Rare Breeds,Inbreeding, andBreed SurvivalD. P'Sponenberg

Managing rare breeds often boils downto the management of inbreeding. Thesame is true of the genetic managementof individual herds. I used to be somewhatcavalier about inbreeding, thinking thatselection for productivity could minimizethe risk of inbreeding depression. With afew more years under my belt, I have morerespect for inbreeding, inbreeding depres-sion, and the potential risk that it poses torare breeds. Inbreeding can be a positiveand powerful tool in rare breed conserva-tion and management, but it needs to beused wisely because it very definitely has apotentially negative downside!

Inbreeding is the mating of relatedanimals. Exactly how to distinguish itfrom linebreeding is subjective, becausethey are both "inbreeding," but they areusually taken to mean different degreesof relationship. One useful distinction isto consider inbreeding to be the matingof first-degree relatives. "First degree"means parent to offspring, full-sibling, orhalf-siblings to one another. More distantrelationships pose less of a threat, andcan be considered linebreeding instead of

inbreeding.The reason inbreeding needs to be taken

seriously is "inbreeding depression," whichrefers to diminished vitality in inbred ani-mals. This occurs to different degrees indifferent populations, breeds, or herds, butusually includes reproductive characters(fertility and prolificacy), and also generalhealth and environmental adaptation. Theperplexing part of this is that some popula-tions tolerate great degrees of inbreed-ing, while others do not. Unfortunately,predicting which is which before undertak-ing inbreeding is impossible. Some linesof rare breeds have been intensely inbredfor several generations, but then suddenlyhit a real wall of inbreeding depressionwhere fertility and vitality reach levels thatthreaten the survival of the line.

Inbreeding within a breed usually oc-curs in one of a few different ways. Insome breeds that are very separated intodifferent bloodlines, inbreeding can occurwithin a bloodline, or multiple bloodlines,but from bloodline to bloodline unrelatedanimals are still available. Pineywoodscattle are an example, with many distinctfamily lines. Each is relatively line-bred, some are highly inbred, but each iscompletely unrelated to other lines of thebreed.

Other breeds, usually through use ofindividual popular sires and their sons, endup with all animals of the breed inbred to

the same individual animal. If memoryserves correctly, Lusitano horses all haveat least some relationship to a popular sirefrom the early 1900s. In this case, com-pletely unrelated matings are no longeravailable to breeders, even though theactual inbreeding level is relatively low,occurred in the distant past, and may poseno real threat. Individual closed herds canexperience the same thing by using onemale for a single generation, because afterthat generation every animal within theherd is related to that same single male,even if other herds in the breed are not.Randall cattle are a good example of this,because Everett Randall used a single bullin the herd for several years, then replacedhim with a son. That pattern was repeatedover several generations, so now all Ran-dall cattle are related.

These two situations (inbreeding acrossan entire breed versus inbreeding limitedto single bloodlines) can have differ-ent significance to the breed. In the firstinstance there are still unrelated matingsavailable for every animal ofthe breed.In the second situation there is no optionfor a completely unrelated mating, andif inbreeding depression occurs, there istherefore no remedy. The important andtricky detail here is that when an inbredanimal is mated to a completely unrelatedmate, the offspring is not inbred at all.

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The Pineywoods cattle breed has many distinct family lines. Pictured: Desoto Piney-woods bull owned by Jess Brown. Photo by Jeannette Beranger.

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Inbreeding andBreed SurvivalContinued from page 3This means that inbreeding can disappearcompletely in one generation if a popula-tion's structure is managed well to ensurethat all animals are not related. The chal-lenge in rare breeds is to manage the entirebreed so that unrelated matings are indeedavailable, because this can take carefulplanning and thinking several generationsinto the future.

Managing inbreeding is always mostdifficult for small populations, becausemaintaining all of the genetic diversity thatis needed for long-term management istricky. Add to that the fact that linebreed-ing certainly does have some advantagesas a breeding strategy by assuring suchthings as predictability and uniformity,and it is important to not throw out thebaby with the bathwater! A few practicalways for breeders and breeds to manageinbreeding can be noted. The main goal isto assure that inbreeding is optional, or, toput it another way, to assure that every ani-mal has a potential mate that is completelyunrelated.

One strategy is to not necessarily avoidall inbreeding, but to try to limit it to lev-els that qualify as linebreeding rather thaninbreeding. This can take careful consid-eration, because the goal is to assure thateach animal in the herd has a mate that isno more closely related than a cousin ofsome sort. As a practical issue, this meansthat several male lines need to be kept go-ing, and, while they can be related, it mustbe kept to a relatively low level. As malesare retained for breeding, their pedigreesmust be compared to other males to ensurethat relationships are kept distant enough.

While inbreeding of first-degree rela-tives does have risks, it can still be used asa last resort to save rare bloodlines withina breed. This can be a very useful breed-ing tool to rescue rare lines. In general, asuccessful strategy is to not inbreed/line-breed for multiple generations in a row.Therefore, follow close inbreeding by anoutcross to another strain, or to a less-relat-ed animal. In that way the inbreeding leveldoes not creep up higher and higher, but istaken down a notch every generation or so.

In order to structure breeds to maintaingenetic distance between herds, one op-

Everett Randall used a single bull in the herd for several years, then replaced him witha son. Pictured: Randall cow owned by Joe Henderson of Chapel Hill Farm. Photo byJeannette Beranger.

tion is for different herds to be linebred indifferent directions. This means that eachherd can be linebred to a different found-ing strain, or a different set of ancestors.In this situation the breed is assured ofseveral completely unrelated choices formates if need be, because animals fromdifferent herds are unrelated. One way tomanage this strategy over the long term isto use an occasional outcross to the herd,but then to take those outcrossed animalsand linebreed back to the original strainfor a generation or two. In this way thepower of the original strain is not lost, butneither is the risk of inbreeding depressionvery high.

An opposite situation, where all breed-ers carefully avoid inbreeding or line-breeding, also has a subtle threat in it. Thisis especially true of rare breeds that havefew options for outside stock. If all breed-ers of a rare breed carefully search out un-related males to bring in every two or threegenerations, the result over time is thatthey have eventually used up the sourceof unrelated males. When done across allherds of a breed, the final result is that allof the herds are at least somewhat relatedto one another. This is because the searchfor unrelated animals takes the breederfurther and further afield, and if this ismultiplied over all the herds of the breed,pretty soon they are all dipping into the

same pool in just about the same way.Depending on numbers in the breed, thiscan eventually result in all animals beingrelated to one another at some level, and atthat point every breeding will be inbred tosome degree or another.

The most challenging situation formanaging inbreeding is small herds, withsingle males used in the herd over severalyears. This situation is all too typical ofseveral species like hogs and cattle. Asnew males are brought in, eventually thesource of unrelated males dwindles, andeventually completely unrelated malesmay no longer be available. "Unrelated"in this case can take on a nearly tyrannicalaspect, and very distant relationships mayactually not count for much in inbreed-ing, but should still be noted for long-termmanagement.

Managing inbreeding can be tricky, andusually takes good communication amongbreeders. Breed associations can be a bighelp in this regard, by keeping up with thedifferent directions that the breeders aretaking their herds and flocks .•:.

D. Phillip Sponenberg, DVM, Ph.D. isa Professor of Pathology and Genetics atVirginia Tech, in Blacksburg, Virginia. Heis a long-time member of The LivestockConservancy and serves as a TechnicalAdvisor. He can be reached via email atdpsponen@vt.edu.

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