The Thoroughbred race horse was established on one singular strategy, running fast. There have been many ways suggested to reach that same end point, some of which make perfect sense and others less so. Here’s a quick survey of Thoroughbred breeding strategies and theories. 

The old adage “Breed the best to the best and hope for the best,” has the right spirit, but it’s a little vague about what constitutes “the best.” Does that mean that only the fastest racehorses should be bred? What about the unraced full sister to Tapit (USA) or Frankel  (GB) or Zenyatta (USA)? What it should really mean is that a breeder should try to be a little selective about breeding stock and use a horse for its redeeming qualities and not just because it has functional reproductive organs or a famous ancestor.

Probably the first breeding strategy was the recognition of “nicks,” instances when one bloodline works remarkably well with another.  These fortuitous crosses have guided, and sometimes misguided breeders for generations.  Bend Or worked with Macaroni mares, Fair Play with Rock Sand, Phalaris with Chaucer, Nasrullah with Princequillo, Sadler’s Wells with Darshaan. The reasons why a good nick works can be mysterious, but it seems to occur when both bloodlines are of the highest quality and very complementary.  For instance, the modern nick of Galileo (IRE) on mares by Danehill (USA) created the great runner Frankel (GB). A closer look at the best runners bred on this cross shows that most of them are out of high class sprinting daughters of Danehill (USA).  Since Galileo (IRE) is a pre-eminent sire of classic stamina, it makes sense that he would cross well with quality mares that add more innate speed from the champion sprinter Danehill (USA). In America, the Pioneer of The Nile (USA)/Storm Cat (USA) nick is looking fierce, producing the 2015 Triple Crown winner American Pharoah (USA) and 2016 Champion 2-Year-Old Classic Empire, out of mares by Yankee Gentleman (USA) and Cat Thief (USA) respectively, both sons of the brilliant Storm Cat (USA).  Pioneerof the Nile is by Empire Maker, whose son Bodemeister (USA), also out of a Storm Cat (USA) mare, sired the recent G1 Kentucky Derby winner Always Dreaming (USA). Empire Maker (USA) is by Unbridled (USA), who also sired Unbridled’s Song (USA), and they all have a history of doing well with Storm Cat (USA)-line mares. There are 2 statistically-based nick rating systems that evaluate any given cross based on its frequency in producing good runners, eNicks and TrueNicks. Surprisingly, if one asks for a TrueNicks rating for the mating that produced American Pharoah (USA), the result is a disappointing C+. The mating for Frankel (GB) is an A+. 

Taking Frankel (GB) as an example again, we can illustrate inbreeding, another principle that may help explain his greatness.  Frankel’s sire Galileo (IRE) is by Sadler’s Wells (USA), a great sire by an even greater sire, Northern Dancer (CAN). Frankel’s dam is by Danehill (USA), a son of another great sire, Danzig (USA), also by Northern Dancer (CAN). This makes Frankel (GB) inbred 3x4 (in his 3rd generation and 4th generation) to Northern Dancer (CAN). A lot of horses, good and bad, have been inbred to Northern Dancer (CAN) this closely, but something obviously clicked in the production of Frankel (GB). Inbreeding is a strategy used to “fix” a type, by doubling up on a desirable ancestor, and concentrating those good genes. Inbreeding in Thoroughbreds is usually not as intense as in other domestic species, where it’s not unusual to have a daughter bred to her father or brother to maximize the effect. 

In Thoroughbreds, duplications within 4 generations appear in about 25% of stakes winners. This means that 75% are not inbred, instead being what we refer to as outcrossed. Outcrossing happens when mating a stallion and a mare who have no common relatives within 4 generations. It is done to produce offspring with a more diversified genetic background, which hopefully translates into physically superior, stronger offspring.  The concept is called heterosis or hybrid vigor.  That gives outcrosses the potential advantage over inbred horses. Most of the great racehorses of recent times have been outcrosses, about 3 to 1. Sea the Stars (IRE) is a prime example, as were Secretariat (USA), Nijinsky II (CAN) and Sea-Bird II (FR).  The problem with these outcrossed horses is that by the nature of their highly diversified genetic background, they can’t “breed true,” which means many of them are disappointing as stallions. 

Inbred horses are more likely to breed true, which is why an inbred super horse like Frankel (GB) makes an exciting stallion prospect.  He is engineered to be a top sire as well, and now he’s proving it, although to be fair, Sea the Stars (IRE) is doing well, too.Dosage is not actually a breeding theory, since it was originally developed as a gambling angle for the G1 Kentucky Derby. 

This modern version comes from an older theory developed by Franco Varola, who placed influential stallions in categories based on their speed or stamina and looked for a balance. Varola based his ideas on an even earlier theory developed by J. J. Vuilliers, which focused on the most important ancestors, stallions and mares, and determined the ideal “dosage” or percent of blood in which they were found in classic pedigrees at the time. The Aga Khan is said to use an updated model of Vuilliers’ theory, and his success may warrant a closer look at this misunderstood concept.The Large Heart Theory, also popularized as the X-Factor, supposes that superiority in heart size is passed along the female X chromosome and translated into greatness. This has been proven invalid by modern genetic research. While heart size has genetic factors, it is not associated with the X chromosome. Although humans are always trying to find easy answers, there are none in horse breeding. 

Even modern genetics offer no guarantees, and the eye of the horseman is still his most valuable tool.