Dietary impacts on rumen microbiota in beef and dairy production

• The rumen harbors diverse microorganisms including bacteria, protozoa, fungi, archaea, and viruses. They play a key role in the breakdown and utilization of feedstuff carbohydrate and protein (from forage, grain) through the process of fermentation, resulting in the production of volatile fatty acids (or short-chain fatty acids) and microbial protein. • The volatile fatty acids provide the ruminant with readily available sources of energy and substrates for the synthesis of lipid and glucose by the animal. Microbial protein provides a highly digestible source of amino acids for muscle growth in beef and milk protein in dairy cows. Therefore, rumen microorganisms play an essential role in optimizing nutrient utilization from feed. • Historically, knowledge about the rumen microbial ecology and the nutrition of microorganisms was obtained using classical culture approaches, which by the 1990s, allowed the characterization of at least 22 major bacteria. • Rapid advances in molecular biology and phylogenetic techniques (e.g., multiple sequence alignment) and the rise of "omic" approaches (e.g., high-throughput sequencing) have allowed for understanding the ecology and function of microbial ecosystems in the rumen. • These technical advances have enabled scientists, for the first time, to have a holistic view of the rumen microbiota and enhanced our ability to explore some of the hidden relationships between an "altered" rumen environment and the development of a disorder such as sub-acute ruminal acidosis or milk fat depression ("low fat milk syndrome"). • Here we briefly discuss recent examples demonstrating how age and nutrition can alter the structure, composition, and diversity of the rumen microbiota in beef and dairy production. We also highlight areas where there are knowledge gaps for further research. • Applying an integrative approach, i.e., systems biology, encompassing nutritional management effects on the rumen microbiota, the tissue responses, and the production outcomes will provide added value to nutritionists attempting to optimize further the production of high-quality beef and milk to meet the demands of a growing population worldwide.