In vivo and in vitro study of the caecal fermentation pattern in rabbits between 22 and 56 days of age

The age dependent caecal fermentation pattern was studied in young rabbits both in vivo and using in vitro batch incubations. Sequential slaughterings were performed in 6 conventional litters in order to collect caecal contents of rabbits from 22 till 56 days of age. In the present paper, in vivo and in vitro observations are linked in order to check correspondence between the two series of data. Age had a significant effect (P<0.01) both on in vivo total VFA concentration and on in vitro,VFA production. A significant correlation (P<0.01) was found between in vivo and in vitro total VFA values (R-2=0.48) and between the latter and daily weight gain (R-2=0.20, P<0.05). In vivo and in vitro VFA values showed a sharp drop on day 36, followed by a plateau. This decrease reflected the significant (P<0.05) drop in daily weight gain that occurred on day 36. Molar proportions of both in vivo and in vitro VFA showed a shift from propionate to butyrate which occurred on day 25. In vivo and in vitro acetate, propionate as well as butyrate were significantly (P<0.01) correlated (R-2=0.31; R-2=0.78 and R-2=0.61, respectively). In vivo molar proportions of isobutyrate, isovalerate and valerate were negligible whereas significant amounts were produced in vitro. In vitro NH3-N showed a significant (P<0.001) decrease with age, whereas in vivo an irregular pattern was obtained. In vivo lactic acid concentration as well as in vitro,production were significantly (P<0.01) affected by age. However, both for NH3-N and lactic acid no significant (P>0.05) correlation between in vivo and in vitro data was observed. In conclusion, in vitro production rates and in vivo concentrations showed significant correlations for those end-products that are not utilized by microbes (VFA), but not for those end-products that are continuously produced and utilized (NH3-N, lactate). Therefore, the applied in vitro incubation technique is of valuable interest in studies related to rabbit digestive physiology.