AS THE LARGEST COMPARTMENT OF THE CATTLE STOMACH, THE RUMEN AND ITS MICROBIOME ARE CRITICAL FOR THE CONVERSION OF LOW-QUALITY FEEDSTUFFS INTO USABLE ENERGY FOR RUMINANTS. THEREFORE, THE RUMINAL MICROBES REPRESENT THE GREATEST OPPORTUNITY TO RAPIDLY IMPROVE BEEF CATTLE NUTRITION AND GROWTH FOR MEETING FUTURE GLOBAL PROTEIN DEMANDS. HOWEVER, IF CERTAIN MICROBES THAT ARE SIGNIFICANT FOR FEED EFFICIENCY, DISEASE RESISTANCE, AND OTHER PRODUCTION TRAITS ARE TO BE MAINTAINED, THERE IS A CRITICAL NEED TO IDENTIFY HOST BEEF CATTLE GENETICS THAT ESTABLISHES AND INFLUENCES THE RUMEN MICROBIOME AND THE POTENTIAL HERITABILITY OF THESE MICROBES. THE OVERARCHING HYPOTHESIS OF THIS PROJECT IS THAT HOST BEEF CATTLE GENETICS ARE ASSOCIATED WITH THE VARIATION OF MICROBES IN THE RUMEN, PRODUCING AN INDIVIDUALIZED RUMEN MICROBIOTA AMONG ANIMALS. TO TEST THE OVERARCHING HYPOTHESIS OF THIS PROJECT AND DETERMINE ITS IMPACT ON FEED EFFICIENCY, THE SPECIFIC OBJECTIVES INCLUDE, 1) DETERMINE THE MICROBES AND MICROBIAL INTERACTIONS IN THE RUMEN OF ANGUS CATTLE AND THEIR RELATION TO FEED EFFICIENCY,2) ESTIMATE THE HERITABILITY OF THE RUMEN MICROBES AND MICROBIAL FEATURES,AND 3) IDENTIFY HOST GENOMIC MARKERS FOR RUMEN MICROBES AND MICROBIAL FEATURES. AT THE COMPLETION OF THIS PROJECT, WE EXPECT TO HAVE IDENTIFIED HERITABLE RUMEN MICROBES AND MICROBIAL FEATURES THAT ARE IMPORTANT FOR FEED EFFICIENT BEEF CATTLE. ALTHOUGH THE STUDY WILL FOUNDATIONALLY DEFINE A HOST GENOMIC LINK TO HERITABLE RUMEN MICROBES, THIS PROJECT WILL ULTIMATELY, A) PROVIDE A STEPPINGSTONE TOWARD MICROBIOME MANIPULATION AND A MEANS TO INCREASE FOOD AVAILABILITY WHILE LOWERING ENVIRONMENTAL IMPACTS,B) DEVELOP MORE SUSTAINABLE COW-CALF PRODUCTION SYSTEMS,AND C) ENHANCE ANGUS BREEDING PROGRAMS,GENOMIC PREDICTIONS, AND US AGRICULTURE. THE PROJECT WILL DRAMATICALLY ADVANCE THE FIELD OF BEEF PRODUCTION AGRICULTURE TO SUSTAINABLY MEET THE PROTEIN REQUIREMENTS OF AN EVER-INCREASING GLOBAL POPULATION