Danehill was one of the most influential stallions the world has seen. He was champion sire nine times in Australia, three times in Great Britain and Ireland, and twice in France, and he sired 89 Group 1 winners. In 2011 he was also the leading broodmare sire in North America. Look closely at his pedigree and you'll find one mare in it twice: Natalma, the dam of Northern Dancer. She appears once on his sire's side through a son, and once on his dam's side through a daughter.
That pattern has a name. It's called the Rasmussen Factor, usually shortened to RF. You'll see it mentioned in stallion promotions, yearling sale notes and pedigree columns, often as a selling point.
Before we start, a word on what this guide is. It's based on my own research and my own interpretation. Pedigree students argue about the Rasmussen Factor, and many people can and will see it differently to me. If you're new to the idea, I hope this gives you a solid place to start.
What is the Rasmussen Factor?
In plain English, a horse has the Rasmussen Factor when:
- The same superior mare (a proven racemare or producer) appears twice in its pedigree,
- once on the sire's side of the pedigree and once on the dam's side,
- through two different children of hers: two sons, two daughters, or one of each (not the same horse twice),
- within the first five generations.
You count generations the same way as in my linebreeding guide: the sire and dam are generation 1, the grandparents generation 2, and so on. So "Natalma 3x3" means she sits three generations back on both sides.
Three examples: Danehill, Victor Ludorum and Winx
Look again at Danehill's pedigree at the top of this guide. Northern Dancer is a son of Natalma. Spring Adieu is a daughter of Natalma. Two different children, one on top and one underneath, both three generations back. That's a 3x3 Rasmussen Factor, and because it comes through a son and a daughter, it's what breeders call sex-balanced (more on that below).
Victor Ludorum, winner of the French 2,000 Guineas in 2020, has an RF to a different mare, Helen Street, the dam of Street Cry:
Helsinki and Historian are both daughters of Helen Street. That's a 3x3 RF through two daughters. It's a perfectly good RF.
Winx has an RF to Natalma, like Danehill, just further back:
Raise the Standard is a daughter of Natalma, and Northern Dancer is a son. That's Natalma 5x5.
An RF doesn't make a good horse. It concentrates a good family, and that can shift the odds in your favour.
Does it matter who the mare is?
The original idea was about doubling up on a superior mare or a blue hen (a dam whose family keeps producing top horses generation after generation). Natalma, La Troienne and Special are famous global examples. In Australia, Shantha's Choice (dam of Redoute's Choice) is a good local one.
What counts as "superior"? Faversham and Rasmussen later treated any mare who turns up twice within five generations as superior: to appear twice, they argued, she must have been influential to get there. Critics say that lets a lot of very ordinary mares count.
Science backs the idea that who you double up on matters. A 2018 University of Sydney study of more than 135,000 Australian racehorses found that not all ancestors are equal: overall, more inbreeding meant worse racing, but doubling up on some ancestors was linked to better racing. So the ancestor you duplicate matters.
Where the name came from
Leon Rasmussen, who wrote the "Bloodlines" column in America's Daily Racing Form, set it out as a defined method in a speech to breeders in Sydney in August 1993. In March 1994 publisher Jack Werk named it "the Rasmussen Factor", and in 1999 Rasmussen and his protégé Rommy Faversham published the main book on it, Inbreeding to Superior Females.
Why breeders should use it: the supporters' case
The supporters' claim is simple: the RF produces better racehorses. Their evidence is that RF horses turn up among top winners more often than among horses in general. The main figures come from Rommy Faversham:
- United States. About 4 in every 100 horses in a general sample had an RF. Among graded stakes winners of the 1990s, it was about 6 in every 100. So RF horses turned up at the top roughly one and a half times as often as you'd expect.
- Australia. Between 2000 and 2022, horses with an RF won about 18 in every 100 Group 1 races. Winx's 25 Group 1 wins count as 25 Group 1 races in that figure.
- Australian Racing Hall of Fame. When Faversham counted in 2023, about 3 in every 10 Hall of Fame horses had an RF (21 horses), including Carbine, Star Kingdom, Danehill, Makybe Diva and Winx.
Faversham himself notes that nobody knows how common RFs are among Australian horses in general, so there's nothing to compare these two figures with.
These figures are debated, and I'll come to the other side below.
Why might it work?
Supporters give three main reasons:
- It doubles up on a proven mare. A great stallion can have thousands of foals, so a few stars are almost guaranteed. A mare only has a few foals in comparison. If two or more of her children became important, her family has typically proven its value, so she's worth having in a pedigree.
- It gives the foal two chances instead of one. A mare's influence fades with every generation she moves further back. Bringing her in twice gives the foal two chances to inherit her best genes.
- It puts the spotlight on mares. Most breeding plans are built around stallions. The RF deliberately builds around the mares whose families keep producing speed, stamina and class.
How strong is the effect?
The useful way to think about it is how much of the mare the foal carries:
- A mare appearing once in a pedigree, three generations back, normally gives a foal about 12.5% of its genes. At 3x3, that doubles to about 25%, as much as a grandparent.
- At 4x4 it goes from about 6% to about 12%.
- At 5x5 it goes from about 3% to about 6%.
So a close RF really does concentrate the mare.
In my view, an RF works much like linebreeding: it's most effective when it's backed up by other linebreeding further back in the pedigree. A lone 5x5 RF only lifts the mare's share to about 6%. But extra lines of her family, or of closely related horses, further back add to that share across the pedigree, and that increases the RF's overall effect.
It also matters what you're reinforcing. Doubling up concentrates whatever is in the family, the weaknesses as well as the strengths. That's why the quality of the mare comes first.
