**AWARDS ISSUED PRIOR TO JANUARY 20, 2025, WERE FUNDED UNDER PREVIOUS ADMINISTRATIONS AND MAY NOT REFLECT THE PRIORITIES AND POLICIES OF THE CURRENT ADMINISTRATION.** THIS 2-YEAR PREDOCTORAL RESEARCH PROPOSAL IS DESIGNED TO UNDERSTAND THE PHYSIOLOGICAL MECHANISMS OF RUMEN UNDEGRADABLE PROTEIN (RUP) ON REPRODUCTIVE PARAMETERS IN BEEF HEIFERS. OUR HYPOTHESIS IS THAT HEIFERS SUPPLEMENTED ANIMAL SOURCED 50% RUP WILL HAVE GREATER ANTRAL FOLLICLES, GREATER NUMBER OF CUMULUS OOCYTE COMPLEXES RECOVERED, HAVE OPTIMAL UTERINE HISTOTROPH, AND IMPROVED EMBRYO QUALITY IN COMPARISON TO PLANT SOURCED RUP. OBJECTIVES INCLUDE 1) EVALUATE SOURCE AND LEVEL OF RUP SUPPLEMENTATION ON OVARIAN DEVELOPMENT, OOCYTE COMPETENCE, AND EMBRYO DEVELOPMENT IN HEIFERS DEVELOPED UTILIZING DRY-LOT ENVIRONMENTS, 2) DETERMINE THE INFLUENCE OF SOURCE AND LEVEL OF RUP SUPPLEMENTATION ON UTERINE HISTOTROPH AND EMBRYO DEVELOPMENT IN HEIFERS DEVELOPED ON DRY-LOT SETTINGS, AND 3) EVALUATE SUPPLEMENTING DIFFERENT SOURCES AND LEVELS OF RUP ON OOCYTE COMPETENCE AND EMBRYO DEVELOPMENT IN HEIFERS GRAZING NATIVE RANGELAND. THE PROPOSED RESEARCH WILL IMPROVE OUR UNDERSTANDING OF THE PHYSIOLOGICALMECHANISMS RUP HAS ON REPRODUCTIVE PARAMETERS. THIS PROPOSAL INVESTIGATES RESEARCH RELATED TO THE NIFA PRIORITY AREA OF ANIMAL HEALTH AND PRODUCTION, SPECIFICALLY ANIMAL PRODUCTION. THROUGH THE COMPLETION OF THIS PROPOSED RESEARCH WE WILL PROVIDE IMPROVED RECOMMENDATIONS IN HEIFER DEVELOPMENT PROGRAMS TO STAKEHOLDERS TO IMPROVE REPRODUCTIVE EFFICIENCY AND DAM LONGEVITY, THEREFORE MAKING BEEF OPERATIONS MORE ECONOMICALLY EFFICIENT.