The X-Factor

How do you breed or find that one horse that has that undeniable quality, that one thing that sets them apart, not only in racing terms, but in how they look and race. Millions are spent every year, looking for the “X” factor, that one thing that makes a horse a superstar. In the equine world, that factor is what translates into the big money.

According to research, one thing has emerged as the prerequisite for the elusive “X” factor. Heart size is something which seems to determine not only the performance of the horse at the track, but its breeding future as well. Research indicates that heart size is carried on the X chromosome alone, so a male horse only has a single marker for heart size, whereas the female horse has two. This doesn’t mean though that she will necessarily inherit a large heart, as the male horse inherits one chromosomes from his dam, and that alone determines his heart size, whereas the female has the two X’s and can have either one or both, or no large heart gene. A mare that carries 2 X chromosomes with markers for large hearts, is the one that breeders look for, she is known as a double copy, and she is the goldmine for breeders, as she is likely to have an exceptional record of producing good large hearted progeny, as every foal she produces will have the gene.

In Australia in the 1950’s, a pioneering study was done by Dr.James Steel, a professor of veterinary medicine at Sydney University. Dr Steel used an ECG on 2500 horses to work out a “heart score”. His testing provided convincing evidence of the link between large heart scores and racing performance. Subsequent studies over the years around the world, have confirmed Steel’s research that larger hearts do make better racehorses.

Various lines throughout the decades seem to confirm this, and one of the most influential lines in terms of large hearts through the pedigree is that of the great foundation and double copy mare, Mahmoud. Through her line (two generations removed on dam Natalma’s side), Mahmoud was responsible for the greats, Northern Dancer and Rahy. Other great and high producing heart score lines are: the Princequillo line; (Secretariat, Mill Reef and Key To The Mint), the Blue Larkspur line (dam five generations removed on dam Gold Digger’s side of Mr. Prospector and Halo), and War Admiral line (Seattle Slew and Buckpasser) they all carry upwards of 140 heart scores. Another successful double copy mare was Weekend Surprise, dam of Secretariat, A.P.Indy and Summer Squall. Weekend Surprises’ dam, Lassie Dear, was herself a double copy mare, and she successfully produced winners throughout her career at stud, something she thankfully passed down to her daughters.

Autopsies on several well known and legendary racehorses seem to verify this. Secretariat was found to have an abnormally large heart (22 pounds) upon his death, and his racing career certainly indicates this worked in his favour. Although a heart score was never given at the time of his autopsy, it is estimated that Secretariat’s heart score would have been 180 at the very least. The same was true for another champion and rival of Secretariat, Sham, whose heart weighed in at 18 pounds.

More recently, renowned equine cardiologist, Dr Frederic Fregin, measured more than 400 horses with an ECG. His findings were extremely interesting, and certainly seem to back up Steele’s early research. Fregin found the X Factor to be 100 percent consistent, a rarity in scientific research. He concluded that through extensive research, we’ve been able to track the large heart from sire to daughter to granddaughter and grandson and to great-granddaughter and great-grandson.

Yes – there are many other variables to consider when looking for a horse with that “X” factor, environment, nutrition, conformation and attitude, are only a few of the many things that could overshadow this possible genetic contribution. However, the evidence is convincing, and its now up to horsemen and woman alike to do their homework.