The Numbers Game

I recently discovered an external hard drive with scans from many books that are out of print or otherwise difficult to get. All of the books were found in Trinity College Dublin's various libraries as TCD is unique in that it has had a legal right to any book published in the United Kingdom and Ireland since 1801. Many of the books I scanned as an undergraduate were related to horse racing. Of these a number stand out, perhaps the most important is Sir Charles Leicester's Bloodstock Breeding as this is the foundational text in pedigree theory.

Chapter Nine, 'Brood Mares', deals with some heuristics regarding breeding. Notably some aspects regarding certain thoroughbred families that I believe Leicester probably knew at the time of writing are left out. Of the heuristics discussed, all deal with empirical evidence observed from race results in contrast to many of the qualitative beliefs that I believe are key to breeding success. A horse breeder's alpha is certainly in their ability to balance empirical facts with some suspicions regarding the temperaments and personalities associated with certain families.

Thoroughbred families are defined by the tail female line so mare selection dominates breeding discourse. Leicester uses the Bruce Lowe number system developed in the 1890s by Charles Bruce Lowe, an Australian accountant, with Family 1 the most successful in terms of English Classic wins¹. There are approximately 50 such Thoroughbred families.

Leicester considers several families but notably skips some of the most significant ones. Of the families he omits from detailed analysis: 2, 3, 4, 5, 7, 8, 11, 15 are worth careful study. These families produced important horses including Eclipse (Family 12), Nasrullah (Family 9), Ribot (Family 4), Stockwell, Tristan, Pretty Polly (Family 14), Bend Or and Gallinule (Family 19). Critically, several of these families have produced numerous Champion sires.

Leicester's study focuses on statistical analysis of genetic success in the following races: the 2000 Guineas, the Epsom Derby, the St Leger, the Ascot Gold Cup, the King George VI and Queen Elizabeth Stakes. These races range between one mile and two and a half miles, with some restricted to three-year-old colts and others open to both three-year-old colts and fillies. All races require horses to carry weights ranging from approximately 8 stone to 9 stone (roughly 50-57kg). These races comprise the richest and most prestigious in the English calendar.

Leicester's Analysis

Taking the breakdown of winners in these races, males have a 91.2% success rate with the conventional wisdom at the time of writing being that fewer fillies were entered as the races were seen as too testing.

Success Rate by Gender in Premier Races

Gender Success Rate Leicester's Interpretation
Males 91.2% Superior racing performance
Females 8.7% Critical for genetic inheritance

Leicester finds that the following families are most likely to produce winners of these five races:

Most successful families in Leicester's analysis:

Hard racing career data Leicester provides:

  • Kincsem: 54 wins from 54 starts, still became great producer
  • Beeswing: Raced 8 seasons, won 52 races including Ascot Gold Cup 4x
  • Pretty Polly: Won 22 of 24 races, produced Donatello II (Italian Derby)
  • Sceptre: Won 2000 Guineas, 1000 Guineas, Oaks, St. Leger in same year

One aspect of the book that has not been absorbed by the sport is that fillies are better producers when they race extensively rather than being kept off the track. Leicester documents that among the great brood mares with hard racing careers, there's a pattern of success continuing into the breeding shed. Of the 21 mares he lists as having extensive racing careers, virtually all became successful producers, with many founding important bloodlines. This contradicts the prevailing wisdom that racing damages a filly's breeding potential.

¹ The English Classic races are the 2000 Guineas, 1000 Guineas, Epsom Derby, Epsom Oaks, and St. Leger Stakes.