Thoroughbred breeding theories

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.[2]

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.[3]

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.[7] 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.[8]

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.[13]

Lowe stated,

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.[14]

During the 1950s Kaziemierz Bobinski and Count Zamoyski produced Family Tables of Racehorses,[15] 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.[16] 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.[17] The current female family tables were updated by Toru Shirai of the Japanese Bloodstock Agency with the latest update occurring in 2004.

Old Bald Peg, dam of the Old Morocco Mare (c.1655)

Thoroughbred families include the following:

• Families 1-43 are described by Bruce Lowe's Breeding Racehorses by the Figure System
• Families A1-A37 descend from Sanders Bruce's American Stud Book, with mares who cannot be traced to Weatherbys General Stud Book (GSB)
• Families Ar1-Ar2 are Argentine families
• Families B1-B26 trace directly to F.M. Prior's Half-Bred Studbook
• Families C1-C16 are described in the Australian Stud Book as approved Colonial Families
• Families C17-C33 descend from Australian and New Zealand mares who cannot be traced to the GSB
• Families P1-P2 are Polish families

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.[18]

The mare Penelope (foaled 1798) produced two Derby winners, and established family 1-o, a branch of family 1. Her female line descendants established further branches over the following decades

The overall structure of family 1 is as follows:[19]

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.[20] 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.[21]