IN-VITRO SIMULATION OF RABBIT CECAL FERME NTATION IN A SEMICONTINUOUSFLOW FERMENTER .3. EFFECT OF THE QUANTITY OF DRY-MATTER INTRODUCED DAILY AND REPRODUCTIBILITY OF THE METHOD

In a preliminary experiment 3 Rusitec-like fermenters of 1 L capacity were operated under identical conditions. The only difference was the quantity of treated substratum introduced daily into each fermentor: 1 5 g/d (M15); 40 g/d (M40) and 60 g/d (M60) on a dry matter basis. The fermentation substratum was a rabbit feed that had been digested with amylase and pepsin. The organic matter was lost over 48 h at a signifi cantly higher rate in the M15 fermentor than in M40 and M60: 30.4%, 19 .7% and 17.3%, respectively. The pH values observed in M40 and M60 (5. 99 and 5.83) were similar to that observed in vivo under ad libitum fe eding conditions. The pH was higher in M15 (6.7), as observed in vivo with restricted animals just before the daily meal. The volatile fatty acids (VFA) proportions for C2, C3 and C4 were similar to the in vivo proportion for the 3 fermentors. The daily total VFA production was t he largest with M40 but was associated with a poor stability and an ex cess of minor VFAs. The introduction of 15 g/d was preferred by the au thors because of organic matter disappearance rate, pH stability and V FA production. In a second experiment 4 fermentors were used in the sa me way as the M15 one, in order to study the reproducibility of the me thod Small but significant differences between fermentors were observe d for all parameters in relation to a high fermenter stability from I day to the next. Nevertheless, the standard deviations between ferment ers of the NDF (4.8) and ADF (4.9) disappearance rates were similar to those observed during digestibility trials performed in vivo by other authors with the same source of fibre: 3.0-5.1 for NDF and 3.7-5.9 fo r ADF. The same was observed far VFA production and proportions. The s tandard deviation of VFA concentration was 5.7 mM/L between fermentors and 13.5 mM/L (and up to 28 mM/L) between rabbits. In addition, the d ay-to-day total VFA concentration variability was 1-1.2% with our ferm enters and 0.9 to 1.4% with Rusitec according to other authors.