HGG Community Forums

Linebreeding is a slippery slope. I could point to dozens of breeds of dogs for example, who have been ruined by breeders who thought they were doing the right thing by linebreeding but instead doubled up on both those wanted genes and genes that cause weakness, poor conformation and death. Any line breeding must be done with rigorous attention to bad genes.. it looks like we now have tools to help with this, DNA tests!

I really appreciate the replies so far!

Here’s a little more background to my questions. Get ready, it’s a frustrating situation!
I know of a herd of horses with an already small gene pool that has been allowed to breed indiscriminately for the last 10 to 15 years. This herd was started in the 50’s, supposedly with 12 horses of unknown breed and pedigree as the foundation. The foundation herd was four stallions and the rest were mares. The only outside breeding was the addition of a mustang stallion sometime in the 90’s and a stallion of unknown breeding introduced sometime in the last 10 years or so. The herd has passed through multiple “breeders” and documentation of the herd has unfortunately gone downhill with each owner.
Recently they were transferred to a new owner who wants to keep the herd together by selling them all to a new “breeder”. The current owner is not intentionally breeding. Although last I heard, the yearling colts hadn’t been separated from their moms, sisters, aunts and granddams yet. A friend of mine is trying to help the current owner understand that the herd needs to be dispersed and not be left to breed. Or at the very least, not left to breed indiscriminately as one large group. Baby steps to start with in this situation. If we push too hard for the end of their breeding all together we might be shut out entirely. We’re hoping that pointing out the genetic risks might help them understand where we’re coming from.
Unfortunately the current owner hates me so I’m trying to help from the shadows by doing research my friend can then pass along. Because of this I feel like I need to be cautious with how many details I give out.

What I can comfortably share is this:
The horses had parentage tests done and many of the younger horses came back inconclusive. Typically with a match for their dam but no known sire, or vice versa. Their sires or dams must have been horses that passed away during those years of essentially being feral. There are a handful of older horses left with known pedigrees or almost fully known pedigrees.
I did enter the pedigrees of the horses that we know through an inbreeding calculator (https://www.nabssar.org/coi_calculator/ ) and got results anywhere from at least 4.10% to 38.50% inbred.
Next I ran a bunch of theoretical foals through the calculator with different sire and dam possibilities. I only did this with mares and stallions with known pedigrees. I was getting results from 6.13% to 61.79% inbreeding depending on the pairing. That worst one would be if a certain mare was bred to her son, yikes!

To me, the overall health of the herd seems good from a glance, but I’m no vet. Most of these horses have never had any sort of vet or farrier assessment. A couple of them have problems with locking stifles, poor teeth and cataracts, but they’re all older horses. I’m not a good judge of conformation so I can’t really comment on that. There’s no data regarding average foal crop so we can’t tell if there’s any decrease in fertility or not. Foals do seem to be born in the winter though, so foal mortality might have been high in the past.

I’m sure a lot of you will be frustrated reading about this herd, and trust me I’m frustrated too. The only chance I see for changing this herd’s future for the better is with scientific, genetic fact. So I’m hoping we can keep this discussion focused on explaining line breeding and inbreeding. The current owner has zero knowledge of genetics, so the simpler the explanation the better.

Again, thank you all for your time!

One of the origin stories for this herd is that it was started from a single herd of mustangs, but there's no documented proof.

Now that you've mentioned the Przewalski's horse I remembered something interesting! I found a genetic variation report most likely from 1990 or earlier for this herd. In the analysis of their serum proteins, specifically on the "XK" system, they found an allele that's found in Przewalski's horses but not reported in domestic horses. Not sure what that really means but I think it's interesting.