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Evaluating Soundness in a Sire’s Progeny: Use of the AEI/SI Comparison

Evaluating SoundnessFew issues wreak more havoc on an owner’s bottom line than the soundness of his horses. The slightest of injuries can add thousands of lay-up dollars to an owners bill, and more serious injuries that end a horse’s career can be catastrophic to an entire program. And yet, soundness in breeding stock is one of the least scrutinized topics by mare owners as they make their breeding plans each year. Traditional methods for evaluating soundness in a sire’s progeny have included percentage of starters from foals and number of starts per foal, though there hasn’t been a push to develop more advanced measures.

Another method of evaluating the durability of a sire’s progeny involves a comparing and contrasting of his Average Earnings Index (AEI) and Comparable Index (CI) to his Sire Index (SI) and Comparable Sire Index (ComSI). In order to understand the importance of these comparisons, it is first important to understand each statistical acronym.

The AEI was developed by the late Joseph Estes during his time as editor of The Blood Horse. A numerical index where 1.00 represents the average for the breed, the AEI tells us the earnings for any horse relative to his or her peers during the same year.

For example, if the average earnings for a five year old mare during 2004 was $10,000, and your five year old mare had earned $15,000 during the same year, her AEI would be 1.50. The obvious flaw with the AEI is that it fails to take into account the number of starts made by each individual. Durable horses are more likely to push their AEI up simply by making more starts. Fragile horses are at a disadvantage in that their lower AEI is merely a function of fewer earnings opportunities.

The CI tries to give us an idea of the quality of foals out of the same mares, but sired by different stallions. The idea here is to gauge whether or not a sire is improving on his opportunities, or pulling his mares down. The CI is derived by taking the average AEI of the foals sired by different stallions. For example, in a hypothetical scenario where a sire was bred to just one mare, and the AEI’s for her previous foals were 2.00, 1.50, and 1.00, the sire’s CI would be 1.50, the average for the mare’s other foals.

The key to understanding the AEI and CI is that both are based on yearly earnings, and ignore the number of starts made by each starter. In this system, a horse who earned $50,000 in a calendar year over the course of 10 starts is assigned the same AEI for a horse who earned the same amount in 20 starts. This is where the SI and ComSI can be useful. A sire’s SI is the average SSI (Standard Starts Index) for his progeny. The SSI essentially tells us the average earnings per start for that particular starter, relevant to the breed average which again is represented by an index of 1.00. In this system, the indexes are not influenced by the number of starts made yearly, and we get a better indication of the progeny’s racing class. Similar to the CI, the ComSI is the average SSI for foals out of the same mares, but sired by different stallions.

Now that we have an understanding of the acronyms, we will illustrate how these numbers can be compared and contrasted to help assess the sire’s aptitude for throwing durable racehorses. This method can also warn mare owners of sires who mask a lack of racing class in their progeny numbers through excessive soundness.

The best way to explain this phenomenon is through example. First, let’s take a look at the father/son duo of In Excess and Indian Charlie. Owners and breeders in southern California are very familiar with the unsoundness that frequently accompanies the brilliance and class of In Excess’s progeny, and now supporters of Indian Charlie are witnessing the same dynamics. When examined from the SI/ComSI perspective, both father and son clearly demonstrate the ability to improve upon their opportunities:

Horse SI ComSI Differential
In Excess 2.74 1.68 +63.0%
Indian Charlie 2.25 1.40 +60.7%

However, when we go to the AEI/CI system, which favors sires whose progeny make more than the average number of starts per year, the picture changes significantly:

Horse AEI CI Differential
In Excess 2.13 1.57 +35.6%
Indian Charlie 1.88 1.40 +34.2%

The phrase ‘like father, like son’ certainly applies here. Both In Excess and Indian Charlie are clearly able to improve their mares in both scenarios, but when examined in part as a function of the number of starts made each year by their foals, the amount of improvement slides significantly.

Then there are sires who are the inverse of In Excess and Indian Charlie. These sires have difficulty getting progeny in the upper levels of racing, but are able to mask this lack of class simply by siring durable types that make an inordinate number of starts each year. The flaw is masked by the AEI/CI system, but revealed by the SI/ComSI. Again, this is best illustrated by example, this time through California sire High Brite:

Horse SI ComSI Differential
High Brite .98 1.20 -22.4%

From this vantage point, where we’re not incorporating the gross number of starts per year, High Brite looks pretty bad. Each time one of his progeny leaves the starting gate, they earn an average of 22.4% less than foals out of the same mares but sired by different stallions. But when we change tools and utilize the AEI/CI system, things don’t appear quite so bad:

Horse AEI CI Differential
High Brite 1.15 1.25 -8.6%

This is an obvious reflection of High Brite’s reputation for siring durable types. Though they aren’t garnishing large checks each start, they are sound enough to make more starts per year than the average racehorse, thereby closing the gap in the AEI/CI system.

Another example is Slew City Slew. Even though he’s had two Grade One winners in 2005, through out the years he’s been known primarily as a sire of tough, durable progeny who are useful at mid-level tracks. Graded stakes production has merely been icing on the cake. An AEI/SI comparison illustrates his propensity for siring sound individuals:

Slew City Slew

AEI = 1.50 CI = 1.47 Differential = +2.0%
SI = 1.29 ComSI = 1.39 Differential = -7.7%

In the SI/ComSI system, it appears that Slew City Slew may actually pull his mares down a bit, though the statistical significance here is debatable. But like High Brite, the AEI/CI system is more favorable since it incorporates a sire’s ability to get sound runners.

Our intention here is not to criticize particular sires. Each of the sires referenced above can be used successfully by mare owners, depending on the level at which they wish to compete. The key is gaining a full understanding of each sire’s aptitudes and using that information to make the best matings possible. By doing a simple AEI/SI comparison, the mare owner has a better idea of what kind of stallion he’s dealing with, and subsequently, will make better breeding decisions. It’s imperative that breeders understand that this is just one of many tools available to resourceful breeders and like other tools, should be used proportionately.

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  • About Jason Hall

    A lifelong student of bloodstock topics as well as being an active owner and breeder, Mr. Hall advocates the importance of empirical research to identify truth in breeding practices. His articles have appeared in such magazines as The Blood-Horse, Thoroughbred Times, The Mid-Atlantic Thoroughbred, The Texas Thoroughbred, The Homestretch, Illinois Racing News, Hoofbeats, The Louisiana Thoroughbred and El Caballista. Mr. Hall holds a degree in journalism from Boise State University.

  • Mariana Lopez

    An enthusiastic and dependable member of our team, Mariana specializes in data collection and interpretation for our statistical research projects.

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