Thoroughbred is a horse breed best known as a race horse. While carefully bred racehorses had existed throughout Europe for centuries prior to this time, the breed as it is known today developed during the 17th century in England when English mares began to be bred to imported Arabian stallions. This addition of verifiable Arabian blood coincided with the creation of the General Stud Book of England and the practice of official registering of horses. Today all modern Thoroughbreds trace to these imported stallions.
Some individuals mistakenly refer to a purebred horse of any breed, or any other type of purebred animal as a "thoroughbred." However, this is incorrect usage: the Thoroughbred is a distinct breed of horse. The proper term for any horse or any other animal that is pedigreed and of a single breed is "purebred." While this distinction is not as widely understood outside the realm of horse breeding, where the terms are at times used interchangably, nonetheless, the term "thoroughbred" is not commonly used to describe purebred animals of other species.
The Thoroughbred stands typically from as small as 15.2 to as large as 17.0 hands (64 inches/1.63 m) high and is usually bay, "brown" (dark bay), chestnut, black, or gray. Less common colors include roan and palomino. White is very rare, but is a recognized color separate from gray. The face and lower legs may be marked with white, but white will generally not appear on the body (although certain color genes, possibly the rabicano or sabino genes, result in white hairs and white patches in the coat—the study of equine coat color genetics is complex). Good quality Thoroughbreds have a well chiseled head on a long neck, high withers, a deep chest, a short back, good depth of hindquarters, a lean body, and long legs.
Thoroughbreds are often crossed with horses of other breeds to add speed and refinement. Thoroughbreds are classified among the "hot-blooded" breeds, animals bred for agility and speed, generally considered spirited and bold.
Unlike most registered breeds today, a horse cannot be registered as a Thoroughbred (with the Jockey Club registry) unless it is conceived by "live cover;" that is, by the witnessed natural mating of a mare and a stallion. Artificial insemination (AI), though legal and commonly utilized in other horse breeds, cannot be used with Thoroughbreds. Originally this was because blood typing and DNA testing had not yet developed to a degree adequate to verify parentage. Today the reasons may be more economic: a stallion has a limited number of mares who can be serviced by live cover. Thus, the practice prevents an oversupply of Thoroughbreds to some extent. (Though modern management still allows a stallion to live cover more mares in a season than once was thought possible.) By allowing a stallion to only cover a couple hundred mares a year rather than the couple thousand possible with AI, it also preserves the high prices paid for horses of the finest or most popular lineages.
All modern Thoroughbreds carry the genetics of three stallions imported to England from the Middle East in the late 17th and early 18th centuries: the Darley Arabian, to whom 95% of today's Thoroughbred pedigrees trace, the Godolphin Arabian, also known as the Godolphin Barb (Because this horse was born in Morocco, there is some dispute among historians whether this horse was a true Arabian or a Barb. However, based on paintings from life, the stallion was clearly Arabian in type, a Barb is built differently), and the Byerly Turk (who may have been a Turkoman Horse rather than an Arabian), together with around 35 mares. There are also other horses of oriental breeding that have been less of an influence but are still noteworthy. One of those is the Alcock Arabian, thought to be largely responsible for the gray coat color in Thoroughbreds. Others include the Unknown Arabian, the Helmsley Turk, the Lister Turk and Darcy's Chestnut.
The first Thoroughbred horse in the American Colonies was Bulle Rock, imported by Samuel Gist of Hanover County, Virginia, in 1730.
Maryland and Virginia were the centers of Colonial Thoroughbred breeding, along with South Carolina and New York.
Although the Thoroughbred is primarily bred for racing, the breed is also used for show jumping and combined training due to its athleticism, and many retired, retrained race horses become fine family riding horses, dressage horses, and youth show horses. The larger horses are sought after for hunter/jumper and dressage competitions, whereas the smaller horses are in demand as polo ponies.
Thoroughbred horse race
The Thoroughbred is bred primarily for racing under saddle at the gallop. Some families of Thoroughbreds are known primarily as sprinters or as distance runners.
Although buyers generally prefer to buy larger individuals, many great racehorses and great stallions have been average or small in size. While Man o' War and Secretariat were famous horses over 16 hands, a significant number of excellent race horses were average to small. These include Northern Dancer (15.1 hands), Hyperion (15.1), Seabiscuit (15.2), and Aristides, who despite standing barely 15 hands was winner of the first Kentucky Derby in 1875.
Many experts who purchase Thoroughbreds attempt to assess a young horse's potential by observing its overall structural balance, the athleticism and willingness of its walk, the perceived intelligence of its outlook, and the correct conformation of its legs. Buyers of more expensive horses often hire veterinary experts to examine and report on the condition of the horse's breathing apparatus, soundness of bone structure, and size of heart.
Thoroughbreds that are born in the Northern Hemisphere technically become a year older on January 1 each year; those born in the Southern Hemisphere turn one year older on August 1 annually. These artificial dates have been set to enable the standardization of races for horses in certain age groups.
