EFFICIENT REPRODUCTION IS KEY TO PROFITABLE DAIRY OPERATIONS, IT REDUCES COSTS FOR DAYS OPEN, INEFFICIENT CULLING, AND PREGNANCY LOSSES. ALTHOUGH SOME IMPROVEMENT CAN BE ACHIEVED THROUGH MANAGEMENT, THIS RESEARCH IS FOCUSED ON LONG-LASTING EFFECTS IN THE HOLSTEIN BREED BY IMPROVING THEIR GENETICS FOR FERTILITY. IN HOLSTEIN CATTLE, UP TO 50% OF PREGNANCY LOSSES OCCUR IN THE FIRST 5-7 POST-INSEMINATION, CONCOMITANT WITH PREIMPLANTATION EMBRYONIC DEVELOPMENT AND INDICATING THAT SUPERIOR REPRODUCTIVE PERFORMANCE OF HIGH FERTILITY FEMALES STARTS FROM A DEVELOPMENTALLY COMPETENT EMBRYO. FEMALES SELECTED FOR HIGH FERTILITY HAVE FASTER ONSET AND LONGER ESTRUS, HIGHER ESTRADIOL CONCENTRATIONS, AND ARE MORE LIKELY TO BECOME PREGNANT AFTER THE FIRST INSEMINATION, HOWEVER, LITTLE IS KNOWN ABOUT THEIR OOCYTE AND EMBRYONIC PHENOTYPES. THE LONG-TERM GOAL OF THIS RESEARCH IS TO IMPROVE THE RELIABILITY OF REPRODUCTIVE TRAITS, BY USING ACCURATE PHENOTYPES TO ESTIMATE FERTILITY. THE CENTRAL HYPOTHESIS OF THIS PROPOSAL IS, MUCH OF THE DETRIMENTS AND IMPROVEMENTS IN FERTILITY IN LACTATING HOLSTEIN CATTLE OCCUR PRIOR TO DAY 7 OF PREGNANCY. THIS WILL BE ADDRESSED IN THE FOLLOWING SPECIFIC AIMS: 1) DETERMINE HOW GENETIC MERIT FOR FERTILITY (DPR) AFFECTS EMBRYO QUALITY AT DAY 7 IN SUPER-OVULATED HOLSTEIN COWS, AND 2) DETERMINE HOW GENETIC MERIT FOR FERTILITY (DPR) AFFECTS OOCYTE COMPETENCE AND SUBSEQUENT EMBRYONIC DEVELOPMENT IN VITRO. THE OVERALL OBJECTIVE OF THIS PROPOSAL IS TO DELINEATE MECHANISMS INVOLVED IN PREIMPLANTATION EMBRYONIC DEVELOPMENT AND THEIR ASSOCIATION WITH GENETIC MERIT FOR FERTILITY. THE SUCCESSFUL COMPLETION OF THIS RESEARCH IS EXPECTED TO PROVIDE FOUNDATIONAL INFORMATION THAT WILL BE CRUCIAL TO ENHANCE SELECTION FOR REPRODUCTION.