DECLINING FERTILITY IN DAIRY CATTLE IS A SERIOUS THREAT TO THE PROFITABILITY OF DAIRY CATTLE. THE HOLSTEIN CATTLE ACCOUNT FOR 90% OF U.S. DAIRY POPULATION BUT HAVE THE MOST SERIOUS FERTILITY PROBLEMS AMONG ALL DAIRY BREEDS. FERTILITY IS ONE OF THE PHENOTYPES THAT BENEFITS MOST FROM CROSSBREEDING, BUT CROSSBRED COWS HAD A LOSS IN MILK REVENUE OF $206 PER COW PER YEAR RELATIVE TO HOLSTEIN COWS ACCORDING TO OUR OWN ANALYSIS. WE HYPOTHESIZE THAT THE UNDERSTANDING WHY CROSSBREEDING RESULTS IN REPRODUCTIVE SUPERIORITY WILL LEAD TO NEW METHOD TO IMPROVE HOLSTEIN'S REPRODUCTIVE PERFORMANCE WHILE MAINTAINING HOLSTEIN'S COMPETITIVE ADVANTAGE IN MILK PRODUCTION. THE OVERALL OBJECTIVE IS TO DISCOVER AND UTILIZE GENETIC VARIANTS AND MECHANISMS UNDERLYING DAIRY REPRODUCTIVE HETEROSIS FOR GENETIC IMPROVEMENT OF REPRODUCTION IN PUREBRED DAIRY BREEDS PARTICULARLY IN HOLSTEIN CATTLE. THIS PROJECT WILL FILL A KNOWLEDGE GAP IN REPRODUCTIVE HETEROSIS AND PROVIDES NECESSARY KNOWLEDGE FOR DEVELOPING NEW STRATEGIES TO IMPROVE HOLSTEIN FERTILITY WHILE MAINTAINING HOLSTEIN'S COMPETITIVE ADVANTAGE IN MILK PRODUCTION, AND IS EXPECTED TO HAVE A SIGNIFICANT IMPACT ON GENETIC IMPROVEMENT OF REPRODUCTION FOR HOLSTEIN CATTLE.