Dosage Index and birthdates for 2025 Kentucky Derby contenders

It’s an annual rite of spring for some Kentucky Derby (G1) handicappers, consulting a contender’s Dosage Index as a shorthand guide to the stamina in his pedigree. I regard it as more akin to reading tea leaves than offering meaningful insights into a horse’s genetic inheritance, or aptitude for 1 1/4 miles on the first Saturday in May at Churchill Downs.

Indeed, although I came of age a long time ago, when the Dosage theory was in its heyday as a Derby handicapping tool, I never embraced it for that purpose. Even as a teenager, I experienced schadenfreude when Creme Fraiche (1985) won the 1 1/2-mile Belmont (G1) with a Dosage Index that was off the charts, and Strike the Gold (1991) captured the Derby with a 9.00 number that was well beyond the theoretical 4.00 threshold.

Critique of Dosage for Derby handicapping 

My opposition was based primarily on how the Dosage Index is calculated. It’s not quantifying the sum total of the ancestors through the first four generations; only a select number of stallions whose influence in pedigrees has put them on the list of “chefs-de-race.” If a stallion is not included in that elite sires’ club, then by definition, he doesn’t count in the formula.

Moreover, any female influence in its own right is ignored. The sensible reason is that Dosage can’t quantify it: mares don’t have as many foals, limiting the ability to assign aptitudes. However, the analysis omits a significant component of the pedigree: the maternal line. 

These limitations created obvious anomalies at the time and occasionally prompted some artful retrofitting of sires. Yet, it’s become glaringly out of touch over the years because the chef-de-race list is not being updated. Such breed-shapers as Storm Cat and Sunday Silence, along with other pivotal players in contemporary pedigrees – e.g., Into Mischief, Tapit, Deputy Minister, and Distorted Humor – are ignored altogether in the Dosage calculation. How can a tool be viable in their absence?

I also disliked this use of Dosage because its re-orientation departed from its historical roots in Europe. (If you’re game to go there, you can find a brief summary of its history in my first jeremiad against the Dosage Index for the Derby.) 

The flawed methodology, now compounded by an obsolete list of chefs-de-race, makes the Dosage Index tantamount to a pedigree horoscope. To be fair, there is logic to Dosage, but it is waning as the qualifying sires are receding further back in pedigrees. Only the first four generations contribute “points” to the formula. 

Interpreting the Indices for 2025

With that context, a Dosage Index (DI) above the magical 4.0 shouldn’t be interpreted as a caveat, let alone put you off an otherwise logical contender. Sovereignty’s DI of 4.33 comes by airbrushing out Into Mischief, Bernardini, and Empire Maker. Tiztastic’s borderline 4.0 completely whiffs on the fact that he’s inbred 3x2 to Tapit.

Citizen Bull (9.0) and Burnham Square’s (7.80) lofty DIs are due to paternal grandsire Unbridled’s Song’s 2019 designation as a chef-de-race with an “Intermediate” aptitude. His sire, Unbridled, had been classified as a “Brilliant/Intermediate” influence, as if he’s responsible for imparting more speed than classic stamina.

Those debatable categories tilt the DI in a speedier direction. You would find lower DIs using the pre-2019 chef-de-race list without Unbridled’s Song. Thus, you’d see Citizen Bull with a 5.0 and Burnham Square’s as 4.60. 

Either way, I agree that Citizen Bull has questions to answer at the Derby distance, but my reasoning has nothing to do with the Unbridled’s Song/Unbridled factor. And I disagree with Dosage’s take on Burnham Square, who has a stronger case to stay. 

Coal Battle’s figure is likewise implicated, with a DI as low as 2.20 without Unbridled’s Song but 3.80 with him. In his case, Dosage is arguably overestimating the stamina influences; he has them, but they’re actually mediated through speedier sires who don’t count. 

Because Dosage doesn’t credit Unbridled as a classic influence, but places him on the speedier side of the equation, it inflates the DIs of Publisher (4.33) and Luxor Cafe (5.00). Both would also benefit if Unbridled’s son Empire Maker factored in the points. 

Finally, note that DIs can be based on a vanishingly small number of points. For example, Rodriguez, Flying Mohawk, and East Avenue have the same DI of 3.00, but that figure leans on very different foundations. 

While East Avenue has more chef-de-race sires, generating a DI from a healthy 24 points, Rodriguez and Flying Mohawk’s DIs are calculated from just four points apiece. Those points trickle down from a pair of great-great grandsires in each case. For Rodriguez, it’s Gone West and Blushing Groom (not Into Mischief or Deputy Minister), while Flying Mohawk can thank Halo and Fappiano (not Storm Cat, Sunday Silence, or Candy Ride). 

Owen Almighty’s DI of 3.57 illustrates how narrow the calculation can be. All of his points come through paternal grandsire Gone West, himself a chef-de-race whose own sire Mr. Prospector and broodmare sire Secretariat are also counted. By the lights of Dosage, the other three-fourths of the pedigree are nonexistent, including his close inbreeding to Storm Cat. 

Derby favorite Journalism has the exact same DI as Owen Almighty, although his pedigree is more robust in the classic department. His paternal grandsire, Smart Strike, provides the lion’s share of his points, and on his dam’s side, A.P. Indy throws in a couple more. His colossus of a sire, Curlin, doesn’t enter the reckoning, other than as a passive vector of Smart Strike. The influence of broodmare sire Uncle Mo is screaming into the void, according to Dosage.

What’s in a date?

Along with the Dosage in the above table, we’ve included the foaling dates of the 2025 Kentucky Derby entrants. 

All things being equal, horses born later in the season could be at a developmental disadvantage compared to their rivals. May foals American Promise, Chunk of Gold, Baeza, and Rodriguez are on an upward curve at the moment, which signals they’re catching up. 

Conversely, horses born earlier may still be later-maturing types based on pedigree, mentality, or physique. The light bulb’s coming on for one of the oldest runners, Publisher, while Final Gambit is rapidly improving, and Sandman has finally put it all together.

As with any tool to evaluate contenders, it’s better to view the birthdays from the perspective of their overall records. 

For further perspective, here are the corresponding data points for Kentucky Derby winners going back to 1997.

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