RECENT REGULATORY CHANGES AND SHIFTS IN CONSUMER PREFERENCE HAVE LED TO AN INCREASE IN ANTIBIOTIC-FREE AGRICULTURAL PRODUCTION, INCLUDING THAT OF MEAT-TYPE (BROILER) CHICKENS. THE ABSENCE OF ANTIBIOTICS CAN NEGATIVELY IMPACT BIRD GROWTH PERFORMANCE, ULTIMATELY LEADING TO REDUCED PROFIT MARGINS FOR POULTRY PRODUCERS AND INCREASED COST OF POULTRY PRODUCTS FOR CONSUMERS. ALTERNATIVE STRATEGIES TO THE USE OF ANTIBIOTICS THAT MAINTAIN BIRD GROWTH PERFORMANCE MUST BE DEVELOPED. FOR THIS TO OCCUR, IT IS NECESSARY TO IDENTIFY BIOLOGICAL MECHANISMS THAT CONTROL METABOLIC EFFICIENCY AND DETERMINE HOW THESE MECHANISMS ARE INFLUENCED BY ANTIBIOTICS AND SUBOPTIMAL CONDITIONS THAT CAN OCCUR IN PRODUCTION SYSTEMS SUCH AS HIGH ENVIRONMENTAL TEMPERATURE. OUR OVERALL HYPOTHESIS IS THAT BIOLOGICAL MECHANISMS REGULATING METABOLISM ARE CLOSELY INTEGRATED WITH THE INTESTINAL MICROBIOME, AND THAT UNDERSTANDING THIS INTEGRATION WILL FACILITATE DEVELOPMENT OF INNOVATIVE STRATEGIES AIMED AT ENHANCING THE PRODUCTION EFFICIENCY OF MODERN BROILERS. THIS PROJECT WILL EVALUATE BIOLOGICAL SYSTEMS ASSOCIATED WITH BIRDS THAT EXHIBIT EITHER HIGH OR LOW FEED CONVERSION RATIO, A COMMON MEASURE OF EFFICIENCY OF FEED NUTRIENT USE IN ANIMAL AGRICULTURE. THE MECHANISMS TO BE INVESTIGATED ARE ASSOCIATED WITH ACTION OF HORMONES REGULATING GROWTH AND METABOLISM, GENES REGULATING NUTRIENT UTILIZATION AND INTESTINAL INFLAMMATION, AND THE COMPOSITION AND METABOLISM OF THE INTESTINAL MICROBIAL POPULATION. THE PROJECT WILL ALSO EVALUATE HOW THESE SAME MECHANISMS ARE INFLUENCED BY HEAT STRESS, BOTH IN THE ABSENCE AND PRESENCE OF ANTIBIOTICS. IT IS EXPECTED THAT BIOLOGICAL SYSTEMS ASSOCIATED WITH METABOLIC EFFICIENCY WILL BE IDENTIFIED, AND THESE SYSTEMS WILL INCLUDE THOSE ASSOCIATED WITH HORMONES, NUTRIENT UPTAKE AND UTILIZATION, AND THE INTESTINAL MICROBIOME. THE IMPACT OF DIETARY ANTIBIOTICS AND HIGH ENVIRONMENTAL TEMPERATURES ON THESE SYSTEMS WILL ALSO BE DETERMINED. TOGETHER, RESULTS FROM THIS PROJECT WILL REVEAL TARGETS THAT CAN BE USED FOR DEVELOPMENT OF NOVEL STRATEGIES TO MAINTAIN OR IMPROVE EFFICIENCY OF MEAT PRODUCTION IN ANTIBIOTIC-FREE SYSTEMS. SUCCESSFUL DEVELOPMENT AND IMPLEMENTATION OF SUCH STRATEGIES WOULD SUBSTANTIALLY IMPROVE PROFIT MARGINS FOR BROILER PRODUCERS BECAUSE FEED ACCOUNTS FOR OVER 70% OF THEIR PRODUCTION COST. AS IMPROVEMENTS IN FEED CONVERSION ARE LIKELY REFLECTIVE OF SHIFTS IN NUTRIENT USE THAT FAVOR MUSCLE ACCRETION OVER FAT DEPOSITION, SUCH STRATEGIES WOULD ALSO ENHANCE MEAT PRODUCTION, AND THIS IMPROVED PRODUCTION EFFICIENCY WOULD BE PASSED ON TO THE CONSUMER IN THE FORM OF A REDUCED COST FOR POULTRY PRODUCTS.