Thoroughbred breeding theories are used by horse breeders in an attempt to arrange matings that produce progeny successful in horse racing. Bloodstock experts also rely on these theories when purchasing young horses or breeding stock. A basic understanding of these theories can also help the racing public understand a horse's theoretical genetic potential. The breeding theories stem from the belief that careful analysis of bloodlines can lend predictability to breeding outcomes. A well-designed mating increases the probability of the offspring's success, although many other factors also come into play.
Many thoroughbred breeding theories are implemented from other animal breeding stock practices, such as the use of inbreeding to "fix a type". Some breeding theories are qualitative, relying on judgement. Quantitative breeding theories usually focus on statistical analysis of the sire and broodmare sires in particular. The best-known classification system for mares was developed in the late 1800s by an Australian named Bruce Lowe, who analyzed the statistics of major race winners and ranked the distaff or mare lines by their degree of success. This and similar ranking systems are still used by some breeders today.
The soundest breeding theory is the simplest one: "Breed the best to the best and hope for the best" is a phrase that probably originated with John E. Madden in the first half of the twentieth century. Studies have proven that, in general, good racehorses make the best breeding stock. While not all top male runners succeed as stallions, it is much more unusual for a poor racehorse to become a good proven sire. A poorly-raced mare does have a better chance to outbreed her own record on the track, especially when she has a good pedigree and is bred to good stock. On the other hand, statistics have shown that high quality racemares produce an inordinately high percent of high class runners.
The Racecourse Test means that the most important selection criteria for breeding the Thoroughbred is the ability displayed on the racetrack. The Racecourse Test measures a horse's ability to win, which requires a certain combination of speed and stamina depending on the race in question. Racing also tests the horse's strength, soundness and will to win, all of which are heritable to some degree. The ultimate goal is to win elite races, especially The Derby in England or the Kentucky Derby in the United States, and the breed has evolved accordingly. Horses that fail the Racecourse Test, either because they are poor athletes or lack racing spirit, are usually poor candidates as breeding stock. When an unproven racehorse becomes a good sire or broodmare, a further look usually shows that he or she showed tremendous potential in training and was retired due to some untimely circumstance, usually an injury. Such horses usually also have above average pedigrees.
An example of the danger of ignoring the Racecourse Test was Sunday Silence, who firmly established his ability on the racetrack as the 1989 Horse of the Year and thus would ordinarily have been given major opportunities as a sire. But his pedigree was not fashionable and there was little interest in America in breeding to him. Thus he was sold to Japan, where he became the leading sire for thirteen straight years.
Because stallions can have hundreds of offspring, it is possible to do statistical analysis to measure their success, which in turn affects their future prospects at stud. The most commonly used is the Average Earnings Index (AEI), which takes the average earnings of all runners for a specific period and evaluates the stallion's average progeny earnings against that figure. A number below 1.00 is below average. A number above 1.00 is above average. Good stallions generally have an AEI of at least 1.50.The advantage of looking at the AEI compared to straight earnings is that the AEI compensates for differences in the number of runners different stallions may have. The AEI can also be used to compare the relative success of sires over different generations, though larger foal crops in recent years have led to a general decrease in the AEI of the best modern sires compared to those of the past.
For context, the AEI should be accompanied by the Comparative Index (CI), which is a way of measuring the quality of the mares to which the stallion was bred. The CI considers all the offspring of the stallion's mates and subtracts out their combined progeny. A stallion with a CI of 2.19 means that when his mares were mated with other sires, the mares produced offspring that averaged 2.19 times the average for the generation in question. If a stallion's AEI is lower than his mares' CI, it is generally a red flag as it suggests that he's not generating the same quality of runners as these mares have produced with other stallions.By contrast, if the stallion's AEI is higher than his mares' CI, the stallion is said to be improving his mares. This generally leads to higher quality mares being bred to him in future years.
