**AWARDS ISSUED PRIOR TO JANUARY 20, 2025, WERE FUNDED UNDER PREVIOUS ADMINISTRATIONS AND MAY NOT REFLECT THE PRIORITIES AND POLICIES OF THE CURRENT ADMINISTRATION.** IN MANY PARTS OF THE UNITED STATES LEVELS OF SELENIUM ARE DEFICIENT IN THE SOILS. THIS TRANSLATES TO DEFICIENCIES OF THIS ESSENTIAL TRACE MINERAL IN THE FORAGES THAT SUSTAIN GRAZING CATTLE. PRODUCERS MUST THEREFORE SUPPLEMENT SELENIUM TO THEIR CATTLE TO ALLEVIATE THE NEGATIVE PHYSIOLOGICAL EFFECTS OF THIS DEFICIENCY. SELENIUM IS NORMALLY PROVIDED TO CATTLE IN AN INORGANIC FORM; HOWEVER, ORGANIC FORMS ARE CONSUMED WHEN CATTLE NATURALLY GRAZE FORAGE. PREVIOUS RESEARCH HAS DEMONSTRATED THAT COMPARED TO SUPPLEMENTING SELENIUM AS A PURELY INORGANIC FORM, PROVIDING SELENIUM AS A MIXED BLEND COMPOSED OF 50% INORGANIC AND 50% ORGANIC SELENIUM ADVANCES THE DEVELOPMENT OF THE CONCEPTUS AT MATERNAL RECOGNITION OF PREGNANCY, THE CRITICAL PERIOD WHEN THE EMBRYO SIGNALS TO THE MOTHER THAT IT IS PRESENT AND PREPARING TO IMPLANT WITHIN THE UTERUS. THIS TIMING IS CONCURRENT WITH CHANGES IN METABOLITES IN THE BLOOD AND TRANSCRIPTIONAL CHANGES IN THE ENDOMETRIUM SUGGESTING THAT THE MIXED FORM OF SELENIUM IS ALTERING AVAILABLE ENERGY AND METABOLITES THAT SUPPORT CONCEPTUS DEVELOPMENT. THIS STUDY WILL DETERMINE THE MECHANISTIC CONNECTION AMONG FORMS OF SELENIUM, METABOLIC CHANGES IN THE LIVER, AND ENERGY AVAILABILITY IN THE ENDOMETRIUM, OPERATING WITH THE HYPOTHESIS THAT THESE CHANGES WILL FAVORABLY FACILITATE AN ENVIRONMENT STIMULATING CONCEPTUS DEVELOPMENT AND VIABILITY. IT IS EXPECTED THAT THE PROVISION OF THE MIXED BLEND OF SELENIUM AS A NECESSARY SUPPLEMENT WILL MITIGATE THE EFFECTS OF OXIDATIVE STRESS IN THE LIVER AND ENDOMETRIUM RESULTING IN INCREASED AVAILABILITY OF METABOLITES AND LIPIDS FOR UTILIZATION BY THE ENDOMETRIUM AND SUBSEQUENTLY, THE ELONGATING CONCEPTUS. THIS STUDY WILL CONNECT THE METABOLIC CHANGES IN THE LIVER WITH THE PHYSIOLOGICAL RESPONSES IN THE ENDOMETRIUM WHEN SUPPLEMENTING SELENIUM AS A BLEND OF ORGANIC AND INORGANIC FORMS, COMPARED TO THE INDUSTRY STANDARD OF INORGANIC SELENIUM ALONE. IMPLICATIONS OF THIS RESEARCH ARE THE WIDESPREAD ADOPTION OF A DIFFERENTFORMULATION OF SUPPLEMENTAL SELENIUM THAT WILL IMPROVE EMBRYONIC GROWTH AND OVERALL FERTILITY IN COW/CALF OPERATIONS, AND HENCE THE PRODUCTIVITY AND SUSTAINABILITY OF THE BEEF INDUSTRY AS A WHOLE.