important applications of Linebreeding

 

from Managing Breeds for a Secure Future
Strategies for Breeders and Breed Associations

pp 67-69

by D Phillip Sponenberg and Donald E Bixby of the
American Livestock Breeds Conservancy

Linebreeding has a few specific powerful benefits for many rare breed conservation programs,  In rare breeds it is common to find that some bloodlines of a breed have dwindled down to a few individuals, and these are the most often females.  This is especially likely to happen in breeds that are widely used for crossbreeding, such as Florida Cracker and Pineywoods cattle.

A major strength of inbreeding is the fixation of traits in a given line of animals.  Even extreme inbreeding can be used as a short term strategy to accomplish the specific goal of making certain traits more prevalent in a breed or herd.  An excellent older female, for example, can be mated back to a son for an inbred replacement.  If the offspring is a son, then he can be widely used to spread the superior genetic traits of the original female more effectively than would be possible without resorting to the close inbreeding.

This strategy can be especially effective when used to correct the under-representation of the genetic influence of certain founders of rare breeds.  Some of these may be female remnants of important bloodlines that have declined as in the preceding example.  Another common situation for using this technique is to enhance imported bloodlines that may have only come through a single sex of animal- more commonly a male via imported semen, but occasionally a female of a rare bloodline.

In situations where a line has dwindled to a few females (and as a practical issue these females are generally closely related) one good strategy is to mate the females to a purebred male from another line (this is by definition a linecross),  Then mate a male offspring back to the original females.  The result is a crop of youngsters that is 3/4 from the original line.  These 3/4 males can be used back on the original females, and this strategy, (young males, older females) can be used until the older females cease to be productive.  The goal is to take genetic material that can only be used to a limited degree by virtue of being in female form, and generate males that can be used more widely to distribute the genetic material in these rare foundation bloodlines broadly throughout other portions of the breed.  It is important to note that while these initial linecross (first generation) and linebred (succeeding generations) males are being produced, so are females that can be effectively used in other portions of a breeding program without endangering the critical role that the foundation females are playing.

Similar strategies can be used with outstanding females from any breed but especially from rare breeds.  Outstanding or genetically unique females can be mated back to sons to produce offspring (hopefully male) that are 3/4 the influence of the original female.  This has been done in many rare breed conservation programs with individual rare line females.  While this strategy cannot be used over several generations without running into inbreeding depression, it is usually a successful strategy for a generation or two, at which point the males produced can be linecrossed to other lines and thereby contribute widely to the breed.

For example, within the Colonial Williamsburg Foundation importation of Leicester Longwool sheep came a single ewe from the Glendhu property in Tasmania.  She was unrelated to all the lther imported sheep.  This ewe was conformationally correct for the breed, strong -wooled (as appropriate to the breed), highly maternal and reproductively fit. On two occasions she was mated back to different sons, and the results included two rams (as well as ewes that were 3/4 her genetic influence.  These rams were then moved to other flocks so that it was possible to more widely disseminate the founder ewe's excellence in type and production throughout the breed in the USA.

This ewe's usefulness to the breed was only possible because of the past efforts of long-term breeders of purebred livestock who make such linebred animals available.  Every breed must have such breeders and such animals.  In general the line becomes associated with the breeder, such as in the case of the Ridley Bronze Turkley, the Conway Pineywoods cattle, and Hillis Cotswold sheep. It is the long term dedication of individual breeders that provides these distinctive genetic packages that are so useful for others to build upon.  Each breed and each generation within the breed needs breeders with this dedication in order to safeguard the genetic heritage of the breed.