Another statistic that is valuable to both the bettor and breeder is the Average Winning Distance (AWD), available through the Online Stallion Register maintained by The Blood-Horse and published in some Past Performance charts.By comparing the values for horses in a given race, a bettor can identify which horses have a more speed oriented pedigree, and which have a more stamina oriented pedigree. From a breeding point of view, stallions with a low AWD number are considered to be speed influences, and may be bred to mares whose broodmare sire has a higher number to inject stamina.
Dosage is a further attempt to quantify the amount of speed versus stamina in a horse's pedigree. Major sires, called chefs-de-race, are placed in one of five categories: Brilliant, Intermediate, Classic, Solid, and Professional. "Brilliant" sires offer the highest speed and least stamina, while "Professional" sires (now quite rare) provide the lowest speed and greatest stamina. A "Classic" sire offers a balance of speed and stamina traditionally associated with winning classic races. For any given horse, the dosage profile is generated by assigning points for each Chef-de-race in the pedigree, with the number of points varying depending on what generation the chef appears in. The Dosage Index can then be calculated, with a higher number meaning the pedigree is more speed-oriented. A Dosage Index of under 4.00 is considered optimal for horses attempting to win classic races.As the Dosage Profile and Index are widely published, breeders may select mates for their mare with these figures in mind, especially if they are intending to sell the foal at auction. As with Average Winning Distance, if the mare has a Dosage Profile oriented to speed, the breeder may look for a sire that provides stamina influences. The resultant foal would then have a more optimal Dosage Profile.
Although much attention is paid to the sire line, Thoroughbred horses are also traced through the distaff line, alternately called the mare line or tail-female line. This maternal line is known as a "family". This practice dates to the beginning of the General Stud Book (GSB). This was done because the mares produce far fewer foals than stallions and many leading breeders maintained and built families all tracing to a single mare. However, modern genetic studies have revealed that there are some cases where the haplotype in the mtDNA of modern Thoroughbreds, which should not mutate or alter, differs from the records in the General Stud Book, indicating that some female families contain deep rooted pedigree errors.
Many horses were inbred in the early years of Thoroughbred development, which increased the chances of early horses appearing in many pedigrees today.One example was Old Bald Peg placed in family 6, one of the earliest tap-root dams, having been foaled around 1635. Most, if not all modern Thoroughbreds trace their ancestry to her through one or both sides of their pedigree.
Around 1895 an Australian, Bruce Lowe, wrote: “Breeding Racehorses by the Figure System”. He formulated a system of family numbers from the mares listed in the General Stud Book. Lowe believed that the three foundation sires of the Thoroughbred were successful largely due to the mares they were bred to, and so predicting race horse quality required identification and assessment of the mare lines.
The figures are derived from a statistical compilation of the winners of the three great English classic races, Derby, Oaks and St. Leger. The family with the largest number of wins is No. 1, the next No. 2 and so on up to No. 43, and include families whose descendants have not won a classic race.
During the 1950s Kaziemierz Bobinski and Count Zamoyski produced Family Tables of Racehorses,commonly known as the Bobinski Tables. This work expanded Bruce Lowe's numbering system of 43 families and identified a total of 74 families tracing to mares in the GSB. They identified mares in several countries whose pedigrees had been lost or whose descendants were unacceptable to the GSB at the time of Lowe’s work. Bobinski later updated his works and split Lowe's families into sub categories. The current female family tables were updated by Toru Shirai of the Japanese Bloodstock Agency with the latest update occurring in 2004.
Over time, the influence of the foundation mares of each family becomes diluted by the presence of all the other horses in a given pedigree. On the other hand, some of the younger mares of the family become so influential that a new family is created to separate out their offspring. For example, Family 1 refers to all horses who trace back in the female line to Tregonwell's Natural Barb Mare, foaled c. 1670. It is estimated that about 15% of modern thoroughbreds belong to family 1 as a whole. Five generations of the family later, the mare Bonny Lass (1723) was so influential that a new family 1-a was designated for her descendants. Family 1-a was subsequently split into families 1-d (Promise, 1768) and 1-b (Morel, 1805). The Promise family 1-d spawned further families and so on to the present, where there are now 22 designated branches (1-a to 1-x) within the overall family 1. It takes many generations for the most influential mares to be identified so most of these family branches date back to the 18th and 19th century. The most recent branch was created for descendants of La Troienne (1926), family 1-x.
