You Learn Something New Every Day

I received the following comment on my post about racing class of dams from a little over a year ago.

“I’m quite convinced on your general refutation (or perhaps debunking) of close inbreeding generally. I do wonder if there is a case in which it might matter. Suppose you compare to individuals to have similarly elite racing class, one is an outcross and one is close inbreed. Do they differ in ability to produce foals of high class? This normalizes racing class and focuses on homogeneity. My hypothesis would be that the inbreeds outproduce but have no idea if that would be born out by the statistics.”

The above is a fair question, and I decided to explore it a bit. Of the 45,562 sales foals of 2008-2111, 6,659 were out of stakes winners. Of those 6,659 foals, only 150 (about 2.25%) were out of mares inbred 3×3 or closer. (The reader specified “close” inbreeding.)

The remaining 6,509 foals were out of mares not inbred 3×3 or closer. Call the former group DI (for dam inbred) and the latter group DNI (for dam not inbred). Below are the prices for the two groups.

Group          Foals          Average          Maverage          Price Index

DI                   150            $68,051            208.15                    1.35

DNI             6,509          $85,442            222.99                    1.45

Totals          6,659          $85,050           222.66                     1.45

As you can see, there is quite a bit of difference between the two groups in terms of prices. The DNI group was more expensive than the DI group ($85,442 to $68,051 by average, 222.99 to 208.15 by maverage, and 1.45 to 1.35 by Price Index).

Below are the racetrack results for the two groups. APPPSW stands for average Performance Points per stakes winner, a measure of the quality of stakes winners involved, with the benchmark now being 693.

Group          Foals          Stakes Winners          %          APPPSW          PPI (Result)

DI                   150                      11                       7.33            649                    2.04

DNI              6,509                  306                     4.70            849                     1.71

Totals           6,659                  317                      4.76            841                      1.71

The DI group was much better than the DNI group by percentage of stakes winners from foals (7.33% to 4.70%). But the DNI group was better than the DI group by APPPSW (849 to 649). Taking both quantity and quality of stakes winners into account, the DI group was decisively better than the NDI group (2.04 to 1.71) despite its lower prices.

So the DI group sold for prices about 35% above average and produced results about 104% above average (an improvement of 0.69). The NDI group sold for prices about 45% above average and produced results about 71% above average (an improvement of 0.26).

So, yes, the DI group was definitely better than the NDI group, lending credence to the theory that inbred (3×3 or closer) dams make better producers than dams not inbred that closely.

At least among dams who were stakes winners. Just for grins I also tracked foals out of mares inbred 3×3 or closer who were NOT stakes winners. There were 1,282 such foals, and their prices are summarized below.

Dams Inbred 3×3 or Closer

Category          Foals          Average          Maverage          Price Index

Not SWs          1,282          $43,608             145.71                    0.95

SWs                    150           $68,051             208.15                    1.35

Totals               1,432          $46,168             152.25                    0.99

The overall average for all 45,562 foals was $46,418, and the overall maverage was 154.0. The 1,432 foals out of mares inbred 3×3 or closer were just below those benchmarks by both average ($46,168) and maverage (152.25, which corresponds to a Price Index of 0.99). The prices for the 150 foals out of mares who were stakes winners and also inbred 3×3 or closer were significantly higher, of course.

Below are the results for the three groups.

Category          Foals          Stakes Winners          %          APPPSW          PPI (Result)

Not SWs          1,282                    34                      2.65%        719                      0.82

SWs                     150                     11                       7.33%        649                     2.04

Totals               1,432                    45                       3.14%        702                     0.94

The 1,432 foals (both stakes winners and not) sold for a price of 0.99 (about 1% below average) and achieved a result of 0.94 (about 6% below average). They were slight underachievers.

The 150 foals out of stakes winners sold for a price of 1.35 (about 35% above average) and achieved a result of 2.04 (about 104% above average). They were definite overachievers.

The 1,282 foals out of mares who were not stakes winners sold for a price of 0.95 (about 5% below average) and achieved a result of 0.82 (about 18% below average). They were more definite underachievers.

The point is that just because mares who were stakes winners and inbred 3×3 or closer achieved much better results than their prices warranted, be wary of theorizing too broadly from that. What works for stakes winners may not necessarily work for mares who were not stakes winners. In this case it did NOT work for mares who were NOT stakes winners. Even with the fantastic results from those 150 foals, the overall results for all 1,432 foals were still slightly negative.

The more important point could be that close inbreeding might indeed intensify the characteristics (both good and bad) of the mare in question. Inbreeding works both ways, in other words, intensifying both the good and the bad.

But it does appear that foals out of stakes winners who are inbred 3×3 or closer do outperform their prices rather significantly. I would not have believed it if I had not done all the research and number crunching myself. You learn something new every day.

