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

Cell wall isolated from grasses as neutral detergent fibre (NDF) is ferment ed more readily than cell wall in the unfractionated forage by rumen organi sms in vitro. In several experiments with Heteropogon contortus and Astrebl a 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 differ ence appearing in the first 24 h of fermentation. Similar in vitro increase s were obtained for Sorghum sp, wheat hay, bagasse and Lolium perenne and l ess 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 stat istically significant there was a significant relationship between NDF cont ent of the unfractionated grass (x) and the ratio of NDF digestibility of i solated 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 0.01 6 to 0.020 h(-1), P < 0.001) rate of production from the slowly-digested po ol. Spear grass NDF incubated in the rumen in polyester bags had a higher N DF loss than unfractionated forage between 9 and 24 h and the difference wa s maintained after 24 h. Treatments in which amorphous silica was precipita ted within NDF or leached from a grass substrate indicated that the enhance d rate in grasses was not due to removal of biogenic silica by the neutral detergent treatment. (C) 1999 Society of Chemical Industry.