Why a son-and-daughter RF might be more impactful
Many pedigree students believe, though it isn't proven, that a duplication works best when it comes through a son and a daughter, because it keeps the mare's full genetic package in play. A son on one side and a daughter on the other has its own name: sex-balanced. Federico Tesio, one of the greatest breeders of all time, favoured duplications that came through both a son and a daughter of the ancestor, and it's the version of the RF I prefer.
How breeders can apply it
As a mating tool. Start with your mare. Find the strong mares in her first four generations, then look for a stallion who carries a different son or daughter of one of them. If you can make it a son on one side and a daughter on the other, you get sex balance as well.
A good modern Australian example is Jacquinot, winner of the Golden Rose and CF Orr Stakes:
Sliding Cube is a daughter of Shantha's Choice, and Redoute's Choice is a son. That gives Jacquinot Shantha's Choice 3x4, sex-balanced. His breeder, Lindsay Maxsted, told ANZ Bloodstock News before the 2022 Golden Rose: "The Shantha's Choice double-up was probably the first thing I saw in the blending of pedigrees."
It doesn't need an expensive stallion. A cheaper stallion carrying the right family close up can create the same pattern as an expensive stallion. That makes the RF something smaller breeders can use too.
The other side of the debate
Not everyone is convinced. Here are the main points.
Compare like with like. The original US figures compared ordinary starters at two Californian race meetings with graded stakes winners from across the country. Pedigree researcher Roger Lyons pointed out that better-bred horses carry more duplications of every kind, so RF horses may win more simply because they're better bred.
The stallion study. When Dr Steven Roman tested all 817 stallions advertised in North America in 2003, RF stallions were no more common among Grade 1 winners (6 in every 100) than among the stallions overall (also 6 in every 100). If the RF made better racehorses, he reasoned, it should have been more common among the Grade 1 winners. He concluded the RF was "unrelated" to racing performance. But he only looked at colts (more on that below).
Cause, or good company? American pedigree consultant Alan Porter argues that RFs naturally turn up in well-planned matings, so an RF may be a sign of a well-bred horse rather than the reason it's good.
Colts and fillies: does it matter which you're breeding?
This is where I think the debate gets interesting. Roman's stallion study only looked at colts. In an unrelated study, New Zealand breeder and researcher Clive Harper looked at fillies with sex-balanced duplications. He tested a few different patterns, and one of them was close to a sex-balanced RF (he counted out to six generations): a son and a daughter of the same mare.
He compared New Zealand stakes-winning fillies with fillies who raced but never placed. About 4 in 10 of the stakes winners had that son-and-daughter pattern in their pedigree, against fewer than 1 in 10 of the maidens.
So the two studies don't really contradict each other. Roman found nothing in stallions; Harper found something in fillies. If Harper is right, a study of stallions alone was never going to find it. Faversham, for his part, saw no difference between the sexes, so this remains an open question.
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So, what can we say?
There's still plenty of debate about what an RF actually delivers. As Clive Harper put it, you can't guarantee a top horse from any mating, and "really you can do no better than improve percentages". That's how I see the RF: as one way of improving the percentages.
My view. I treat the RF as a form of linebreeding. I like to see one when:
- the mare is genuinely elite,
- she comes through a son and a daughter, and
- the RF is backed up further back in the pedigree, ideally as part of a pedigree "recipe" of ancestors with a proven affinity for each other.
Together, these reduce the genetic variation across the pedigree and stacks the deck with aces. I pay most attention to it in fillies, where I think Harper's evidence is strongest. An RF to a moderate mare, with nothing else going for it, doesn't excite me. An RF is one ingredient in a pedigree recipe, not a defining trait.
Champions usually have something unique in their pedigree. If they didn't, every horse would be a champion.
If you'd like to dig deeper, Faversham and Rasmussen's book has been reprinted by Bluebloods and is the place to start. Read it alongside Roman's study.
Michael MarosaThe Pedigree Nerdmichael@digitalbloodstock.com
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Sources
- Rasmussen L (1993). Speech to the New South Wales Bloodhorse Breeders' Association seminar, Sydney, 8 August 1993. Republished in Bluebloods, 2024, with an introduction by R Faversham ("Leon Rasmussen's wise words remembered: commentary on Leon's speech", Bluebloods 2024 Issue 3).
- Faversham R & Rasmussen L (1999). Inbreeding to Superior Females: Using the Rasmussen Factor to produce better racehorses. Australian Bloodhorse Review (reprinted by Bluebloods).
- Faversham R (2001). "Inbreeding to superior female lines: the performance of the Rasmussen Factor in 2000." Thoroughbred Times, 7 April 2001.
- Faversham R (2021). "The Triple RF." Bluebloods, January/February 2021.
- Faversham R (2023). "Dams getting duplicated: Down under." Bluebloods, 2023 Issue 5, pp. 78 to 80.
- Roman SA. "Rasmussen Factor (RF) Inbreeding in Stallions: The Effect on Their Racing and Breeding Career." In Dosage: Pedigree & Performance (revised edition, 2016), Appendix VII, pp. 286 to 289. Includes a summary of Lyons R (2001), Teamwerk 2001, Werk Thoroughbred Consultants.
- Porter A (2013). "Complacent about the Rasmussen Factor?" Pedigree Consultants, 28 September 2013.
- Todd ET et al. (2018). Founder-specific inbreeding depression affects racing performance in Thoroughbred horses. Scientific Reports 8: 6167.
- "It's In The Blood" (2022). ANZ Bloodstock News (Jacquinot and Lindsay Maxsted); Jacquinot pedigree and race record, Magic Millions catalogue.
- Danehill: Australian Racing Hall of Fame profile, Racing Victoria; Wikipedia (Danehill).
- Winx and Victor Ludorum pedigrees: Wikipedia (Winx; Victor Ludorum).