REPRODUCTIVE PROBLEMS AND LOW MILK PRODUCTION ARE THE TOP CAUSES FOR PREMATURE REMOVAL OF COWS FROM DAIRY HERDS. ONE OF THE MAIN FACTORS LEADING TO THESE ISSUES IS POOR NUTRIENT PARTITIONING DURING LACTATION, LEADING TO INCREASED ADIPOSITY AND SUBSEQUENT METABOLIC DISORDERS AT NEXT CALVING. BECAUSE RAISING REPLACEMENT HEIFERS FOR REMOVED COWS IS BOTH ECONOMICALLY AND ENVIRONMENTALLY COSTLY, ENSURING EXCELLENT PERFORMANCE AND LIFETIME PRODUCTIVITY FROM COWS ALREADY IN THE HERD IS CRITICAL. ULTIMATELY, OUTCOMES IN ADVANCING THE UNDERSTANDING OF DAIRY COW NUTRIENT PARTITIONING FROM THIS FUNDAMENTAL RESEARCH PROJECT WILL SERVE TO IMPROVE FARM EFFICIENCY, PROFITABILITY, AND SUSTAINABILITY.MANIPULATION OF THE SEROTONERGIC SYSTEM MAY IMPROVE BOTH THE IMMEDIATE FEED-TO-MILK PRODUCTIVE EFFICIENCY, AS WELL AS LIFETIME PRODUCTIVE CAPACITY OF DAIRY COWS, BY INCREASING NUTRIENT PARTITIONING TOWARDS THE MAMMARY GLANDS AND LIMITING ADIPOSITY DURING LACTATION. THE SEROTONIN PRECURSOR 5-HTP IS RAPIDLYCONVERTED INTO SEROTONIN BY SEROTONIN-PRODUCING TISSUES WHEN INTRAVENOUSLY INFUSED INTO LACTATING DAIRY COWS, ENABLING USE OF 5-HTP FOR IN VIVO STUDY OF THE SEROTONERGIC SYSTEM.WE HYPOTHESIZE THAT INTRAVENOUS INFUSION OF 5-HTP WILL ALTER GLUCOSE-STIMULATED INSULIN SECRETION, HEPATIC AND ADIPOSE METABOLISM, MAMMARY BLOOD FLOW AND NUTRIENT UTILIZATION, AND MILK PRODUCTION IN LACTATING DAIRY COWS. TO TEST THIS HYPOTHESIS, SPECIFIC OBJECTIVES FOR THIS PROJECT ARE TO:DETERMINE THE EFFECT OF INTRAVENOUS 5-HTP ON GLUCOSE-STIMULATED INSULIN SECRETION, THROUGH ADMINISTRATION AND ANALYSIS OF A GLUCOSE TOLERANCE TEST;INVESTIGATE THE EFFECT OF INTRAVENOUS 5-HTP ON ADIPOSE, HEPATIC, AND MAMMARY EXPRESSION OF GENES INVOLVED IN KEY ENERGETIC PATHWAYS, THROUGH ANALYSIS OF BIOPSIES TAKEN FROM THOSE TISSUES; ANDEVALUATE THE EFFECT OF INTRAVENOUS 5-HTP ON BLOOD METABOLITES, MAMMARY BLOOD FLOW, NUTRIENT FLUX, UPTAKE, AND PRODUCTION OF MILK COMPONENTS, THROUGH ANALYSIS OF BLOOD AND MILK SAMPLES.