Rasmussen Factor (RF) Inbreeding in Stallions:
The Effect on Their Racing and Breeding
The "Rasmussen Factor" (or RF) is a term used to describe inbreeding to superior
female families through different individuals. As defined by the originators and
developers of the idea, the late Daily Racing Form "Bloodlines" columnist Leon
Rasmussen and his colleague, Rommy Faversham, the inbreeding must occur through
the sire and the dam (i.e., be on both sides of the pedigree) and the
duplication of the inbred female must be within five generations.
Thus, inbreeding to full or half siblings within four generations would
qualify, while inbreeding to the same son would not.
Rasmussen and Faversham claim an advantage for RF inbreeding based on a study of
starters at the 1996 Del Mar and Oak Tree meets.
They found 4.0% of the starters exhibited the RF in their pedigree while 6.0% of
graded stakes winners in the 1990s did the same.
Their conclusion was that RF inbreeding occurred 50% more often among the
top class runners than it did in the general population.
This would be equivalent to an Impact Value (IV) of 1.50.
In 2001, pedigree expert Roger Lyons questioned the significance of the
Rasmussen/Faversham study by noting that inbreeding is not distributed evenly
across the Thoroughbred population, with graded stakes winners having more fifth
generation and beyond inbreeding of all types than less talented runners (http://www.werkhorse.com/Teamwerk2001.pdf).
According to Lyons, this results from the fact that "ancestries that are
swirled into the shallows of the breed have less in common with one another than
ancestries that are swept along in the mainstream".
In other words, higher-class runners descend from a narrower range of
breeding stock than the population at large.
His evidence comes from data on RF frequencies in yearling sales in 1998
and 1999. Lyons found that the percentage of RF yearlings at the
Keeneland July, OBS August and Keeneland September sales exceeded the percentage
of RF yearlings at the FTK July, Texas August, Louisiana August, Washington
August and CTS August sales. He
noted that the breeding of yearlings at the first three, higher quality sales
was more typical of graded stakes winners than the breeding of the yearlings at
the other sales, especially the California sale.
It should not have been surprising then that Rasmussen and Faversham
found the result they did.
Unfortunately, they were comparing dissimilar populations that make their
observed IV unreliable. At the same time, Lyons was also quick to point out that
the superior performance of graded stakes winners is not necessarily related to
their enhanced inbreeding.
Since the RF seems to be of general interest to the Thoroughbred community at
large and breeders in particular, and because meaningful research on the RF is
relatively limited, we have undertaken a study of our own that examines the
importance of the RF in the pedigree of stallions, both on their racing career
and, to a lesser extent, their breeding career.
The population in question is all of the stallions found in the Register of
Advertised Stallions in the The Blood-Horse Stallion Register for 2003.
Although not a truly random population, it is certain that their
inclusion is independent of any association with the RF.
In all, there are 817 stallions included in the analysis.
Of these, 49 display the RF pattern while 768 do not.
Furthermore, we will not compare separate and distinct populations as did
Rasmussen and Faversham. Rather we
will measure the distribution of effects within the single population of
advertised stallions. In this
manner, we hope to avoid any potential issues arising from the improper use of
For this exercise, racing class was divided into seven categories designating
the highest level of racing success:
1 – Grade 1 winner
2 – Grade 2 winner
3 – Grade 3 winner
4 – Non-graded stakes winner
5 – Allowance winner
6 – Maiden winner
7 – Non-winner
A separate group comprised stallions that were unraced.
Table 1 summarizes the results for those stallions that did race and includes
information on starts, earnings and racing ability.
Table 1. Racing Statistics for RF and Non-RF Stallions
|NUMBER OF EXAMPLES
AVG NUMBER OF STARTS
|AVERAGE RACING CLASS
The only apparent differences are in average earnings and average earnings per
start, where it seems that RF stallions have a distinct advantage.
However, the respective median figures are virtually identical.
If we remove the top two leading earners in each group, the averages
shift dramatically to $620,554 for the RF stallions and $666,177 for the non-RF
stallions. Clearly the averages are
skewed by unusual earnings at the top end.
The leading two in the RF category are Fantastic Light and Singspiel
while those in the non-RF category are Skip Away and Silver Charm.
The distortion in average earnings compared to median earnings may be
attributed to the inflated purse structures of several key races in the US,
Japan, Hong Kong and Dubai.
Overall, however, the data indicate no difference in racing ability between the
RF and non-RF stallions.
A more detailed examination of RF and racing class is presented in Table 2.
Table 2. RF Stallions by Racing Class
The variation in RF horses by racing class is minimal, although the 6.3% average
for graded stakes winners is slightly larger than the 5.3% average for
non-graded stakes winners.
Nevertheless, a chi-square test involving the two populations (RF and non-RF)
affords a P-value of 0.58, indicating that the groups are not different in a
statistically significant sense. In
addition, the increase in %RF from G1 to G2 to G3 winners is the opposite of
what one expects from a RF effect favoring performance.
The last analysis of RF and racing performance for stallions involves the
distribution of racing class within each group, as shown in Table 3.
Table 3. Distribution of Racing Class for RF and Non-RF Stallions
Here again, the distributions are essentially indistinguishable and further
suggest no effect on racing performance resulting from the presence or absence
of the RF.
Finally, we will examine briefly the relationship between RF and stallion
breeding performance. In this case,
we isolate those stallions from within each subpopulation that have a
Proficiency Index (PI) of 2.00 or more through 2001.
PI is a measure of stallion performance including an earnings and a stakes
production component. Fewer than
200 sires of North American runners normally qualify with a PI of at least 2.00.
Table 4 summarizes the results.
Table 4. Leading Sires From Among RF and Non-RF Stallions
||%AMONG LEADING SIRES
Among the 49 RF stallions, only one (2.0%), Quiet American, has a PI of 2.00 or
This contrasts with 8.7% for the non-RF stallions.
Although it may appear as if the RF in a sire's pedigree is detrimental
to stud performance, we observe once more through the chi-square test that the
difference is not statistically significant (P-value 0.12, which is greater than
the P-value of 0.05 required for statistical significance).
This study confirms
that the racing performance of stallions is unrelated to the appearance of a RF
in their pedigree. Stud performance seems to be similarly unaffected.
In general, the results support the arguments made earlier by Lyons and, more
importantly, raise serious issues about the actual validity of the RF concept.