Repeat matings of a pairing that has proven successful seems like mere common sense. The 17th Earl of Derby, one of the most influential breeders of all time, not only followed up proven matings in this fashion but advocated the position that if one believed in a mating enough to try it at all, one should repeat it twice more before moving on. On the other hand, some breeders voice a fear of “going to the well too often” and are reluctant to repeat a mating more than once.

Which is the better strategy for producing good runners?

Anecdotal evidence is rich on both sides. For instance, California matron Cee’s Song threw multiple grade II winner Budroyale as her second foal by Cee’s Tizzy. Her subsequent seven foals by the same sire included 2000 Horse of the Year Tiznow, grade II winner Tizdubai, and grade II-placed stakes winner Tizbud; by comparison, her five foals by other sires (including three by two-time leading sire Storm Cat) yielded only four winners, one of them stakes-placed.

On the other hand, 1937 Triple Crown winner War Admiral had five full sisters that managed to win just one race among them. Native Dancer also had five full siblings, of which only one was a winner. Citation had three full siblings, of which one won. The list of high-profile failures that were full siblings to a top horse is long and seemingly endless.

In truth, of course, the odds are stacked against a good runner being duplicated by a sibling. On average, full siblings share just 50% of their genes, meaning that siblings can vary widely in genetic potential. Add to this the influence of environmental factors—training, nutrition, general handling, early experiences, and so on—and it’s easy to see why more full siblings to good horses fail than make names for themselves.

Still, common sense would indicate that full siblings to a good horse have a better chance of being good runners themselves than half siblings, all other factors being equal and the respective sires being of roughly equal merit. As a test of this hypothesis, consider the produce records of the dams of horses named Horse of the Year since official championships began in 1936 up through 2004. (As many repeat matings are done on the basis of an early foal’s success, the cut-off year was chosen to exclude mares whose produce records are not yet close to complete.) While this is not a random sample of the Thoroughbred population by any means, or even a random sample of all good runners, the resulting pool of horses may still yield some useful information.

Collectively, the dams of the 60 champions studied were an outstanding bunch of broodmares. In total, they produced 554 named foals of racing age, of which 340 (61.4%) were winners and 87 (15.7%) were stakes winners. Those are excellent figures, and the percentage of winners would probably be even higher were it not for owners’ tendency to retire untalented sisters and half sisters to good horses to the breeding shed after just a few races rather than drop them into the claiming ranks where they might have a realistic chance to win.

Sixteen of the champions studied had no full siblings; the remaining 46 champs had 120 full siblings among them. (Interestingly, 36 of these elite runners were the firstborn among their sets of full sibs, which is natural if most repeat matings are made because of the production of a top runner or at least a very promising foal.) By contrast, there were 434 half siblings to champions in the studied group. But if these results are any indication, perhaps breeders should have tried repeat matings more often. Of the half siblings, 261 (60.1%) won and 59 (13.6%) became stakes winners. But the full siblings yielded 79 winners (65.8%) and 28 stakes winners (23.3%).

The proverbial lightning seldom strikes twice, and the odds a truly great horse will be duplicated in a sibling are slim even under the best of circumstances. But at least from the evidence presented here, repeating a successful mating is a good way of substantially improving the odds of getting a good horse, and that’s all any breeder can reasonably ask.