Verification of a Pregnancy by the First Mated Stallion in Successive Estrus Cycles After Stallion Change by DNA Parentage Testing

DNA parentage testing using microsatellite markers confirmed that a Thoroughbred foal conceived from a mare that was bred with two different stallions at an interval of 22 days in successive normal estrus cycles was not sired by the last mated stallion. The results of the stallions were confirmed by retesting DNA parentage test and video recording during the mating sessions. Clinical examinations excluded the absence of an embryo in the uterus after the first mating but confirmed the presence of twin embryos after the last mating, despite confirmation of only one ovulation. One of the twin embryos was crushed by hand, and the other embryo was normally delivered at 332 days after the last mating (354 days after the first mating). Furthermore, to identify similar cases in Thoroughbred mares, we retrospectively reviewed all breeding records over the previous 16 years in Japan and identified three similar cases among 7,787 foaled mares by stallion change. Because the mating interval was approximately one estrus cycle in these cases, pregnancy seemed unlikely to have resulted from prolonged survival of the sperm. Embryonic diapause characterized by delayed implantation of an embryo after the first mating should be strongly considered for proper identification in Thoroughbred lines.