Thoroughbred breeding theories, or racehorse 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.
Breed the best to the best 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
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.
Stallion statistics 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
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