Inbreeding and the Axis of Production

In an earlier post entitled Inbred Thinking, I noted that the cattle industry was the first to create a formal registry of breeding stock, and that this industry was also the first to note that milk and meat production dropped when animals were inbred very deeply or for very long.

Because farmers had a clear "steak and eggs" axis for evaluation of stock, they were ready and willing to outcross to achieve the best results for their needs and their land. Consumers, after all, do not much care what breed of chicken their eggs come from, or what "champion" bull sired their steak.

Through experimentation, farmers discovered that outcrosses and hybrids of two "pure" types produce as well or better, while remaining more disease resistant, more fecund, and longer-lived than deeply homogeneous stock.

Of course, what was true 150 years ago is still true today!

Anyone who wants to look at the deleterious effect of inbreeding animals beyond the single-digit threshold need only look at performance records for racing animals or farm stock where there is a real axis for evaluation.

As I have noted in the past, winning race horses are not heavily inbred, nor are winning Greyhounds.

In both animals, a Coefficient of Inbreeding of less than 5% is the norm.

What about production farm animals? Here too we find rising levels of inbreeding correlating perfectly with declining performance. As The Dairy Site notes:

The effects of inbreeding have been so much more negative than positive in animal breeding that the term "inbreeding depression" was coined. Table 1 shows inbreeding depression for lifetime and individual lactation traits of Holsteins from a recent study by Smith, et al. at Virginia Tech. The changes are expressed "per 1% increase in inbreeding." This means that the lifetime economic loss for a mating producing 6.25% inbreeding would be $24 X 6.25 = $150 expected loss from such a mating. Notice that, with the exception of somatic cell score where inbreeding has no apparent effect, all consequences of inbreeding in Table 1 are undesirable. Age at first freshening goes up, length of productive life goes down, all production traits are reduced, and first calving interval is lengthened as inbreeding increases.

The table referenced in the paragraph, above, is appended below:

What about egg production and beef cattle?

Here too we find exactly what we have found with racing dogs, racing horses, and mik cows: increased inbreeding results in less productivity.

The bottom line is a simple one: breeding any animal beyond the single-digit threshold is a recipe for infecundity and sickness.

As Heather Lorimer, associate professor of genetics at Youngstown State University notes:

Mammals, most other animals, and higher plants as well, have evolved mechanisms to avoid inbreeding of any sort. Some, like sweet cherries, have even evolved elaborate biochemical mechanism to ensure that their flowers can not be fertilized by themselves or by very genetically similar individuals.

Most pack animals (like lions, primates, and dogs), kick young males out of the pack so as to prevent them from mating with female relatives. Humans have very strong taboos against mating with relatives. Even fruit-flies apparently have a sensing mechanism to avoid too close of inbreeding; even in a closed population they maintain more genetic diversity than they ought to by random mating.

Professor Lorimer goes on to detail a quick list of the negative consequence of inbreeding:

1. Elevated incidence of recessive genetic diseases
2. Reduced fertility both in litter size and in sperm viability
3. Increased congenital defects such as cryptorchidism, heart defects, cleft palates.
4. Fluctuating assymetry (such as crooked faces, or uneven eye placement and size).
5. Lower birthweight
6. Higher neonatal mortality
7. Slower growth rate
8. Smaller adult size, and
9. Loss of immune system function.

And what are the positive benefits according to Kennel Club rosette chasers?

Simple: They can incrementally increase their chance of winning a $2 rosette made in China.

That's it!

Want to read more? Try these links:

• Dairy Crossbreeding: Why and How: There has been renewed interest in dairy crossbreeding as new advantages come to light.... Crossbreeding is more widely practiced by dairy producers every year.
  
  
• Inbreeding in Cattle: Inbreeding does not create undesirable recessive genes, but it does tend to bring to light these unfavorable genetics. This leads to a decline in average phenotypic performance called inbreeding depression. This phenomenon is well documented in all the major livestock species....
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