“…Cellulolytic bacteria emerge around the fourth day. Protozoa become established later (2 wk) and their populations have maintained by 4 mo [87]. In field-raised lambs, individual bacteria sequancially colonize the rumen: stric anaerobes prevail at day 2, from day 2 to 10, the strict aerobes and facultative anaerobes are 10 to 100-fold lower than the anaerobes and continue to decrease thereafter; methanogens and cellulolytic bacteria appear by day 3 and reach adult levels by the end of the first week [88].…”