Isolation of grass cell walls as neutral detergent fibre increases their fermentability for rumen micro-organisms

Abstract

Cell wall isolated from grasses as neutral detergent fibre (NDF) is fermented more readily than cell wall in the unfractionated forage by rumen organisms in vitro. In several experiments with Heteropogon contortus and Astrebla squarrosa the NDF digestibility at 48 h for the isolated cell wall was 8–18 units higher than for the unfractionated forage, with most of the difference appearing in the first 24 h of fermentation. Similar in vitro increases were obtained for Sorghum sp, wheat hay, bagasse and Lolium perenne and less so for Panicum maximum. The difference was not significant for Chloris gayana, nor for the tropical legumes Stylosanthes hamata, Clitoria ternatea and Lablab purpureus. For those grasses in which the above effect was statistically significant there was a significant relationship between NDF content of the unfractionated grass (x) and the ratio of NDF digestibility of isolated NDF to that of the unfractionated grass, as described by y=0.018x (P<0.01). For H contortus, kinetic analysis of gas produced from 0 to 48 h of in vitro incubation with rumen microbes showed an increased (from 0.016 to 0.020 h −1, P<0.001) rate of production from the slowly-digested pool. Spear grass NDF incubated in the rumen in polyester bags had a higher NDF loss than unfractionated forage between 9 and 24 h and the difference was maintained after 24 h. Treatments in which amorphous silica was precipitated within NDF or leached from a grass substrate indicated that the enhanced rate in grasses was not due to removal of biogenic silica by the neutral detergent treatment. © 1999 Society of Chemical Industry