The overall structure of family 1 is as follows:
Today, these numbers often follow a horse’s name in sale catalogues and pedigrees, much like a numerical surname and are used for checking the accuracy of pedigrees and comparing the contributions made by various mares and families.Horses that come from more highly respected families will usually command better prices than those from less respected bloodlines, although they may not prove to be better as racehorses or sires/broodmares.
Specific affinities of stallions of one male line for mares from other sire lines—commonly called Nicks—have made a profound impact on the development of the Thoroughbred. Compatibility of stallions from one male line with mares from other sire lines has shaped the breed since the cross of Eclipse with mares by Herod in the late 18th century. These successful crosses–Hermit/Stockwell, Lexington/Glencoe, Bend Or/Macaroni, Phalaris/Chaucer–have made a profound impact on the development of the Thoroughbred.While modern calculations of nick ratings use more complete databases, there has been criticism of nicks and nick ratings within the Thoroughbred industry.
Inbreeding is the mating of two closely related individuals. It is known as one of the quickest ways to "fix" desired characteristics into a bloodline of any species. The risk is that the inbreeding will reveal negative recessive characteristics; thus, "closebreeding" is usually avoided. Continued inbreeding over a series of generations also has a negative impact referred to as "inbreeding depression." The lines may become dominant for certain characteristics, but the offspring also tend to become weaker, less vigorous individuals than animals that are not as inbred.
In the thoroughbred industry, inbreeding is used to focus specific genes by using superior, prepotent individuals, usually within the fourth and sixth generations. Inbred animals are likely conduits for certain specific characteristics coming from their inbred ancestor. Too much inbreeding is not desirable and rarely produces the superior runner. But inbred animals frequently make outstanding breeding stock because when outcrossed, superior hybrid individuals often result. Secretariat is an example of such hybrid vigor.
Horse breeding is reproduction in horses, and particularly the human-directed process of selective breeding of animals, particularly purebred horses of a given breed. Planned matings can be used to produce specifically desired characteristics in domesticated horses. Furthermore, modern breeding management and technologies can increase the rate of conception, a healthy pregnancy, and successful foaling.
Eclipse was an undefeated 18th-century British Thoroughbred racehorse who won 18 races, including 11 King's Plates. After retiring from racing he became a very successful sire and today appears in the pedigree of most modern Thoroughbreds.
Potoooooooo or variations of Pot-8-Os was an 18th-century thoroughbred racehorse who won over 30 races and defeated some of the greatest racehorses of the time. He went on to be a sire. He is now best known for the unusual spelling of his name, pronounced Potatoes.
Messenger was an English Thoroughbred stallion bred by Richard Grosvenor, and imported into the newly-formed United States of America just after the American Revolution.
La Troienne (1926–1954) was one of the most famous and influential Thoroughbred broodmares in twentieth century America. She produced 10 winners including two Hall of Fame inductees while at stud, while her daughters in turn produced many notable offspring. In 2000, pedigree expert Janeen Oliver designated her as the taproot of family 1-x, a designation that was implemented by the Pedigree Online Thoroughbred Database in 2003. Recent matrilineal descendants include 2003 Horse of the Year Mineshaft and Kentucky Derby winners Smarty Jones (2004) and Super Saver (2010).
Somethingroyal was an American Thoroughbred racehorse best known as the dam of U.S. Triple Crown champion and Hall of Fame inductee Secretariat. She also produced three other stakes winners and was named the 1973 Kentucky Broodmare of the Year.
Flying Childers was a famous undefeated 18th century thoroughbred racehorse, foaled in 1714 at Carr House, Warmsworth, Doncaster, and is often cited as the first truly great racehorse in the history of thoroughbreds and the first to catch the public imagination.
The Thoroughbred is a horse breed best known for its use in horse racing. Although the word thoroughbred is sometimes used to refer to any breed of purebred horse, it technically refers only to the Thoroughbred breed. Thoroughbreds are considered "hot-blooded" horses that are known for their agility, speed, and spirit.
