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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!

8 thoughts on “100 years of Derby data: inbreeding sire line x broodmare sire line”

  1. Sid,

    Thank you for the article on inbreeding. I found it interesting that Burchard Von Oettingen reached a similar conclusion in his book “Horse Breeding In Theory And Practice,” published 1909 by Sampson Low, Marston & Co.

    “…There has been also much damaged by exaggerated inbreeding in the American Thoroughbred breeding. Among the many instances of 0 free generation I have not found a single one in the American Stud Book which has been of importance to breeding. I have only found one good racehorse in the American Breeding, and that is Henry 1819 by Sir Archy-Doimed (i.e. 0 Diomed), celebrated by his famous match with the American Eclipse in New York, 1823.” – page 233

  2. Calvin, thanks for that. It’s ironic that his comments in 1909 begin one year before my look at the Derby from 1910 to 2009.

  3. Interesting. Do you have any data on how many horses have attempted the feat? Or what percentage of the Thoroughbred population is inbred in this manner? If only 4% of Thoroughbreds are inbred sire line x broodmare sire line then this data doesn’t mean much. If 20% are then perhaps it becomes more meaningful. Then we also have to wrestle with the issue of sample size. These are all reasons why I have always disliked the traditional use of Derby history as a predictor of the future. These are all independent running with countless independent variables. They simply do not offer a proper forum for “scientific” study.

  4. Roger Lyons is going to get me a statistically meaningful sample tonight, extracting data from the 2009 September sale. However, my little “study” covering 100 years of Derby data was meant as nothing more than a cautionary tale; after all, there’s been only 1 Derby winner inbred as close as 3×3 sire line x broodmare sire line, so if you were looking at a Derby prospect in a yearling sale inbred, say, 3×2 to Mr. Prospector in this fashion, or perhaps planning a classic mating the same way, perhaps you’d need to think twice? Or at least do more research.

    FYI: This post was actually inspired by a pedigree guru who is planning a “Derby horse” for a client that will be inbred 3×2 to Mr.P!

  5. Sid,

    Yes, I did notice that – I thought you would find it interesting. Again, thanks for the article.

  6. sid,

    interesting piece leaves me in a quandary about the subject i had with a knowledgeable friend. i read (and tested myself by random selection of such, weekly for a year) the theory that no more than 20-25% of graded stakes winners were a result of inbreeding within 4 generations. My study results were consistent with that. So i posed the question to my friend: why inbreed if 3 out of 4 occurences fail to produce a graded SW. His reply: Why not if you can expect a graded SW 25% of the time! What’s your take on this?

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