMD of the silage showed that ensiling was a more effective method to preserve fresh grain sorghum residues than it was to preserve it as hay or as foggage.Keywords: carbohydrate; chemical composition; crude protein; cultivars; dry matter digestibility; energy; ensiling; fermentation; fibre; foggage; grain; hay; in vitro dry matter digestibility; leaves; neutral detergent fibre; nitrogen; phosphorus content; protein; quality; silage; soil types; sorghum; south africa African Journal of Range & Forage Science, 12(3): pp. 116-12