What Heritability Tells A Horse Breeder

Heritability is a ratio: genetic variance divided by total variance, which includes the effect of environment. For horse performance traits the published values are mostly modest and wide-ranging. They describe one population at one time, not a promise about any foal.

What the numbers look like

A veterinary text gives ranges of 0.15 to 0.55 for flat racing, 0.05 to 0.28 for show jumping and eventing, and 0.11 for dressage. Muscle fibre composition falls at 0.12 to 0.28, and walk and trot variables at 0.05 to 0.44. Conformation traits such as height and weight range from 0.25 to 0.89. A Thoroughbred review reports racing time mostly at 0.1 to 0.2, and earnings or handicap measures at 0.3 to 0.4. A Brazilian study of 23,482 Quarter Horse sprint records from 5861 animals found 0.10 to 0.37.

Why estimates vary

The Thoroughbred review notes that estimates differ by method, age, sex, track and distance. For final race time they ranged from 0.06 to 0.84 across studies, though most were below 0.20. The veterinary text adds that an estimate applies only to the breed and population where it was calculated, and should be recalculated periodically in an active breeding programme.

What breeders can take from it

A value of 0.40 means genetic influence that is substantial but not deterministic. Environment, training and management still count. Generation intervals are long, at 11.2 years for male and 9.7 years for female Thoroughbreds, which slows progress. The review also notes that winning times have been stable for decades, even though genetic variation exists. Researchers debate why. Selection on performance records is practical in horses because both sexes can be tested and repeated records collected, according to the review. I found no heritability estimate for polo performance in the sources retrieved.

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 does a heritability of 0.3 mean? Roughly, that about 30% of the variation in that trait, in that population, is linked to genetic differences. The remainder includes environment.

Is there a heritability figure for polo performance? I did not find one in the sources retrieved, so racing and sport-horse figures are only a proxy.

Why do estimates differ between studies? Method, age, sex, track, distance and the population studied all shift the result.