While inbreeding is a form of line-breeding and the two are related, no pun intended, they are different.
According to Jim Lents, owner of the Anxiety 4th line of horned Hereford cattle in Oklahoma, a line not outcrossed since the 1870’s, the difference between inbreeding and line-breeding is the amount of genetic influence any single animal plays in any descendent’s genetic makeup.
For instance, any individual is made up of 50% of each of its parent’s nuclear DNA (Mitochondrial DNA plays a role here; however, they are passed down only through the egg. In order to keep things simple we’ll ignore mitochondrial DNA for the time being). In other words, the sire and dam of your “best” cow each contributed 50% to that cow’s DNA. A daughter to that cow receives 25% each from her dam’s parents and 25% each from her sire’s parents, potentially watering down the quality and predictability of subsequent offspring.
In line-breeding the idea is to always keep the amount that any one animal contributes to the DNA of any descendent at or below 50%. With inbreeding you regularly will find a higher degree of influence. For instance, a sire/daughter mating will result in an offspring which carries 75% of it’s DNA from the sire and only 25% from the maternal dam. Interestingly, before the advent of genetic testing for recessive traits the only way to statistically ensure genetic “purity” of a bull/ram/buck etc. was to breed that bull to 35 of his own daughters concurrently. If no genetic defects show up in any of the offspring, the bull is 99.7% likely to be genetic defect free.
In any case, mating two full siblings together does not qualify as inbreeding because the level of influence any one of the grandparents exercises is still only 50% on the individual resultant calf from that full-sib mating.
In conversation with Dr. tatiana Stanton (yes, she spells her given name entirely in lower case) from Cornell University, the optimal level of relationship across a given herd/flock of animals is 12.5%. tatiana is the NY State Small Ruminant Specialist with Cornell Cooperative Extension and she is a breeder of goats and a goatherd in her own right. This 12.5% relationship ensures consistency of type and kind, uniformity of animals, while also allowing for enough diversity to avoid inbred suppression across the herd.
Wye Angus is a herd, really a program, owned and operated by the University of Maryland Foundation. The herd has had no new introductions of genetics since 1957. Many of the cattle within the herd will have individuals occurring hundreds of times in a given pedigree, yet the individual whose pedigree we are looking at may only carry an inbreeding coefficient of six or seven percent.
For my purposes, having certain extremely high quality individuals appearing multiple times up close in a pedigree is important. Most of my bull customers have completely heterogeneous herds of cows. By using a high quality bull that has been selected from proven lines of maternal efficiency, good disposition, longevity and phenotypic correctness, they imbue far more predictability and consistency into their subsequent calf crops.
One key component to any breeding program is culling. As a breeder of registered parent stock which I sell to commercial cattlemen and women, it is incumbent upon me to identify and remove any faults that may be hidden within my herd of cattle. I sincerely hope never to sell problems into someone else’s herd, especially genetic problems which are insidious in a total outcross environment.
If you have not read the book, Battle of Bull Runts, I highly recommend it. This is the story of how the simple recessive, lethal defect of snorter dwarfism (colloquial, I know) was not just propagated within the Hereford breed, but nearly brought the breed as a whole to commercial irrelevance from the 1920’s to the 1960’s. Through much investigative work looking through reams of pedigree documents the “index” animal was identified as a bull born in 1901. That bull was used in a complete outcrossing program for eight or nine years, only by natural service. Yet, the results of his offspring never being purposely and purposefully bred to one another allowed this simple genetic defect to go undetected until the 1920’s and unrecognized for what it was until the 1940’s.
Interestingly, the Anxiety 4th Herefords were one clean repository of genetic material from which the Hereford breed was able to rebuild. Why? The cattle in the herd have never been outcrossed. Any defects that might be present are detected early and culled.
These same breeding principles, including heavy culling, are found in every breed and species of animal currently in domestication. The principles are actually quite close to what happens in isolated populations of wild animals, as observed and pondered by Charles Darwin. Also interestingly, Mr. Jim Lents, the current owner/breeder of the Anxiety 4th Herefords, is a devout Christian who looks to the Book of Genesis for his guidance in breeding. So whichever side of that particular discussion you come at breeding your own animals from, line-breeding works.
Check out the whole series on breeding matters by Morgan Hartman: