A summary by Karen Gerhart, drawing heavily from A Conservation Breeding Handbook, 1995, D.P. Sponenberg and C.J. Christman.
Breeders of purebred animals commonly employ one or more of the following breeding systems: inbreeding, linebreeding, and linecrossing. Each has advantages and disadvantages, and a breeder may wish to use only one system, or all three at different points to reach different goals.
Inbreeding: Defined as “mating together animals which are related so that the resulting offspring have one or more ancestors that occur on both the sire’s side and the dam’s side of the pedigree.” Close inbreeding would include the mating of full siblings, or father to daughter; more distant inbreeding might involve second cousins.
While inbreeding can be an emotionally charged subject, in itself it is neither good nor bad. Inbreeding tends to bring recessive traits (in St. Croix, these would include horns and colors other than white) to the surface; because of this, it should always be coupled with selection for excellent breed characteristics and the strict culling of individuals with undesirable traits. Used in this way, inbreeding tends to increase uniformity and consistency within a flock, and it has been used in the formation of most breeds. The St Croix breed started with only 22 bred ewes and 3 rams, and therefore all mating within the breed today involves some degree of inbreeding. However, for practical purposes, recent relatives have the most genetic impact and usually only the first 5 generations of the pedigree are considered when inbreeding calculations are made.
Inbreeding can lead to “inbreeding depression”, a reduction in vigor, fertility and disease resistance. Studies of wool sheep suggest that each 1% increase in inbreeding corresponds to a reduction of 1.4 lambs born per 100 ewes bred, a reduction of 2.78 lambs weaned per ewe lambing, and a loss of 2.44 pounds per lamb weaned (Lamberson et al. 1984, as presented in the Sheep Production Handbook, 1995). However, breeds and populations differ in their tolerance to inbreeding depression; a general guideline used by the American Livestock Breeds Conservancy (ALBC) is to keep inbreeding below 5% per generation, and 30% in any individual.
Learn more about a Coefficient of Inbreeding and software for calculating COI’s
Linebreeding: A form of inbreeding, linebreeding involves concentration of a particular ancestor within a pedigree (rather than several ancestors, as in other forms of inbreeding). Usually, this individual is a particularly excellent representative of the breed. The goal is to create a flock as much like this individual as possible, so matings often involve breeding half-brother to half-sister.
Like inbreeding, linebreeding reduces the variation within the flock, making the individuals more uniform and therefore more predictable. Again, the possible risk is inbreeding depression, including reduced vigor and reproductive performance. Strong selection and strict culling are necessary in a successful linebreeding program.
Linecrossing: A line (or strain) is a group of animals that are more closely related to each other than to the population as a whole. They might be the product of an inbreeding or linebreeding program. Linecrossing is the mating of individuals from one such line to those of another line. Generally, linecross individuals will show greater vigor, better growth, and more ‘bloom’, or ‘presence’ than individuals from either of the parent lines for at least the first generation. Thus, linecross individuals are more likely to succeed in the showring. Linecrossing can also be used to bring new vigor into an inbred or linecross flock.
Which is the best?
Each of these three breeding systems have advantages and disadvantages. If the goal is a flock with high predictability and low variability between individuals, then some degree of inbreeding or linebreeding will help the breeder achieve this goal. Individuals from such flocks will produce lambs much like themselves.
If, however, the breeder’s goal is to produce excellent individuals, then linecrossing may be the better approach. These outstanding individual sheep may not be consistent in the types of lambs that they produce, however, and may not be able to produce lambs as good as themselves.
One advantage of a Conservation Breeding plan is that it can allow a breeder to include advantages of both linecrossing and linebreeding within a relatively simple program.
Part 2: What is Conservation Breeding?
Part 3: An Example of a Conservation Breeding Plan
Part 4: Three Disadvantages of Conservation Breeding