Dosage Index, birthdates for 2026 Kentucky Derby contenders

Every year around Kentucky Derby (G1) time, the old talking point about the “Dosage Index” resurfaces, as if it’s an accurate measure of a contender’s distance capacity for 1 1/4 miles. 

While we’re providing this data for handicappers who continue to find it of interest, I must reiterate its inherent limitations. Indeed, I’ve never been a fan of Dosage, in its modern incarnation applied to the Derby, and conscience obliges me to go on my annual rant about the calculation. 

What is Dosage?

The Dosage theory has a fascinating history in Europe, where it began as a method of identifying elite stallions (chefs-de-race) and quantifying their proportions in the pedigrees of classic winners. In a subsequent development, those elite stallions were classified according to the aptitudes they transmitted to their progeny, on a broad spectrum ranging from speed to stamina.

The American variation of this theory, created by Dr. Steven Roman, made significant modifications. Points were assigned to the chefs-de-race in the first four generations, divvied up according to their classifications, which served as the basis for calculating the Dosage Index (DI). The putative cutoff for Derby winners was a DI of 4.00 – anything higher was supposed to indicate lack of stamina to negotiate 1 1/4 miles on the first Saturday in May.

But the theory kept generating exceptions when horses above the 4.0 threshold won the Derby! Looking at just the past decade or so, five of the last 11 Derby winners defied their Dosage Index, including Triple Crown winner American Pharoah and 2025 Horse of the Year Sovereignty. 

Dosage limitations

One of my longstanding concerns about Dosage from a handicapping perspective is its strict limits on which ancestors count. Stallions who are not labeled as chefs-de-race are simply ignored, as if they contribute 0 to the contender’s staying capacity. 

For years, the Dosage calculations were outdated because the list of chefs-de-race was outdated. Older stallions disappeared beyond the fourth-generation horizon, and the new classic sires like Into Mischief, Empire Maker, and Tapit were literally not in the equation.

This particular criticism has just been addressed recently, with 23 contemporary sires now added as chefs-de-race. Even so, a few glaring omissions remain (no Sunday Silence, Storm Cat, Seeking the Gold, Deputy Minister). 

Armed with these updates, if you recalculate the DI for those allegedly defiant Derby winners, they nearly all meet the 4.00 qualification. American Pharoah’s DI today drops to 1.13, Always Dreaming is halved to 2.50, Mandaloun plummets to 1.86, and Sovereignty becomes 2.16. (We are preserving their original DIs in the table above, to reflect the numbers designated on their actual Derby Days.)

Alas, poor Nyquist’s updated DI remains above the cutoff at 4.33, and that brings us to the bigger issue with Dosage. While certain ancestors are superior influences on the breed, it’s too reductionist to envision them as the only ones making any meaningful contribution to an individual contender’s makeup. 

Aside from the ability of less fashionable sires to factor in pedigrees, up-and-coming stallions will make an impact long before they can be assessed as chefs-de-race. In other words, Dosage will always be playing catch-up to incorporate successful sires. The contender that you doubt today, thanks to Dosage, could be the Derby winner whose DI gets lowered retroactively.

Another limitation of Dosage is its complete avoidance of the female factor. Not a single mare counts toward the calculation, never mind the deep matrilineal influences that endure in families over centuries. The same tail-female lines keep producing, underscoring that they’re contributing something of their own, not just passing on whatever they’ve received from stallions.

With all of those caveats, here are the Dosage Indices for the 2026 Kentucky Derby contenders, along with their birthdates.

Dosage and the 2026 Kentucky Derby

All but two meet the desired 4.0 criterion, Fulleffort (4.09) and So Happy (7.00). Each is a case in point about how to treat Dosage with a degree of caution.

Fulleffort is by Liam’s Map, whose son Burnham Square just crushed the 1 1/2-mile Elkhorn (G2) by a stakes-record margin at Keeneland. Burnham Square brought an absurd 7.80 DI into last year’s Derby, where he finished a troubled sixth. His recalculated DI is now 3.62 because of adjustments involving his grandsires.

Fulleffort’s half-sister, Power Squeeze, won the 1 1/4-mile Alabama (G1) at Saratoga. Their dam is by chef-de-race Awesome Again and out of a mare by Roanoke, who does not qualify as a chef-de-race. Yet is it conceivable that Roanoke, by chef-de-race Pleasant Colony and out of a mare by stout chef-de-race *Sea-Bird, isn’t imparting some stamina to the mix?

The most persuasive piece of evidence is Fulleffort’s racetrack performance, where his Jeff Ruby Steaks (G3) breakthrough suggests that he needs at least 1 1/8 miles to bring out the best in him. If you like Fulleffort in principle, don’t let the Dosage put you off. He won’t lose for lack of stamina. 

So Happy is by champion sprinter Runhappy, but the irony is that’s not the reason for his lofty DI. Runhappy’s DI is 3.00, and on pedigree, he should have been a router. Not only is he a son of 2010 Derby winner Super Saver (himself out of a mare by chef-de-race A.P. Indy), but Runhappy’s dam has chefs-de-race Unbridled and Nijinsky II as prominent factors.

So Happy’s main Dosage problem is that there are no chefs-de-race in his dam’s half of the pedigree that are close up enough to count. She’s by champion Blame, who will go down in history for beating Zenyatta in the 2010 Breeders’ Cup Classic (G1). As a blueblood from an excellent family, Blame is emerging as a fine broodmare sire, and he presumably helped So Happy to stay 1 1/8 miles in the Santa Anita Derby (G1). 

Yet as Runhappy himself demonstrates, having classic influences on paper doesn’t necessarily translate to classic aptitude in an individual. It depends upon which genes are being expressed. 

Dosage strikes me as far too mechanistic in assigning specific points in a given category, as if grandsire X is uniformly responsible for Y amount of some quality in his grandchildren. The kaleidoscopic marvels of genetics can’t be neatly apportioned in that way. 

That brings us to the opposite problem with Dosage for the Derby, with its potentially misleading sense of security. Six Speed appears to be a slam-dunk with a 1.61 DI, reflecting grandsires Giant’s Causeway and Medaglia d’Oro, and A.P. Indy lurking in the third generation. Yet his Dubai career, and aggressive running style, give cause for pause.

Six Speed’s dam, Browse, was a sprinter-miler who’s produced a stakes-winning sprinter in Pipit (by Quality Road). Is she transmitting more of the quality speed in her pedigree? 

Six Speed’s sire, Not This Time, doesn’t give a clue one way or another because of his extraordinary range. Not This Time can give you world-class sprinters like Cogburn, 1 1/4-mile stars like Epicenter and Magnitude, and a dirt marathoner like Next. 

You can’t guess which ones are which from their DIs. Next is 1.77, Magnitude is 1.93, Epicenter is 1.11, and Cogburn’s is as low as 1.18.