Rangifer tarandus (reindeer) – is actively bred in the northern regions of different countries. Therefore, an urgent task is to deepen information about the features of reindeer adaptations. Rumen symbiotic microorganisms play an important role in the life of Rangifer tarandus, allowing animals to efficiently use scarce nutrient resources of the tundra and forest-tundra. The microbial community of the reindeer rumen, as well as its age-related changes, are the least studied compared to other ruminants. The comparative analysis results of rumen bacterial community composition of calf (4 months), young animals (1–2 years) and adults (3–6 years) Rangifer tarandus of the Russian Arctic are presented for the first time. The reindeer ruminal bacterial community composition was analyzed in the laboratory of the ‘BIOTROF+’ Ltd by T-RFLP method. In the ontogenesis, significant changes in the microorganism representation were noticed, the greatest of which was noted in microorganism involved in carbohydrate fermentation. The content of cellulolytic Clostridia and the acid-utilizing species of the Negativicutes (P is less than 0.05) decreased with age, but bacteria with the amylo- and cellulosolytic properties of the phylum Bacteroidetes increased (P is less than 0.05). A wide range of microorganisms which traditionally belong to the pathogens of various animals and humans diseases was revealed. With age, a tendency to increase the number of pathogens, including the bacteria of the families Campylobacteraceae, Burkholderiaceae, phylum Fusobacteria, and the genus Staphylococcus was noticed. The greatest percent of opportunistic microorganisms, including phylum Actinobacteria and the family Enterobacteriaceae, were detected in young animals.

Ruminants produce a large amount of methane (CH4 ) and carbon dioxide (CO2 ) in their foregut. These gases cause greenhouse effect. There are a lot of studies about different feed additives which can reduce the production of greenhouse gases in ruminants. Prebiotics can also change the amount of bacteria in animal gastrointestinal tract and reduce the occurrence of diarrhoea. The aim of this study was to test whether the prebiotic inulin affects the production of CH4 and CO2 in calves’ rumen and whether it affects the bacteria count in the rumen fluid and bacterial overgrowth in intestines. We used the flour of Jerusalem artichoke (Helianthus tuberosus L.) containing 50% of inulin. Approximately fifty days old, Holstein Friesian crossbreed calves were used in this study. Eight were in the control group, 8 received 12 g of flour and 8 received 24 g per day. On the 28th and 56th day of the research, we measured the amount of CH4 and CO2 in calves’ rumen took rumen fluid samples for bacterial analysis and urine to measure the level of phenol and indican. We concluded that adding the flour of Jerusalem artichoke at doses 12 g and 24 g did not significantly impact the production of CH4 and CO2 in calves’ rumen, the prebiotic inulin may suppress the growth of anaerobic microorganisms in the rumen at concentration 12 g of inulin reaching 56th day of experiment. The amount of phenol and indican in calves’ morning urine did not correlate with the faecal consistency of calves.