B. Bivolarski, G. Beev, S. Denev, E. Vachkova, T. Slavov

Abstract. The experiment was conducted to study the post-natal development of the caecal microbiota in rabbits weaned at different age. A total of 60 healthy New Zealand White rabbits of both sexes, born the same day, were used in the experiment (after controlling for the effect of litter origin and weaning weight and variability). Rabbits were weaned both at 21 days (W21 group, 30 litters) and at 35 days (W35 group, 30 litters) of age. The weaned animals were randomly housed in wire net cages measuring in well-controlled experimental facility. They received standard commercial pelleted diet without antibiotics. Feed and drinking water were available ad libitum. Results of the microbiological examination of the caecal contents indicated that rabbits weaned at 35 day had higher total bacterial count (TBC) per g of caecal content, in comparison with rabbits weaned at 21 day (P<0.001). The TBC in the caecum of earlier and later weaned rabbits after weaning increased significantly (P<0.001). The obligate anaerobic bacteria, particularly Bacteroides spp. constitute an important group of microorganisms in the rabbit caecum. The population of Bacteroides spp. increased with advancing of age. The differences between groups on days 35, 42 and 49 were statistically significant (P<0.001). Sporulating bacteria and especially Cl. perfringens was present in low variable amounts in all the caecal samples obtained from healthy animals. Caecal counts of Cl. perfringens at weaning (21 and 35 day) were very low (1.656 and 1.654 log10 CFU/g, respectively) and not affected by weaning age. To the end of the study, earlier weaned rabbits had higher caecal count of Cl. perfringens (P<0.01). Enterococcus spp. and coliforms, including E.coli are an important part of the caecal microbial population of rabbits. The caecal number of coliforms was considerably high at weaning, then decreased linearly and stabilized on low level at day 49. Our study demonstrated the absence of Lactobacillus spp. in the rabbit caecal tract. The pH of the caecal content fell linearly throughout the experiment – there are not significant differences between groups at days 21 and 49. Compared to the W21 group, rabbits in the W35 group, had a higher live body weight (P<0.001) and low mortality during the trial