D. Phillip Sponenberg, DVM, PhD., has served as Science and Technical Advisor to The Livestock Conservancy since 1978. He provides counsel and mentoring to Conservancy staff, breeders, breed associations, scholars, and NGO partners. Dr. Sponenberg established both the Conservation Priority List and the standards for rare breed inclusion on that List. Because of the quality and originality of his approach to conservation, Phil’s expertise is internationally renowned. He is the author of several books on color genetics and conservation, and is a sought after speaker domestically and abroad. Dr. Sponenberg recently retired as Professor of Pathology and Genetics at Virginia Tech University. On his own farm, he is a conservation breeder of Tennessee Fainting goats, and enjoys playing with color genetics in his Brahma chickens.
Breed maintenance and breed conservation strategies are unfortunately not “one size fits all.” The details of a breeding program will vary from breed to breed and herd to herd, so a variety of plans can be effective, but it is possible to outline some overarching principles.
Breeding strategies vary greatly by the type of breed that is involved. A productive standardized breed that has (or has had) commercial relevance needs an approach that safeguards both the productive potential of the breed and its genetic conservation. For adapted feral populations that are now in captivity, genetic conservation takes precedence over any selection for performance.
For adapted landraces, a balance must be met in which selection for production, adaptation, and genetic maintenance all have a share in the priorities. One principle that underlies just about all effective strategies is that genetic variation must be maintained. This means that a broad range of breeding animals must be kept and used and must each contribute to the next generation of breeding animals. In most situations, this means that more males will be used in an effective conservation breeding program than might be used in more routine programs targeting mainstream production goals. The use of only a few males for several years will rapidly narrow the genetics of a population and is a main reason for the precarious status of many rare breeds. Reversing that situation by using many males for short periods of time can provide the population with a much broader genetic base, and with that comes enhanced survival of the population as a viable genomic resource.
The use of few males with many females is inherent in the thinking of most production-oriented animal breeders because they believe the male is the main improver and strengthener of the population. One strategy to counter this thinking is to explain (and accept!) that the females are actually the sex in which most of the genetic variation is contained, and that it is necessary to use them effectively to assure their contribution is not overwhelmed by a few males. This involves seeking out and using older, productive, adapted females to their maximum potential by preferentially using replacement stock produced by them. Ideally these replacements come from a variety of sires and the same excellent female.
Part of the attraction of “male centered” programs is a subtle consequence of using many fewer males than females – a very disparate selection differential, which basically means that, in most cases, the male is going to be more above the herd average than the females. The males used are generally phenotypically superior to the average of the females, and this ends up driving decisions about male retention, female retention, and the use of the various animals in the herd for generating replacement stock.
Within the breeding plan of using more males for shorter periods of time is the principle that recruitment of replacement males must come from varying portions of the population. If this does not occur, then the families that the few select males were in become the genetic bottleneck the more usual contemporary situation.
Additionally, it important to target animals within the breed or herd that are most genetically unique. Their contribution is likely to be lost unless they are carefully monitored. It is tempting to say that genetically unique animals (usually determined by pedigree analysis) are more important to the breed than are the genetically more common animals. While at one level this is true, at another level it can be reassuring to the breeders of the genetically common animals to realize that they have done a good job in assuring that those lines are, indeed, well represented. This can be especially important in landrace breeds as the few “outliers” of very rare strains have an important task to do in their contribution to the breed’s genetic variation. Unless they are used wisely, their unique genome is likely to be diminished or become extinct within the breed.
A “worksheet” approach can help determine strategies for specific situations. This is very heavily weighted towards a “genetic conservation” approach, and can be modified to include important production aspects.
First, identify founder animals and their contribution to the population. This is most easily done by using software programs that can quickly accomplish this electronically rather than by doing it manually. Answers to the questions “which animals are founders?” and “how broadly are they represented in the present population?” can be incorporated into a spreadsheet that lists founders, and then characterizes each animal within the population as to its percentage of each founder. Averages of all living animals can then give a snapshot of the living population. Founders that have a “low” contribution can then be targeted for selected retention in the next generation.
An example is Randall/Randall Lineback cattle. There were only 12 foundation animals (animals beyond which it is impossible to go in the pedigrees), and they generally include animals used to start a population, or their immediate ancestors, if these are known and if relationships are present between the animals that start the populations. For example, if the source herd only used single bulls at a time, then within the span of any year, the animals are all half- siblings through a common sire. In that case, the founders would be the dams of each animal, but only that one common sire.
Due to using single sires for multiple years, the founders in the Randall breed varied in contribution from 35% (high, coming from the one common sire), to only 1.1% of the entire herd. In this situation, it is recommended to select replacements that decrease the 35% founder and some that increase the 1.1% founder. In this example, the herd is some 200 animals and a few daughters and sons of the “1.1%” founder are available.
Second, determine what to do with the results of step one, generally to correct the imbalance of founder animals.
For the Randall example, a few animals had their own individual percentage of the “1.1%” founder at 50% (his sons and daughters), and it can be easily seen that by using a bull with 50% representation of that founder it is possible to boost the representation of that animal in the next generations. So, basically, this step assures that the underrepresented lines get a chance at selected retention and use.
All of this must be done with a good look at the animals involved and their conformation and performance. Some animals are rare because they are genetically unfit, and it is important to sift through those and not retain the weaknesses. Equally, some are rare simply because they were overlooked or were in peripheral herds and not widely used. This is typical of landraces, and targeting the use of these animals assures that their contribution does not wane and disappear.
If the rare animal is a female, she can be mated back to sons to (hopefully) generate linebred (inbred) sons that can see broader use. In the case of males, these can be more widely used themselves, or if elderly can be mated to daughters that produce sons for wider use.
Third, if step two succeeds, and this might take a few generations, then in step three a plan can be devised with recurrent use of different sire lines in the herd or across the whole breed in order to even out genetic contributions and assure that matings are not closely linebred. If this step succeeds across the breed, the result can easily look like the “conservation breeding program” across several breeds, with different sire lines going through the population.
This article appeared in the 2022 Directory: Rare Breeds and Products Resource Guide. The Livestock Conservancy’s annual Directory listing members and livestock across America is one of our most popular membership benefits. Join or renew today to enjoy our upcoming 2024 issue.
