Monday, March 9, 2009

Line breeding, close breeding, inbreeding in sport horses

Wow, I just read a great article in the Fall 2002 issue of Holsteiner magazine: Linebreeding, Inbreeding, Close Breeding A primer covering the Tesio Method, the Rasmussen Factor, and other breeding theories. I anticipated a dry but instructive read, but it was actually fascinating. I'm going to share a few of the basics from the article, but I recommend you read it yourself if you're interested in the subject.

"Isn't that like marrying your cousin?"
Several years ago, when a friend first described line breeding to me, it seemed kind of creepy. If you haven't heard this term, the average person would probably think of it as inbreeding--when a sire or dam appears more than once within a pedigree. The goal of line breeding is to concentrate bloodlines to bring out desirable traits in the resulting offspring.

It's all about what genes influence the offspring. Geneticists say that a parent must carry 10% of a desired bloodline (e.g., Bold Ruler) in order for that blood to have any influence on the offspring. A son or
daughter will have 50% of a sire's blood, a grandson or granddaughter will have 25%, and a great grandson or great granddaughter will have 12.5%. Blood from the great-great grandsire would be less than 10% (no influence). Line breeding or inbreeding will increase the genetic influence of a bloodline. Here are the definitions of line breeding and related terms.

Line breeding
A horse is considered "line bred" or the product of line breeding when he or she has a common ancestor outside the fourth generation. According to the article, linebreeding "is duplicating key forbears in the fourth to sixth generation." An example of line breeding is the thoroughbred Fairway. If you look at his pedigree you'll see where horses are repeated. It isn't just the presence of the horses that matters, it's where they appear (sire or dam line, etc). Theories of line breeding can be very, very involved.

In the article Pedigree generation position, the author Kathleen Kirshan states that "the focal linebreeding should be in what many call the engine room, which is the 4th through 6th generation positions." If you do choose to read more, it helps to know there is a notation used to describe the pedigree in terms of line breeding: 4X4 indicates the same horse is presesnt in the fourth generation two times; 3X6 means the horse is present in the third and the sixth generation; and so on.

Inbreeding in the vernacular is a general term for what happens when you marry a relative. In horse breeding, it means duplicating an ancestor in the first four generations of an animal's pedigree. An example of inbreeding is found in the warmblood stallion Matcho.

Close breeding
Mating between brothers and sisters, or between parents and their off-spring, is referred to as close breeding. It is seldom done in real life breeding, although the spectacular jumper Rex Z is the product of a full brother/full sister mating. Also a few years ago I read in the USDF Connection that Hilda Gurney bred a daughter to hers sire. I believe this horse was a year end award winner -- but it's still creepy in my book. Close breeding is often used in genetic research to expose genetic weaknesses quickly.

Risks of inbreeding/close breeding
The same mechanism that strengthens the desirable traits also exposes weaknesses in the genetics of a bloodline. Too much inbreeding in labrador retrievers and german shepherds led to the problem of hip dysplasia. Inbreeding can cause runty, unhealthy, infertile, bad-tempered animals. Most breeders try hard to balance line breeding with outcrossing.

Secretariat was outcrossed, meaning he was the product of two unrelated strains. This results in what they call "hybrid vigor," and in the horse world it can produce super-horses like Secretariat. But while outcrossing is more likely to produce healthier more fertile, and stronger individual, it's less likely to produce a good sire. Because their genes are heterozygous it is harder to predict what their offspring will be. In contrast, line bred horses will have homozygous genes. There is less variaility in their genetic makeup so their offspring will more predictably inherit the traits of their sire or dam. A stallion that produces the same traits consistently is said to be prepotent. A prepotent stallion consistently passes on desirable characteristics to offspring. The trait can be speed, endurance, jumping talent, dressage gaits, or beauty/conformation, for example.

Takeaway message
While homozygous genes (from line breeding) make it easier to predict what the offspring will be like, heterozygous genes (from outcrossing) make for a healthier animal. Most breeders will agree that a pedigree should be balanced with respect to outcrossing and line breeding. Avoid extremes in either direction!

Reading pedigrees from The Horse

Chief Bearheart: The power of line breeding

Is sire line outcrossing possible any more? from the Blood Horse


  1. Very interesting about Secretariat and perhaps why he never seemed to produce another super horse from his breedings--nice horses with good conformation, but not superstars.

  2. My filly's grand-dam is all of the above:

  3. This was very interesting. Thank you for posting it.

  4. Secretariat's first test breeding was on a quarter horse mare whose son tore up the Quarter Horse racing tracks.

    Some traits in horse genetics tend to skip generations, like the Dr. Faeger granddaughters who were terrific producers of good running horses.

  5. Barbara, that's interesting! Do you happen to recall the name of the QH? Secretariat had some QH qualities, I seem to recall he had a fairly straight hind leg--not criticizing, but so many tbs have more angulation.

  6. Actually, Secretariat's first foal was an Appaloosa, First Secretary.

  7. The Arabians follow the tail female line and trace the family pedigree back through the mare side. Interestingly, the Bedouins value their mares more than their stallions. There are legends about the five foundation mares.

  8. Secretariat did produce great horses, fillies most of them. Secretariat's offspring can be explained through the x-factor which is passed by the mare AND the stallion. Think of terlingua and Weekend Surprise, two of secretariat's doughters and the offspring they produced (Storm Cat, AP Indy, Summer Squal)

  9. I was looking into a peedigree and seen the inbreeding...they bred a mare to her son...guessing try to preserve poco bueno blood...eeeks...i agree creepy...

  10. The great Arabian sire Raffles was by Skowronek out of daughter of Skowronek - father to daughter. Very common in Arabian breeding. Quiet American is probably the best example in modern Thoroughbred breeding. He is a son of Fappiano, whose dam is by Dr. Fager, out of a Dr. Fager daughter. Leo, a foundation sire of the Quarter Horse breed, was by Joe Reed 11, a son of Joe Reed; Leo's dam was Little Fanny by Joe Reed. I could cite many more examples - in Saddlebreds, Standardbreds, etc. I don't find it creepy at all. When done right it is very effective.

  11. Great article which reinforces the importance of striking a balance between line breeding and out crossing.

  12. A good article but not completely accurate. Author states close breeding "is seldom done in real life breeding". Apparently, little attention was paid to Welsh breeding, Arabian breeding, AQHA etc where you can see brothers being bred to sisters, 1/2 siblings to half, dam bred back to sire, etc. I see it as a bit mroe common then we would like to acknowledge.


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