Breeding the Thoroughbred is highly regarded as both an art and a science by breeders and buyers globally. Even the term Thoroughbred stems from the words “thoroughly-bred” from the horse’s Arabian Stallion ancestors, considered the best racing horses of the time.

There are several theories on how to best breed this exquisite and desirable horse, and here we examine three of those theories.

Breed the Best to the Best

The most popular theory is simply that the best runners generally make the best sires and the best race mares make the best broodmares. Critical to this theory is the Racecourse Test, which measures a horse’s ability to win.

It measures the horse’s speed and stamina and tests its strength, soundness, and will to win. It truly “bets on the horse” versus the statistics attributed to that horse. The most desirable horses, of course, are those who win Britain’s prestigious The Derby and our own Kentucky Derby.

The motto for this theory is “breed the best to the best and hope for the best.” Critics of the theory believe that is its problem. It is not a foolproof method, and high costs can be an issue.

Without quantitative data backing this theory, it’s difficult to know whether you’re really getting out what you’ve put into the breeding. Still, it reigns as the MVP of theories.

By the Numbers

Many breeders look at four sets of statistics when determining whether a stallion or mare will make a good mate. The advantage of using these numbers is simply that numbers don’t lie—they offer quantitative data in a world where every theory is being tested and serious dough is on the line.

But those who prefer to focus on judgment-based choices will likely shy away from using these top stats. Many breeders choose to use both data and personal judgment to make their choices.

The Average Earnings Index (AEI), according to The Blood-Horse, measures the earning power of a sire’s progeny by comparing the average earnings of his runners with all other runners of the same age that raced in the same country during a given year.

The advantage to using this popular number is that it compensates for the difference in the number of runners different stallions may have.

When used with the Comparative Index (CI),which measures the quality of the mares bred to that stallion, breeders want the stallion’s AEI to be higher than the mare’s CI to show that he’s generating the same quality of runners as the mare has produced with other stallions.

Another stat, the Average Winning Distance (AWD), is available through the Online Stallion Register and shows a horse’s pedigree for speed versus those whose pedigree focuses on stamina.

Dosage is an alternative to breeding the best to the best, as it interprets pedigrees in the context of inherited speed and stamina.

According to Dr. Steve Roman, the originator of the contemporary Dosage system and creator of the Dosage Index, the output is a Dosage Profile (DP), a series of five numbers that reflect the relative proportions of each of five aptitudes contributed by chefs-de-race (influential sires in a horse’s pedigree).

These aptitudes are expressed in the order Brilliant-Intermediate-Classic-Solid-Professional. These aptitudes constitute the full spectrum of pure speed on the left to pure stamina on the right, and its pattern symbolizes the aptitudinal type of the pedigree.

Horses With Big Hearts

One of the most controversial breeding theories is the X-factor, or large heart theory. The large heart gene, called the X-factor because it’s on the X-chromosome, is passed down through Thoroughbred families.

According to Laurie Ross of Horse Racing Nation, a large heart in a thoroughbred is a positive attribute, giving the horse greater stamina and strength. Ross notes that a colt can only inherit an over-sized heart through his dam, and a stallion can only pass on the gene to his daughters.

This theory is often maligned, as heart size alone doesn’t guarantee a stakes winner. A supposedly easy way to tell if a horse has the X-factor is the presence of curly-tipped ears. True or not, this theory is a curious one. Note that the following horses are considered to have the X-factor: Eclipse, Secretariat, California Chrome, and American Pharaoh.

These three theories constitute just half of the popular breeding theories used to develop the gorgeous modern Thoroughbred, which is of course bred for racing performance.

Be sure to stop by next week as we discuss the three other theories, as well as the pros and cons of each.