Characteristics of neutral detergent-soluble fiber fermentation by mixed ruminal microbes

A new method for determining neutral detergent-soluble fiber (NDSF) estimates the non-starch polysaccharide present in 90% ethanol-insoluble residue (EIR) but absent from neutral detergent residue (NDF). Unextracted feeds, EIR and NDF of feeds high in pectic substances were fermented in vitro with mixed ruminal microbes and gas production was measured. Soybean hulls were fermented for 48 h, and all other feeds for 24 h. Fermentation kinetics of NDSF were determined from gas curves produced by point by point subtraction of the gas production curves of NDF from those of EIR. Single-pool logistic fermentation rates and lag times for NDSF and NDF were determined from the gas production data. The single-pool logistic rates for NDSF for dried citrus pulp, dried beet pulp, soybean hulls, mature alfalfa stems and leaves and immature alfalfa stems and leaves were 0.13, 0.16, 0.05, 0.11, 0.14, 0.18 and 0.13 h(-1), respectively. Fermentation rates tended to be faster and lag times shorter for NDSF than for NDF. Acetate to propionate ratios were numerically higher for EIR fermentations than for;unextracted feeds or NDF, The fermentation characteristics of NDSF for the feeds in this study are consistent with reports of pectic substance fermentations indicating that NDSF does describe the soluble fiber fraction. The NDSF and gas production measurement methods offer nutritionally relevant ways to estimate soluble fiber content and its digestion characteristics in feedstuffs. (C) 1998 Elsevier Science B.V.