by Thomas M. Young III and Anne Peters ©

The Good, the Bad, and the Inbred: why stallions succeed

One of the most compelling questions in Thoroughbred breeding is why an outstanding racehorse can fail as a stallion while others can succeed. If race record doesn't dictate success, what does?

To find an answer, a study was undertaken to see if there are any pedigree differences between a top-siring group of stallions and a group of stallions that have disappointed. Tom Young compiled the data for the study, diligently researching the pedigrees of current stallions, and noting inbreeding, linebreeding, and male and female sources of inbreeding as they occurred. He also prepared the data in the tables to create a composite profile of the stallions in each of two groups. (Complete lists of the sires used can be sent upon request, but it didn't seem appropriate to include the lists here, especially the list of stallions considered unsuccessful.)

The top-siring group, here referred to as Group A, was easy to narrow down. In the 1999 The Blood-Horse Stallion Register, there is a list of leading sires in order of AEI (Average Earnings Index), with the highest belonging to Danzig (5.17). The first 50 stallions were taken for the study, ending at an AEI of 2.24. The average AEI for the group was 2.91, with an average CI (Comparable Index) of 2.73, an extremely superior lot. Their racing ability was usually of the highest class, but not critical to the study, since the point was their proven superiority at stud.

Group B was more difficult to generate. For fairness, it was decided to find 50 current stallions with enough progeny to ensure their mediocrity. Stallions appearing in same The Blood-Horse Stallion Register were considered if they had at least 75 foals to race through 1999. The cut then came down on the basis of low AEIs. In this group, they range from 1.25 to .48, and all but one had an AEI lower than their CIs, meaning they were downgrading their mares.

Lest one presume Group B was stacked with dregs, it was, in fact, dominated by superior racehorses. Of the fifty stallions, 80% earned more than $100,000 on the track, and 30% earned over a million dollars. All but eight were stakes winners, and those eight included sons of Northern Dancer, Mr. Prospector, Seattle Slew, Storm Bird, and Dixieland Band. In other words, they owned the highest qualifications on the track, in pedigree, or most often, both. Half stood part of their stud career in Kentucky and were sent packing after disappointment. The average AEI for the 50 Group B stallions was 1.0001, their average CIs 1.40.

The pedigree of each stallion was examined for five generations using the following definitions. Inbreeding is considered to be duplications that appear 4x4 or closer, and 3x5, and only if they appears in both parents. Linebreeding is considered to be duplications appearing beyond 4x4, such as 4x5 and 5x5. Inbreeding was also noted for each stallion, his sire, dam, broodmare sire, and second dam, also at five generations.

The results of this initial analysis appears in Table I, which focuses exclusively on inbreeding, not linebreeding. Note that the degree of inbreeding was identical between the stallions in Group A and Group B, both with 42% inbreeding. This is high, since the breed average has been guestimated to be around 25 percent inbred individuals.

Table I. Percent of inbred individuals in the pedigrees of two groups of stallions (Group A=top sires; Group B=poor sires)

Differences began to appear when looking at the stallions' ancestry, however. In Group A, the sires of the stallions included fewer inbred individuals (24%) than the sires of stallions in Group B (32%), but a reverse trend appeared with their dams. The dams of Group A stallions were almost twice as often inbred (42%) than the dams of Group B stallions (22%).

In the next generation, the percentages for broodmare sires and second dams were similar between both groups. Thirty-two percent of the Group A broodmare sires were inbred while 30% of the Group B broodmare sires were inbred. Thirty percent of the Group A second dams were inbred while 28% of the Group B second dams were inbred.

What this seems to show is that while the percent of inbred individuals in both groups averaged out to be the same, relatively high at 42% (compared to the estimated breed norm), the way they got there was dramatically different. In composite, top siring stallions (Group A) tended to have sires who were more inbred than average and out of dams who were much more inbred than their mates. At the same time, the sires still had enough of the same bloodlines to maintain the dam's high level of inbreeding in the resulting foal. Since the dams were themselves out of moderately related parents, the stallions in Group A were the result of the following pattern: moderately inbred second dam, highly inbred dam, highly inbred foal.

