“Unlike a significant number of registered breeds today, a horse cannot be registered as a Thoroughbred unless conceived by live cover, the witnessed natural mating of a mare and a stallion. Artificial insemination (AI) and embryo transfer (ET), though commonly used and allowable in many other horse breed registries, cannot be used with Thoroughbreds. One reason is that a greater possibility of error exists in assigning parentage with artificial insemination, and although DNA and blood testing eliminate many of those concerns, artificial insemination still requires more detailed record keeping.”
https://en.m.wikipedia.org/wiki/Thoroughbred
