FOR BEEF PRODUCERS, FERTILITY IS A LIMITING FACTOR CONTROLLING THE PRODUCTIVITY, PROFITABILITY ANDSUSTAINABILITY OF THEIR LIVELIHOOD. WE HAVE DETERMINED THAT THE FORM OF SUPPLEMENTAL SELENIUM (SE) PROVIDED TO THE DIET OF BEEF CATTLE TO ENSURE A SE-ADEQUATE STATUS CAN BE MANIPULATED TO INCREASE SYSTEMIC LEVELS OF PROGESTERONE (P4) ANDFERTILITY. OUROVERALL GOAL(S) ARE TO DETERMINE THE CELLULAR MECHANISM(S) BY WHICH THIS OCCURS. OUR FIRST OBJECTIVE WILL DETERMINE HOW THE FORMOF SE PROVIDED TO COWS AFFECTS THE PRODUCTION OF PROGESTERONE (P4) BY THE OVARY, ANDCATABOLISM OF P4 BY THE LIVER. OUR SECOND OBJECTIVE WILL DETERMINE HOW THESE FORM OF SE-INDUCED CHANGES IN P4 AFFECT UTERINE FUNCTION ANDDEVELOPMENT OF THE EMBRYO.OUR RESULTS WILL ALLOW FORM OF SE-SPECIFIC RECOMENDATIONS TO BE MADE TO THE CATTLE PRODUCER TO INCREASE THE FERTILITY OF THEIR HERDSAND THE PRODUCTIVITY/PROFITABILITY OF THEIR BEEF CATTLE OPERATIONS.