Lesser stallions (Group B) were likewise the result of mating moderately inbred sires but with even less inbred dams, although the parents had enough shared bloodlines to produce a foal with a higher level of inbreeding than either parent. Their second dams were also moderately inbred, so the pattern in Group B is: moderately inbred second dam, moderately inbred dam, highly inbred foal.

So, the Group A stallions appear to be the result of two generations of accumulated inbreeding while maintaining the level of found in the dam. Group B stallions are the result of only one generation of increased inbreeding.

Could we say the Group A pattern establish a stronger pattern of genetic strength than the Group B pattern, and therefore provides a genetic consistency or prepotency not found in Group B?

Sex Balancing factors:

A further classification of inbreeding and linebreeding involves factors more commonly known as "sex balancing." This is a concept developed by Harold Hampton and further investigated by Clive Harper and Les Pratt in more recent years. Having a son and daughter strain of any duplicated parent, by providing "balance," is believed to be desirable, and having an overabundance of either colt or male factors or filly or female factors is an inbalance believed to directly affect the performance and breeding potential of the resulting foal, good or otherwise.

The concepts of sex balancing used here were taken from Les Pratt's Thoroughbred Breeders Guide (1985-1986), although the definitions used below are Tom Young's interpretations of Mr. Pratt's writings. For the two groups of stallions, the duplications were broken down into the following subgroups. Male factored inbreeding is considered to be duplications through sons of a sire or very overbalanced to male, such as three or more sons in combination with a daughter of a sire. Balanced factored inbreeding has duplications through a son and a daughter of a sire, or two sons and a daughter of a sire (not enough extra sons to be male factored), or one son and two daughters of a sire. Female factored inbreeding is considered any combination of sons and daughters of a mare, and two or more daughters of a sire. The pedigrees of both groups of stallions were evaluated using these guidelines.

Table III shows the sex factored inbreeding for Group B stallions, the disappointments. This group owned a similar profile to Group A, but had significantly more balanced factored inbreeding, slightly more male factored inbreeding, and very slightly less female factored inbreeding.

Group B stallions' sires' profiles are almost identical to their own, except for a noticeable rise on the female factored inbreeding. Their dams, on the other hand, are very different, with lower male and balanced factors, but dramatically higher female factors.

What's really interesting is what happens when you look at the next (second) generation. In Group A, the broodmare sires have a radically different profile than the sires, being lower in all factors, especially female factors. On the other hand the second dams of Group A stallions are noticeably higher in all three factors, especially balanced and female factors.

In Group B, as in Group A, the broodmare sires have lower male factors (compared to the sires), but here, they are much higher than the Group A broodmare sires in balanced and especially female factors. The Group B second dams are also similar to Group A second dams in male and balanced, but are much lower in female factors.

What conclusions, if any, can be drawn from this part of the study? The biggest difference is that Group A stallions have broodmare sires who are dramatically low in female factors. At the same time, Group B stallions have broodmare sires that are dramatically low in male factors. This implies that broodmare sires of top sires should be overtly male in influence, especially since failed sires have broodmare sires who are more overtly female in influence.

Conclusions

Given this study of composite successes and failures, we could draw several conclusions that might be used to predict which stallions will prove outstanding and which will be duds.

1.) The stallion should have an inbred dam, and should have the relatively same percent of inbreeding as his dam (see Table I).

2.) A pattern should exist with a moderately inbred second dam, highly inbred dam, highly inbred foal.

3.) The stallion should have the right kind of broodmare sire, i.e., a horse who is higher in male factors than female factors, and very low in female factors.

[Note from Tom Young: "Many thanks to Anne Peters for suggesting this study and the help with hard-to-locate horses, although she tripled the amount of work by suggesting that a control group (of poorly performing sires) would provide more meaningful observations, which it did. If anyone would like a copy of the 100 stallions used in this study, please email your address or fax number. My email address is tyoung@flash.net"]

[This article first appeared in the June 2000 issue of Owner-Breeder International, Vol 13, No. 4.]

September 2, 2001. Copyright by Thomas M. Young III and Anne Peters