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12 Responses to You Learn Something New Every Day

  1. jim culpepper says:

    Inbreeding doesn’t work for horses because it serves best to bring out undesirable recessives which are marked more plentiful than desirable, especially in populations which have not been culled by careful inbreeding over a number of generations. Presumably the influence of stakes winners indicates a serendipitous absence of deleterious recessives which permits the useful ones to be the greater influence as indicated by ancestors with class. Great post, I appreciate your work.

  2. Allison Roulston says:

    David, as you know as well as anyone, genuinely positive proof of inbreeding success has long proven to be highly elusive and scant at best. Congratulation on coming up with this welcome addition to the latter.

    I’ve always maintained that inbreeding should be practiced only through superior performing animals in an effort to filter out negative recessives. That’s why, despite a lack of evidence, I’ve half-heartedly clung to the notion that stakes-winning broodmares bearing the Rasmussen Factor might proved to be somewhat better producers.

    Do you have any information on the quality of the inbred components of the 150 mares in your project?

    Good work, as usual!

  3. Rob says:

    Really gratified to hear this amounted to something…

    I wonder if the outperformance diminishes (or increases) with relatively less inbreeding.. It certainly must at the limit, when the inbreeding is so remote it begins to resembble the general population. But I wonder is 4×4 better or worse than 3×3? Is 4 x 5 twice, e.g. two different ancestors, better, worse? Assuming a similar relationship holds with less inbreeding, you might get a large enough sample from which to see if there are any patterns, e.g., does the Mr. Prospector continue to be representative, was the lack of Northern Dancer a function of small sample (has to be, right?).

    Thanks again for digging into the original thought! I find your quantitative approach to these questions distinctively insightful.

    • ddink55 says:


      It is funny that you should inquire about 4×4 or closer. Because I originally tested your hypothesis at that level. The results were only very slightly in favor of the group with dams inbred.

      I am going to boil the numbers down to the basics here. The group with dams inbred 4×4 or closer posted a price of 1.45, a result of 1.79, and a difference of +0.34. The dams not inbred posted a price of 1.44, a result of 1.70, and a difference of +0.26. So the dams inbred group was only 0.08 better than the not inbred group.

      I looked at those numbers and wrote a post, but I was not really satisfied with it. It was just too inconclusive.

      Then I looked back at your original comment and saw that you specified “close” inbreeding. That led me to test it at the 3×3 or closer level, with the results you have seen, much more conclusive. Also one of the reasons it took me so long to respond.

      The numbers quoted above for dams inbred 4×4 or closer include the excellent results for dams inbred 3×3 or closer. Removing those 150 foals from the equation yield the following result.

      Now the group with dams inbred 4×4 or closer (but NOT 3×3 or closer) posted a price of 1.46, a result of 1.75, and a difference of +0.29. The numbers for the rest remain the same, a price of 1.44, a result of 1.70, and a difference of +0.26. So now the dams inbred group is only 0.03 better than the other group (0.29 to 0.26). The dams inbred group is still slightly better, but even less slightly better than before.

      I hope this answers at least some of your questions.


  4. Rob says:

    Do you happen to have handy the price index for the 12 di winners?

    • ddink55 says:

      2.02. Stakes winners are always more expensive than all foals. The important number is the Price Index for all 150 foals (1.35).


      • rob says:


        Not sure I totally understand. what I was trying to get at is if price as yearling for the 12 was materially higher than price as yearling for the remaining 148.

        I may need to go back and look at price index. Possible I am asking a non-sense question.

        What I am really trying to ascertain is if there are obvious .. meaning reflected in price .. characteristics of the 148 DI that did not perform. are there recessives that a good eye can see..

        Make sense? If not, I’ll try again

      • ddink55 says:

        Perhaps an analogy will help.

        A buys 20 foals (weanlings, yearlings, or two-year-olds) for a total of $1,000,000.

        B also buys 20 foals (ditto) for a total of $1,000,000.

        Both A and B are rewarded with one SW from among the 20 foals they bought. For the sake of argument let us say that the two SWs are of equal quality.

        A might think to himself, “I am smarter than B. My SW cost only $20,000. His SW cost $100,000.”

        Well, no, not really. A might be luckier than B, but not smarter.

        Both A and B shelled out $1,000,000 for 20 foals. Both were rewarded with one SW of equal quality. Both A and B performed the same (achieved the same results).

        That is what I men when I say that the prices of the SWs are not nearly as important as the collective prices for each group of 20 foals.


  5. Rob says:

    That makes sense

    I still wonder, within the 150 population of di, what is the r square of the price index/ PPi line

  6. Pingback: By a Safety in Overtime | Boojum's Bonanza

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