Inbreeding is a complicated process. It has gained concern over the past decade as it’s become even easier to distribute sought-after genetics into more and more populations. Occasionally, when these desirable genetics are used heavily, the repetitiveness of some alleles can create problems. This is known as inbreeding depression. In a way, it is the opposite of the hybrid vigor that results from crossbreeding.

Farmers can use an inbreeding calculator on projected matings in hopes of avoiding inbreeding depression. The question that may arise is just how far back in a pedigree we should monitor for common ancestors to avoid detrimental inbreeding.

Researchers at Wageningen University & Research in the Netherlands compared the effects of recent inbreeding (about four to five generations removed) to more distant inbreeding. Yield (milk, fat, and protein kilograms), fertility, and udder health traits were analyzed on over 38,000 genotyped Dutch Holsteins.

More recent can make more problems
They found that inbreeding in more current generations was responsible for more pronounced inbreeding depression than that in older generations. “New inbreeding significantly reduced milk, fat, and protein yield, whereas ancestral inbreeding did not,” the report stated.

In fact, data from this study suggested a reduced production of about 80 pounds of milk per 305-day lactation for each 1 percent uptick in pedigree-based inbreeding in the most recent generations. There was also evidence that fat and protein pounds declined.

The fertility and udder health traits that were tested did not significantly differ between more recent and older inbreeding. Still, the findings on yield reduction offer considerations for dairy farmers to make when creating new pregnancies.

Why does this happen?
The differences in the effects of new and ancestral inbreeding could be explained by selection, both natural and artificial. Imagine, for example, that two animals are mated that share an ancestor eight generations back. A deleterious allele that the common ancestor possessed has probably been bred out of the genome, either by human selection or natural selection, somewhere along the last eight generations. It would be unlikely that, in the eighth generation, both or even one of the mated animals would pass on a copy of that allele. There is only a very small chance that a potential calf between the two relatives would exhibit the undesirable trait.

However, if the mating partners are fewer generations removed from the common ancestor, there has been less time for an undesirable allele to be selected out of the population. Therefore, the chance of one or two copies of that allele being passed on to a resulting calf is much more likely.

The study reinforces the idea that it is important to manage the inbreeding rate in the dairy herd so as to avoid the negative impacts of inbreeding depression. Due to natural selections over time, controlling inbreeding in animals more recently related should be of a higher concern than in animals sharing more distant common relatives.