About 35,000 Thoroughbred foals are registered each year in the U.S. The largest number of foals are born in Kentucky, Florida, and California. The Thoroughbred industry is a large agribusiness. It supports tens of thousands of jobs in each of these states, from jockeys, trainers, starters, grooms, and kitchen employees at the race tracks, to farm hands assisting with birth and rearing, grooms of yearlings, feed growers, veterinarians, drivers, auctioneers, and employees of racing-related companies developing products such as newly designed saddles and synthetic turf to reduce incidence of serious injury to horses and people who work with them. Parimutuel gambling on races also provides not only purse money to participants but considerable state tax revenue.
The Thoroughbred in other disciplines
Naturally athletic, with a generally strong work ethic, the Thoroughbred excels in many equestrian sports. While other breeds are currently more popular than the Thoroughbred in dressage and show jumping, certain individuals of the breed are competitive at the top levels. Flowing, long gaits, good jumping form and the ability to go with speed makes the Thoroughbred a top show hunter as well.
The Thoroughbred mare Touch of Class helped win the show jumping gold medal for the United States Equestrian team at the 1976 Summer Olympics, and the Anglo-Arabians on the French dressage team helped earn that nation a bronze medal at the 1936 Olympics.
Of all the equestrian sports, the Thoroughbred is probably most suited for eventing, and dominates the highest levels: almost all Olympic or World Championship horses are full or part-Thoroughbred. The breed is most suited for the cross-country phase, due to its long stride, natural speed and stamina, as well as its athletic jump.
Thoroughbreds are also the most common breed for use in polo. They are seen in the fox hunting field as well.
Although Thoroughbreds are seen in the hunter-jumper world and other purposes, modern Thoroughbreds are primarily bred for speed and racehorses have a very high rate of accidents. This has created a number of concerns and significant controversies.
Current estimates indicate that there are 1.5 career-ending breakdowns for every 1000 horses starting a race in the United States, an average of two horses per day. The state of California reported a particularly high rate of injury; 3.5 per 1000 starts. As a ratio (of injuries with eventually fatal complications to total competitions), this is far in excess of all other legal human and animal sports, including boxing, motorsports and greyhound racing.
Selective breeding theory
One argument for the high accident rate amongst racing Thoroughbreds suggests that selective breeding is the culprit. It is suggested that capability for speed is enhanced in an already swift animal by raising muscle mass, a form of selective breeding that has created animals designed to win horse races Thus, goes the theory, the modern Thoroughbred travels faster than its skeletal structure can support. As a result, all competitive modern Thoroughbreds are muscularly powerful yet osteologically delicate creatures, significantly more so than any equid, fossil or living, found in the wild. "We have selectively bred for speeds that the anatomy of the horse cannot always cope with."
Excess stress theory
The accident rate may also occur because Thoroughbreds, particularly in the United States are first raced as 2-year-olds, well before they are completely mature. Though they may appear full-grown and are in superb muscular condition, their bones are not fully formed. Other theories suggest that track surfaces, shoes with toe grabs, use of certain legal medications, and too intense a racing schedule may also contribute.
One of the most promising trends is the development of synthetic surfaces for racetracks. One of the first tracks to install such a surface, Turfway Park in Florence, Kentucky, saw its rate of fatal breakdowns drop from 24 in 2004-2005 to three in the year following Polytrack installation. The material is not perfected, with some areas reporting problems related to winter weather, but studies are continuing.
Difficulties treating injured horses
The level of treatment given to injured Thoroughbreds is often more intensive than for horses of less financial value, but also controversial, due in part to the significant challenges in treating broken bones and other major leg injuries.
Leg injuries that are not immediately fatal still may be life-threatening because a horse's weight must be distributed evenly on all four legs to prevent circulatory problems, laminitis and other infections. If a horse loses the use of one leg temporarily, there is the risk that other legs will break down during the recovery period because they are carrying an abnormal weight load. A horse cannot simply lie down in the equivalent of a human's "bed rest"—an animal of this size will literally crush the internal organs when lying supine for extended periods. If a horse loses the use of one leg for a long period, its other legs will ultimately break down as well, with euthanasia the only possible outcome.
Whenever a racing accident severely injures a well-known horse, such as the case of Kentucky Derby winner Barbaro breaking his leg during the 2006 Preakness Stakes, animal rights groups tend to target the thoroughbred racing industry. The bioethics are seldom clean-cut, however. While thoroughbreds can be delicate and horse racing is hazardous, veterinary science is also developing, so that previously hopeless cases can now be treated successfully.  Thoroughbreds are arguably as much helped as harmed by the racing industry. Research in veterinary medicine that benefits not only Thoroughbreds but all horses is largely funded and driven by the horse racing industry. If horse racing did not occur, advocates argue that there would be far less funding and incentives to pursue medical and biomechanical research on horses.
The Thoroughbred in breeding
The Thoroughbred remains one of the most important breeds used in modern horse breeding. They have been incredibly influential on many of the favorite breeds of today, including the American Quarter Horse, the Morgan (a breed that went on to influence many of the gaited breeds in America), the Standardbred, and others. Along with the Arabian, the Thoroughbred continues to be a favorite as an improver of breeds. This is most notable in the Warmblood breeds, which occasionally infuse the hotter, leaner Thoroughbred blood when needed.
Favorite crosses to the Thoroughbred includes breeding with an Arabian to produce the Anglo-Arabian (which has a special registry of its own within the Arabian Horse Association), as well as with the Irish Draught to produce the Irish Horse