Inbreeding And Genetic Diversity In A Small Herd

In a small herd, inbreeding builds up faster, and the cost can show as reduced fitness. Conservation guidance treats 1 to 4% inbreeding per generation as acceptable for endangered livestock. That guidance assumes far more animals than a small herd holds.

What inbreeding does

Inbreeding raises the chance that an animal receives the same gene copy from both parents. Parents who share a grandparent produce an inbreeding coefficient of 6.25%, according to a dairy cattle article, which also treats recent inbreeding as riskier than distant inbreeding. In 6128 Thoroughbreds, a 10% rise in genomic inbreeding was linked to a 7% lower probability of ever racing. One haplotype, when carried in two copies, was linked to a 32.1% lower probability. These are associations in one breed.

How small is too small

FAO guidance says diversity is lost faster when effective population size falls below 100. To hold inbreeding at 1 to 4% per generation, it suggests 12 to 25 males and 100 to 250 females. For a new conservation herd it suggests founders of at least 50 males and 50 females. Horse generation intervals run to 9 to 12 years, so each generation takes a long time to produce. Effective population size is also hard to pin down. Across 140 populations, estimates ranged from 15 to 133,056 depending on breed and method.

Where the evidence stops

A Thoroughbred review cites older US estimates of average Thoroughbred inbreeding of about 0.8 to 1.4%, and notes concern over lost variation in a closed population. It also says the inbreeding rate usually practised in the breed does not fix many genes, but advises avoiding close inbreeding. None of the figures above is a polo-specific or Sahel-specific threshold. They come from conservation work and racing, and a small herd falls outside the numbers in the FAO guidance.

What this means for the programme

The programme is in formation. It crosses Sahel mares with thoroughbred sires, using frozen semen. It uses microchip identification, DNA parentage testing, veterinary screening and a written start record. No ponies are for sale and no results are claimed yet.

Frequently asked questions

What inbreeding rate is considered acceptable? FAO guidance for endangered domestic breeds cites 1 to 4% per generation. It is a conservation benchmark, not a rule for polo.

Does inbreeding affect racing ability? In the 6128 Thoroughbreds studied, higher inbreeding was linked to a lower probability of ever racing.

Can a small herd avoid inbreeding entirely? Not by numbers alone. The FAO figures assume 12 to 25 males and 100 to 250 females, which small herds do not reach.