Longitudinal Changes of the Ruminal Microbiota in Angus Beef Steers

SIMPLE SUMMARY: The rumen is a crucial organ in the digestion process of bovines; however, in beef cattle, it is not fully developed until sometime after weaning. In the same way, the microbial population that inhabits the rumen is constantly changing as animals age. The initial inoculation of the rumen of beef calves is heavily influenced by the environment, including the presence of adult animals in the pasture such as the cows. This study investigated the longitudinal changes that occur in the ruminal microbiota of Angus beef steers from weaning to slaughter. Ruminal samples were collected from 12 cows and their steer calves on weaning day, followed by subsequent collections on the same group of steers as they entered the feedlot, upon leaving the feedlot, and at the slaughterhouse. Results revealed that the ruminal microbial composition of the steers at a younger age (at weaning) was very similar to the ruminal microbiota of the adult cows; more so than it was from their own microbiota at later ages. ABSTRACT: The ruminal microbiota of Angus cows and steers were characterized using 16s rRNA gene sequencing, and the expression of their metabolic pathways was predicted. Samples were collected on weaning day from the steers and the cows, and subsequently on three other occasions from the steers. Results showed that microbial richness, evenness, and diversity decreased (p < 0.001) in the rumen of the steers as they were weaned and transitioned to a high-concentrate feedlot diet. However, on the day of weaning, microbial evenness was similar to that observed in the rumen of cows (p = 0.12). The abundance of archaea was similar (p = 0.59) between the cows and steers at weaning, but it decreased (p = 0.04) in the rumen of steers after weaning, and remained stable (p ≥ 0.44) for the remainder of their lives. Likewise, no difference (p = 0.51) in the abundance of Bacteroidetes was detected between the cows and the calves on the day they were weaned, but the abundance of this phylum increased (p = 0.001) and remained stable after that. These results suggest that cows may have a strong influence on the composition, and help modulate the ruminal microbiota of young calves; however, following weaning, their ruminal microbiotas tend to differentiate from that state observed at earlier ages.