HEAT STRESS (HS) AFFECTS US DAIRY HERDS OF ALL SIZES AND IN EVERY REGION OF THE COUNTRY COSTING US AGRICULTURE $1.7 BILLION ANNUALLY. IN DAIRY COWS, HS REDUCES MILK PRODUCTION EFFICIENCY BY AFFECTING MILK YIELD AND MILK COMPOSITION. MUCH OF THE LOSS IN MILK PRODUCTION IS DUE TO REDUCED FEED INTAKE. HOWEVER, REDUCED FEED INTAKE ONLY EXPLAINS ABOUT 50% OF THE OBSERVED DECREASE IN MILK PRODUCTION SUGGESTING THAT HS ITSELF AFFECTS MILK PRODUCTION. WE HYPOTHESIZE THAT HS CAUSES LEAKY GUT IN HIGH-PRODUCING DAIRY COWS RESULTING IN IMMUNE SYSTEM ACTIVATION THEREBY PARTITIONING NUTRIENTS AWAY FROM MILK PRODUCTION. FURTHERING THE EXPLANATION OF DECREASED MILK PRODUCTION DURING HS, WE HYPOTHESIZE THAT AMINO ACIDS (AA) SUPPLIED BY THE DIET AND BODY STORES DO NOT KEEP PACE WITH DEMAND FOR MILK PRODUCTION AND THE IMMUNE RESPONSE TO LEAKY GUT. USING A PAIR-FEEDING MODEL TO ADJUST FOR FEED INTAKE, HIGH-PRODUCING DAIRY COWS WILL BE USED IN EXPERIMENTS TO ADDRESS TWO SPECIFIC AIMS: 1) DETERMINE HOW HSAFFECTS GUT INTEGRITY, IMMUNE SYSTEM FUNCTION, AND WHOLE BODY AA UTILIZATION. 2) DEMONSTRATE THAT SUPPLEMENTATION OF SPECIFIC AA CAN AMEND NEGATIVE EFFECTS OF HS AND IMPROVE WHOLE BODY AA EFFICIENCY BY BALANCING THE EFFECTS OF BODY AA RELEASE, GUT INTEGRITY AND FUNCTION, IMMUNE SYSTEM REQUIREMENTS, AND MILK PRODUCTION REQUIREMENTS. THIS PROJECT WILL IMPROVE US AGRICULTURE THROUGH THE ENHANCEMENT OF AA UTILIZATION FOR MILK PRODUCTION IN HIGH-PRODUCING DAIRY CATTLE UNDER HS CONDITIONS, ALIGNING WELL WITH THE ANIMAL NUTRITION, GROWTH AND LACTATION (A1231) PROGRAM AREA.