Host Specificity of the Rumen Microbial Community and Its Relationship to Feed Efficiency

The microbial community of the rumen is diverse, containing perhaps thousands of individual species. These communities are highly individualized (i.e., vary among animals with respect to relative abundance of individual species), but display functional redundancy that allows conversion of feedstuffs to somewhat similar concentrations and proportions of fermentation end products (volatile fatty acids [VFA] and fermentation gases). The communities are also highly resilient (resistant to change), as evidenced by poor success in establishing exogenously inoculated strains, unless those strains fill an unoccupied niche in the community. Resilience is also demonstrated by reestablishment of the host bacterial community within a few weeks following near-total exchange of ruminal contents between cannulated cows. Contents exchange experiments have also revealed that restoration of an individual’s ruminal chemistry (pH and VFA profile) occurs much more rapidly than does the reestablishment of the bacterial community, suggesting that the animal has substantial control over its own ruminal chemistry. Feed efficiency (FE) in both beef and dairy cattle is only moderately heritable, suggesting that other factors may contribute to FE, including inter-animal differences in microbiome composition. Characterizing FE in individual animals needed for microbiome studies is complicated by changes in FE over the life of the animal (for example, compensatory growth in steers, or varying metabolic demands over the lactation cycle in dairy cows). When such factors have been successfully controlled, several studies in both beef and dairy cattle have demonstrated significant associations between FE and the ruminal abundance of individual microbial taxa. However, the identities of these taxa have not been consistent across studies, and a causative role of the microbial community as a determinant of efficiency differences has resisted demonstration. Recently, near-total exchange of ruminal contents between dairy cows that differ in milk production efficiency (MPE, calculated as energy corrected milk per unit dry matter intake) resulted in transient changes on MPE that disappeared as the ruminal community reestablished its original composition. These data suggest that MPE within individual animals might be improved by establishment of communities from high-MPE animals. Successful establishment of such communities in recipient animals will require novel strategies to overcome the high degree of resilience of the ruminal microbial community.