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7 June 2008


By Tom Reed
Horse International Vol 5 2008.

Do you remember those Paint by Number kits that promised to make "Every man a Rembrandt!"? The kits contained a board with numbered spaces, some brushes, and scores of numbered dollops of paint. All you had to do to become a modern-day Rembrandt was to match each space with the corresponding dollop and to let your conformist juices flow. Just be sure to stay within the lines!

We are now seeing the same phenomenon with horse breeding. Spurred by a new wave of semen agents with little or no actual experience breeding competition horses, novice breeders are told they are "guaranteed to produce a showjumper" or a dressage horse or an eventer if they use this or that stallion with high breeding value index scores or a high ranking. (Of course the agent just happens to sell semen from that stallion.) You too can be a Leon Melchior or a Hermann Meyer or a Sam Barr but only if you order by midnight tonight! Credit cards accepted! A free Ginzo knife is included with every purchase!

Breeding indices and rankings can play a useful role in breeding but only if used correctly and their limitations are understood. In this essay I will briefly discuss some of the major indices and rankings, point out some key issues that breeders should remember when consulting them, and offer a few suggestions on how they could be improved.

Understanding Index Scores
Although each studbook uses slightly different methods when constructing their breeding index the general idea is the same. The progeny of stallions are assessed along the dimensions covered the index. Typical dimensions are front legs, hind legs, topline, walk, trot, canter, etc. The average Partial Index Score for each dimension for the population of stallions is set at 100 and usually scores above 100 indicate better than average and scores below 100 indicate worse than average. So a stallion with a topline score of 115 produces progeny that on average have better toplines then a stallion with a score of 90. (The KWPN index system is different and I'll discuss the differences below.)

Along with these Partial Index Scores for individual attributes the studbooks produce an overall Breeding Value Index, which is a summary measure of the genetic value of the stallion. As with the Partial Index Scores, with the Breeding Value Index the greater the score is above 100 the more genetic value the stallion has and the lower the score is below 100 the less genetic value he has.

How can these index scores be misinterpreted? First, be careful if you are tempted to think there are meaningful differences between two stallions whose index scores are similar. A stallion with an overall Breeding Value Index of 120 is probably no better than one whose index score is 117. However both are likely better than a stallion with an index score of 100.

Second, remember that Partial Index Scores for conformation suggest measurement with great precision but in reality our tools for measuring conformation features are rather imprecise and subject to human error and subjectivism. So again if one stallion has a Partial Index Score 97 for his progeny's hind legs and another stallion has an index score of 101 there is probably not much of a difference between the two stallions: both are average producers.

Third, these index scores assess the stallion's genetic worth in relation to a specific population of mares. So a stallion that has high Partial Index Scores or a high overall Breeding Value Index score in one studbook may have very different results in another studbook's index. I have seen this often especially with thoroughbred sires that service a variety of European studbooks.

Fourth, index scores cannot be compared across studbooks because the population of stallions, mares and progeny are probably not comparable. Let's assume that both the Holsteiner Verband and the Trakehner Verband produced index scores for jumping ability. Would we expect a Holsteiner stallion with a jumping index of 100 to be equivalent to a Trakehner stallion index with a jumping index of 100? Of course not. The average Holsteiner jumper is much better than the average Trakehner jumper.

Let's take a look at several examples of index scores.

KWPN Index
The KWPN has a marvelous index that can be a very valuable tool for breeders. Most indices produced by studbooks take a conformation feature of the progeny (let's say correctness of the font leg) and assigns scores above 100 for correct front legs and scores below 100 for incorrect front legs. However, the KWPN system establishes the correct front leg with a score of 100. A stallion with an index score of 90 produces, on average, progeny that are over at the knee while stallions with an index score of 110 produces, on average, progeny that are back at the knee. So when examining the KWPN Partial Index Scores one needs to have the definitions at hand since a higher score does not necessarily imply better conformation.

The KWPN index includes Partial Index Scores for many conformation features, all three gaits (assessed by three dimensions for each gait), and jumping ability (a total of eight dimensions of jumping ability). Simply stated, the KWPN is light-years ahead of any other studbook in its breeding indices. This is a strength of the index but it is also a potential stumbling block for some breeders.

