STUDIES INVESTIGATING THE GUT MICROBIOME HAVE DEMONSTRATED THAT HOST GENOTYPE INFLUENCES MICROBIAL SPECIES COMPOSITION. HOWEVER, THE RELATIONSHIP BETWEEN RUMEN MICROBIOTA FUNCTION, HOST GENOTYPE AND HOW SUCH RELATIONSHIPS EFFECT FEED EFFICIENCY IN THE RUMINANT ANIMAL IS POORLY UNDERSTOOD. THE FUNCTIONAL GENE PROFILE OF THE RUMEN MICROBIAL COMMUNITY CAN BE VIEWED AS A COMPLEX TRAIT THAT IS INFLUENCED BY HOST-GENETICS AND ENVIRONMENTAL FACTORS. ONE OF THE FACTORS THAT HAVE YET TO BE EXPLORED WITH RELATION TO FEED EFFICIENCY IS THE INTERACTION BETWEEN HOST GENETICS AND THE RUMEN MICROBIOTA FUNCTION. CONSEQUENTLY, THE GOAL OF THIS APPLICATION IS TO IMPROVE THE EFFICIENCY IN BEEF CATTLE BY INVESTIGATING THE INTERACTION BETWEEN HOST GENETICS AND RUMEN MICROBIOTA FUNCTION. TO THIS END, WE WILL PURSUE THE FOLLOWING OBJECTIVES: 1) ANALYZE A LARGE BEEF CATTLE POPULATION (N=750) AND CHARACTERIZE THE RUMEN MICROBIAL METABOLIC GENES, SEARCHING FOR FUNCTIONAL FEATURES AND PATHWAYS OF THE MICROBIOMETHAT MAY INFLUENCE FEED EFFICIENCY; 2) TEST FUNCTIONAL GENES OF THE RUMEN MICROBIOTA ASSOCIATED WITH FEED EFFICIENCY FOR GENOME-WIDE ASSOCIATIONS ON ANIMALS IMPUTED TO 770K SNP GENOTYPES TO DEVELOP A SUBSET OF SNPS THAT CAN BE USED FOR QTL DETECTION. THIS PROJECT WILL RESULT IN NEW GENETIC MARKERS BASED ON RUMEN FUNCTION FOR MARKER ASSISTED SELECTION TO IMPROVE FEED EFFICIENCY, IDENTIFY TECHNOLOGIES THAT PROMOTE FEED EFFICIENCY BY IDENTIFYING MICROBIAL GENES AND PATHWAYS TO USE AS TARGETS FOR ISOLATING DIRECT FED MICROBIALS.