Thoroughbred - Registration, Breeding, and Population

Registration, Breeding, and Population

About 37,000 Thoroughbred foals are registered each year in North America, with the largest numbers being registered in the states of Kentucky, Florida and California. Australia is the second largest producer of Thoroughbreds in the world with almost 30,000 broodmares producing about 18,250 foals annually. Britain produces about 5,000 foals a year, and worldwide, there are more than 195,000 active broodmares, or females being used for breeding, and 118,000 newly registered foals in 2006 alone. The Thoroughbred industry is a large agribusiness, generating around $34 billion in revenue annually in the United States and providing about 470,000 jobs through a network of farms, training centers and race tracks.

Unlike a significant number of registered breeds today, a horse cannot be registered as a Thoroughbred (with The Jockey Club registry) 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 AI, and although DNA and blood testing eliminate many of those concerns, AI still requires more detailed record keeping. The main reason, however, may be economic; a stallion has a limited number of mares who can be serviced by live cover. Thus the practice prevents an oversupply of Thoroughbreds, although modern management still allows a stallion to live cover more mares in a season than was once thought possible. As an example, in 2008, the Australian stallion Encosta De Lago covered 227 mares. By allowing a stallion to cover only a couple of hundred mares a year rather than the couple of thousand possible with AI, it also preserves the high prices paid for horses of the finest or most popular lineages.

Concern exists that the closed stud book and tightly regulated population of the Thoroughbred is at risk of loss of genetic diversity because of the level of inadvertent inbreeding inevitable in such a small population. According to one study, 78% of alleles in the current population can be traced to 30 foundation animals, 27 of which are male. Ten foundation mares account for 72% of maternal (tail-female) lineages, and, as noted above, one stallion appears in 95% of tail male lineages. Thoroughbred pedigrees are generally traced through the maternal line, called the distaff line. The line that a horse comes from will often determine the price paid regardless of the actual talent or potential of the horse.