Investigation of the estrous status in Thoroughbred mares coming to a stud farm for breeding

In the Thoroughbred industry, artificial insemination is prohibited. Veterinarians examine mares to determine the time to breed; however, some mares refuse breeding at the stud farm. Estrous status was investigated at breeding in 12,388 mares by measuring blood progesterone (P4) concentrations, and conception rates between mares in estrus and in the luteal phase were compared. Mares that refused breeding were examined by rectal palpation and P4 measurement to identify the causes of non-receptiveness. Mares with high P4 concentrations (≥2.0 ng/ml) showed lower conception rates, and 124 mares of the 177 mares that could not be bred had high P4. Measuring P4 concentrations is useful for evaluating mare estrous status and contributes to the safety of live cover in Thoroughbreds.