FEED IS A MAJOR EXPENSE THAT ACCOUNTS FOR 60-70% OF THE TOTAL COST OF PRODUCTION IN THE SWINE INDUSTRY. THEREFORE, IMPROVING FEED EFFICIENCY CAN HAVE A MAJOR IMPACT ON PROFITABILITY. STUDIES HAVE FOUND THAT THE REARING OF PIGLETS OUTDOORS DURING LACTATION IMPROVES FEED EFFICIENCY. HOWEVER, THIS REARING ENVIRONMENT IS NOT FEASIBLE FOR THE VAST MAJORITY OF PRODUCERS IN THE US, PARTIALLY DUE TO INCREASED SPACE REQUIREMENTS AND MORE COMPLEX MANAGEMENT PRACTICES NECESSARY FOR THE OUTDOOR SYSTEMS. WE HAVE DEVELOPED A NOVEL MODEL TO MIMIC THE OUTDOOR ENVIRONMENT IN A CONFINED INDOOR SETTING BY EXPOSING PIGLETS TO TOPSOIL DURING LACTATION, WHICH SIGNIFICANTLY INCREASES FEED EFFICIENCY. OUR OVERARCHING HYPOTHESIS IS THAT EARLY-LIFE TOPSOIL EXPOSURE IMPROVES FEED EFFICIENCY THROUGH MODULATION OF THE TRILLIONS OF MICROORGANISM IN THE GASTROINTESTINAL TRACT (I.E. GUT MICROBIOME), WHICH RESULTS IN INCREASED NUTRIENT DIGESTIBILITY AND ABSORPTION AND INCREASED MUSCLE AND ADIPOSE GROWTH. TO TEST OUR HYPOTHESIS WE PROPOSE TO: 1) DETERMINE THE EFFECTS OF EARLY-LIFE EXPOSURE TO TOPSOIL ON THE ESTABLISHMENT OF THE GUT MICROBIOME PRE-WEANING, THE SUBSEQUENT POST-WEANING EVOLUTION DURING NURSERY PHASE AND HOW CHANGES IN THE GUT MICROBIOME CORRELATE WITH FEED EFFICIENCY; 2) DETERMINE HOW CHANGES IN GUT (I.E. SMALL INTESTINE) MICROBIOME CORRELATE WITH GUT PHYSIOLOGY AND NUTRIENT DIGESTIBILITY, ABSORPTION AND AVAILABILITY; AND 3) ASSESS HOW CHANGES IN GUT MICROBIOME CORRELATE WITH MUSCLE AND ADIPOSE GROWTH. THIS STUDY WILL FILL CRITICAL GAPS IN THE FUNDAMENTAL KNOWLEDGE OF HOW CHANGES IN ENVIRONMENT AFFECT THE GUT MICROBIOME, WHICH IN TURN INCREASE NUTRIENT DIGESTION AND ABSORPTION, AND ULTIMATELY TISSUE ACCRETION AND FEED EFFICIENCY. THIS STUDY WILL ALSO PROVIDE NOVEL STRATEGIES (E.G. PROBIOTICS) AND/OR PRACTICE (EXPOSURE TO TOPSOIL) THAT WILL SIGNIFICANTLY INCREASE FEED EFFICIENCY.