I have found that some breeders tend to focus on one conformation attribute of the mare and try to "fix" it. So, for example the mare has a short neck and come hell or high water the breeder wants to ensure that the foal has a longer neck. So he searches for a stallion that tends to produce long necks. The problem is that the length of the neck is probably not an important determinant of athleticism or success in sport and by fixating on this one feature other critical attributes (such as athleticism) become less important.

How could the KWPN index be improved? I wish the Dutch index included an overall index score for rideability comprised of partial scores for submission, ease of the mouth, the extent to which the horse is responsive to the rider, etc. In practice many of these dimensions of rideability would be difficult to measure with reliability and validity but if anyone can do it I believe the KWPN can.

Holsteiner Verband Index
The Holsteiner Verband's breeding index is interesting for me because it includes along with the standard conformation and movement variables three features missing in some other indices: canter, loose-jumping ability, and rideability of the progeny. The Holsteiner breeders know what helps to make a good showjumper and they have included these three variables in their index.

Irish Horse Register Index
The Irish Horse Board (IHB) produces two indices and the most critical difference between them is that one is for stallions with progeny competing in showjumping and the other is for stallions without progeny in sport. Unfortunately the IHB publicizes the latter index.

Under this index a stallion that showjumps national Grand Prix (typically 1.35 - 1.45 m) and has no progeny competing at any level in sport -- such as the current incumbent of the top-rated position in the index – is regarded more highly than a stallion that has international Grand Prix progeny but who himself did not pursue a showjumping career in Ireland.

Every genetic index has strengths and weaknesses but the IHB's most widely disseminated index is problematic since it excludes progeny from the analysis. Another problem is that the conformation, temperament, and athleticism scores are based on the stallion's own inspection scores and the few mares that may have been inspected as part of the IHB's grant scheme. I believe there are substantial problems with the reliability and validity of these Partial Index Scores and they should be taken by breeders with several grains of salt.

WBFSH Rankings
The World Breeding Federation for Sport Horse (WBFSH) has been publishing rankings of showjumping, dressage, and eventing sires for about a decade. In the early years the rankings were published in book form and there was nothing that I liked better than to delve into the densely-formatted pages. But in recent years the rankings are published on a CD-rom and for me this technology makes the data, and the underlying trends and hidden surprises, less accessible.

In collaboration with the FEI, which supplies the data to the WBFSH, these rankings only consider results from upper-level international competition so the rankings are skewed toward established sires whose progeny are old enough to compete at those levels. So for all practical purposes the highly-ranked sires will be 15 years or older and, in fact, many of the sires at the very top of the ranking will be aged, dead or have age-related fertility problems.

The temptation for breeders is to look at the top 10 or 20 sires and to chase after their semen. This can be a mistake because the sire in question may be totally unsuitable for the mare. An example of this happened in Northern Ireland when the UK government funded breeding venture Irish Sport Horse Developments inseminated a large number of Irish mares with Burggraaf. A great sire in the Netherlands was used on a completely different and inappropriate mare base in Ireland and the results were, as I predicted at the time, a disappointment.

I cannot stress enough how important it is to understand what type(s) of mares – with respect to genotype, phenotype, and athletic expression – when matched with the stallions in question produced the international athletes whose success in sport led to their sire's top ranking. The mare contributes 50% of the foal's DNA. Those highly-ranked sires were produced by skilled breeders who were able to match the genetic potential in the sires with the genetic potential in their mares. They did not go to a CD-rom and pick out a Top 10 sire or worse, take advice from a semen agent who himself has never bred a competition horse.

How can the WBFSH rankings be made more valuable and useful to breeders? I have two suggestions that could be implemented very easily by the WBFSH and the FEI. First, a new ranking should be created that takes into account the number of progeny produced by the stallion. A sire that has had 2,000 foals has had many more chances to produce international athletes than a stallion with 500 foals. Ideally this new ranking would take the total points earned by each sire (the basis for the conventional ranking produced now) and divide it by the number of progeny that are over eight years of age. (I recommend eight years of age because that is the youngest that we would normally see a horse competing at the levels that are captured by the WBFSH/FEI data.)

The second suggestion is to create two rankings of dam-sires, the first of which is based on total points and the second based on total points divided by the number of grand-children eight years of age and older.

Conformation indices, breeding value indices and WBFSH rankings can play an important role in breeding, stallion selection, and mare selection. But we all must be discerning consumers of the data and the statistics.