100 years of Derby data: inbreeding sire line x broodmare sire line

By Sid Fernando

Inbreeding is a viable part of thoroughbred breeding that has been practiced from the earliest days of the breed’s inception (when it was unavoidable) to the present day, where the duplication of ancestors is mostly unavoidable beyond 5-generations and frequently planned within and beyond 5 generations. Within the parameters of 5-generations, a common practice these days is to inbreed to the sire line with the broodmare sire line. An example of this would be to breed a Mr. Prospector-line mare to a Mr. Prospector-line sire, which happens frequently to stallions such as Mr. Prospector and Northern Dancer (and Nasrullah and Bold Ruler, etc.) because they were such dominant sires of sires. With so many sons and daughters at stud, much of this type of inbreeding would happen randomly if it were not planned, anyway.

One of the most extreme—and successful—cases of this type of inbreeding took place in France in 1946, when Coronation was foaled. Celebrated French breeder and iconoclast Marcel Boussac planned Coronation’s pedigree by breeding the Tourbillon mare Esmeralda to the Tourbilllon sire Djebel, which resulted in Coronation being inbred 2×2 to Tourbillon. The filly won the French Guineas and Arc among other races.

The closest case of inbreeding I’m aware of in the US involving a high-class sire was a mare that was inbred 1×2—yes, that’s correct!—to Mr. Prospector, named Winloc’s Millie. I’ll spare you the details, but needless to say Seth Hancock, the master of Claiborne, was not pleased that a shareholder in the sire decided to follow that course. Winloc’s Millie was unplaced in 4 starts, and to date she has not been a successful producer, either.

The closeness of the inbreeding, for this particular variation of inbreeding, is something that bears monitoring, especially for racing in this country. At WTC, for example, we’ve found that inbreeding to Mr.Prospector on the sire x broodmare sire cross has worked at generational distances of 3×3 or more, but has not been successful at less. Indeed, there are no unrestricted SWs inbred this way to Mr. Prospector at 3×2 or less. (There have been three G3 winners at 2×3.)

Fact: From 1910 to 2009, there have been only four Kentucky Derby winners inbred to the sire line with the broodmare sire line. That’s 4 of 100, or 4 percent. The most recent was last year’s winner, Mine That Bird, who is inbred 5×3 to Mr. Prospector. Mine That Bird is by Birdstone (by Grindstone, by Unbridled, by Fappiano, by Mr. Prospector) out of a mare by Smart Strike (by Mr.Prospector). The 2008 winner Big Brown is inbred 3×3 to Northern Dancer (by Boundary, by Danzig by Northern Dancer out of a mare by Nureyev, by Northern Dancer). Winning Colors, the 1988 Derby winner, was inbred 4×4 to Nasrullah, and before her the only other instance of this variety of inbreeding was in the pedigree of 1951 Kentucky Derby winner Count Turf, who was inbred 3×3 to the brothers Sunreigh and Sun Briar (both by Sundridge from Sweet Briar).

Statistically, then, inbreeding to the sire line with the broodmare sire line has not been historically successful in the Derby, but it’s been trending that way based on the pedigrees of the last two winners. However, not a single Derby winner has been inbred this way at a degree closer than 3×3. Proceed at your own risk!

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