Relative efficacy of inocula from rumen fluid or faecal solution for determining in vitro digestibility and gas production

Abstract

Three experiments were conducted to study the effects of substituting rumen fluid (RF) with faecal suspension (FS) as an inoculum for determination of in vitro dry matter digestibility (IVDMD, experiment 1) and gas production (exp. 2). Barley grain (BG), Persian clover (PC), alfalfa (ALF), bromegrass (BR) and barley straw (BS) were used to evaluate the efficacy of the two inocula. In exp. 1, IVDMD was determined using RF or FS containing 3, 6 and 9% fresh cattle faeces as inocula. Except for BS, IVDMD values obtained with either RF or FS containing 3% faeces were not different (P > 0.05). In contrast, FS containing 6 or 9% faeces resulted in lower (P < 0.05) IVDMD than those obtained with RF. Total VFA, butyric and valeric acids of in vitro supernatant were higher (P < 0.05) in RF than FS incubated buffers. Molar proportions of acetic and propionic acids were variable among feeds. Mean acetic:propionic ratio was similar (except for BG) for RF or FS. In exp. 2, substitution of RF with FS containing 6, 11 or 16% cattle faeces as inoculum generally resulted in an increased lag time for four feeds (PC was not included). The rate of gas production was lower when FS was used instead of RF for ALF and BR. For BG however, use of FS resulted in a higher (P < 0.05) rate of gas production than when RF was used. Total gas production values were similar within all the feeds regardless of the source of inoculum. In experiment 3, the relative size of the total and cellulolytic populations were higher for RF than for FS, with a greater diversity of genera of bacteria isolated from RF. Results from this study indicate that FS has the potential to be used instead of RF to obtain IVDMD and gas production for BG and forage hay but not for poor quality roughages such as BS. However, more research with FS is required to confirm these findings. Key words: IVDMD, gas production, rumen fluid, faecal suspension