Symposium review: Host-rumen microbe interactions may be leveraged to improve the productivity of dairy cows.

The rumen is a large bioreactor that enables dairy cattle to derive nutrition from otherwise indigestible plant polymers and compounds. Despite the direct contribution of the rumen's microbial community toward the nutrition of the dairy cow, only a general knowledge has been gained of the metabolic processes within the rumen, and less still is known about most of the individual microbial species that colonize the organ. What has been discovered is that the rumen contains a diverse community of microbial species from all of the major domains of life, and that the contents of the rumen can vary greatly among individual animals. Preliminary evidence also indicates that rumen microbial profiles are heritable and sustainable within an individual, and that rumen microbial community structure can revert to its original profile within a short period following substantial perturbation. Much progress has been made in recent years to identify the diversity of microbial species in the rumen; however, the most popular methods used to identify microbial species often lack the predictive power necessary to associate particular microbial profiles with rumen metabolic activity. This represents the most significant barrier to the design of models that can estimate the direct effects of rumen microbial content on downstream dairy production traits. If such challenges can be overcome, it is possible that rumen microbial content could be assessed as a new phenotypic trait in cattle. In the future, we may estimate dairy production using a "genotype × environment × microbial" interaction model that accurately combines all factors affecting milk production.