In vitro study of the age-dependent caecal fermentation pattern and methanogenesis in young rabbits.

The caecal fermentation pattern, including methanogenesis, was studied in young rabbits using in vitro batch incubations. Six conventional litters of eight rabbits each were used. At the age of 22, 25, 28, 32, 36, 42 and 56 days, an animal was slaughtered from each litter and its caecal contents were used for in vitro batch incubations at 39 degrees C/24 h. The incubated samples were analysed for volatile fatty acids (VFA), methane, hydrogen, ammonia nitrogen (NH3-N) and lactic acid (LA). The net total in vitro VFA production did not differ clearly with age, although a significant decrease was observed on day 36, reflecting the reduced zootechnical performances probably related to an infection with Clostridium spiroforme that occurred in the same period. The molar proportions of butyrate and propionate formed a change in the opposite direction with age, starting with a sudden shift from propionate to butyrate at day 25. In vitro NH3-N production was suggestive of a progressive and significant decrease with age; in vitro LA production was always low. Methane production was almost absent from fermentation until 32 days of age, after which it suddenly shifted from 1.6 to 52.0 mumol/flask/day and increased further with age. A significant litter effect on methanogenesis was observed which suggested the existence of a genetic effect. The hydrogen production was quite low and decreased significantly from day 36 with increasing methanogenesis. The calculated hydrogen recoveries showed a gradual increase from day 32 and were positively correlated (r = 0.92) with methane production. In conclusion, it would seem that in young suckling rabbits, reductive acetogenesis is a major characteristic of caecal fermentation, to be replaced gradually and partially by methanogenesis with the increasing intake of solid feed.