Tapit is an American Thoroughbred racehorse. Out of the Unbridled mare Tap Your Heels, he was sired by Pulpit. He had a modest racing career, winning three of six total races, but became the leading sire in North America in 2014, setting an earnings record that he broke in 2015 and again in 2016. For the 2015 season, his stud fee was raised to $300,000, the highest in North America.
Eleanor was a British Thoroughbred racehorse bred by Charles Bunbury and was the first female horse to win The Derby. Eleanor also won the 1801 Epsom Oaks among many other races before retiring from racing at age eight to become a broodmare for Bunbury. She produced the stallion Muley, which in turn sired the mare Marpessa and the influential stallion Leviathan which was exported to the United States in the early nineteenth century. Through the produce of her daughter Active, Eleanor is present in the pedigrees of 19th-century American Standardbred racehorses.
Noble was a British Thoroughbred racehorse and sire. In a career that lasted from May 1786 to May 1788 he ran at least three times and won two races. He won the seventh running of The Derby as a 30/1 outsider in what was probably his first race. His only other success came at Newmarket later that year. He was retired to stud where he stood as a stallion for several years but made little impact as a sire of winners.
Bartlett's Childers was an important Thoroughbred sire in the 18th century.
Rowena was a British Thoroughbred racehorse and broodmare that won the classic 1000 Guineas at Newmarket in 1820. On her only other appearance, she finished second in the Oaks Stakes. She later became a successful broodmare.
Cobweb (1821–1848) was an undefeated British Thoroughbred racehorse and who won two British Classic Races as a three-year-old and went on to become a highly successful broodmare. Cobweb's racing career consisted of three competitive races in the early part of 1824. After winning on her debut she claimed a second prize when her opponents were withdrawn by their owners. She then won the 1000 Guineas at Newmarket Racecourse and the Oaks Stakes at Epsom Downs Racecourse before being retired to stud.
Spilletta was a British Thoroughbred racehorse. She only raced once and is best known for being the dam of the undefeated Eclipse.
Coronation was a French racehorse. In a racing career which lasted from the spring of 1948 until October 1950, she ran thirteen times and won six races. As a two-year-old she was one of the best fillies of her generation in Europe, winning the Queen Mary Stakes in England and the Prix Robert Papin in France. In the following year she dead-heated for the Poule d'Essai des Pouliches, but was beaten in both the Oaks Stakes and the Irish Oaks. In October 1949 she established her reputation as one of the best fillies to race in Europe in the 20th century when she emphatically defeated a strong international field in the Prix de l'Arc de Triomphe. Her subsequent racing career was disappointing and she was a complete failure as a broodmare.
Imperatrix was a British Thoroughbred racehorse. She raced only twice, with her only win coming in the 1782 St. Leger Stakes. She was bred and owned by John Pratt. As a broodmare, Imperatrix produced nine foals.
What a Pleasure (1965-1983) was a Thoroughbred stallion bred at Claiborne Farm in Kentucky for the Wheatley Stables of Glady Phipps. Sired by the leading stallion Bold Ruler, and out of a Mahmoud mare Grey Flight, he excelled on the track and in his stud career. Like his sire, he would go on to become a leading North American stallion; producing Eclipse Award winners and Kentucky Derby winners.
Lady Angela (1944–1966) was a British-bred Thoroughbred who became the foundation mare of E.P. Taylor's Windfields Farm in Canada. She is most notable as the dam of Nearctic, the Canadian Horse of the Year in 1958 and seven-time leading sire in Canada. Nearctic's most famous offspring was the great Northern Dancer, a champion in both Canada and the United States and subsequently a leading sire in both North America and Europe. For her worldwide influence on the breed, Lady Angela was inducted into the Canadian Horse Racing Hall of Fame in 2010.
Australian was a British-bred Thoroughbred racehorse and sire. He was exported to the United States where he had modest success as a racehorse but became a very successful and influential breeding stallion.