STUDY: How Thoroughbred sire line success rates vary by racing surface

Many of the best Thoroughbred racehorses in North America descend from three dominant stallions of the 20th century: Northern Dancer, Mr. Prospector, and Bold Ruler.

Northern Dancer, Mr. Prospector, and Bold Ruler sired many sons who became successful sires in their own right, and this pattern has repeated down through generations, producing uninterrupted streaks of male-line descendants known as sire lines.

The Northern Dancer, Mr. Prospector, and Bold Ruler sire lines have exerted great influence on North American racing and breeding. But is their influence equal across all areas of racing? Or does it vary according to circumstances, such as the surface over which a race is conducted?

It’s well known that some stallions find more success siring dirt runners than turf horses, and vice versa. Others sire top-notch performers over synthetic (all-weather) surfaces.

Do descendants of Northern Dancer, Mr. Prospector, and Bold Ruler exhibit surface preferences depending on which sire line they hail from? Let’s crunch some numbers and find out.

Sire line success rates by surface: research methodology

To determine the most successful sire lines across the three major racing surfaces, we have reviewed the lists of leading North American sires by progeny earnings compiled annually by Bloodhorse.com, focusing our attention on the 10-year span from 2015-2024.

We have analyzed the leading sires by progeny earnings in four different categories: dirt earnings, turf earnings, synthetic earnings, and overall earnings. Any stallion who ranked in the top five in a given surface category in any given year is included in the dataset for that surface category.

After compiling the names of stallions who achieved top-five rankings between 2015-2024, we have grouped them by sire line to see which sire lines are most represented across each racing surface.

Sire line success rates by surface: data analysis

We’ll start with a look at the leading sires by overall progeny earnings, regardless of surface:

Leading sires by overall progeny earnings, sorted by year

Leading sires by overall progeny earnings, sorted by sire line

Mr. Prospector
Candy Ride (Fappiano branch)
Curlin (Smart Strike branch)
Gun Runner (Fappiano branch)
Pioneerof the Nile (Fappiano branch)
Quality Road (Gone West branch)
Speightstown (Gone West branch)
Unbridled's Song (Fappiano branch)

Northern Dancer
Giant's Causeway (Storm Cat branch)
Into Mischief (Storm Cat branch)
Justify (Storm Cat branch)
Kitten's Joy (Sadler's Wells branch)
Medaglia d'Oro (Sadler's Wells branch)
Scat Daddy (Storm Cat branch)

Bold Ruler
Constitution (A.P. Indy branch)
Mineshaft (A.P. Indy branch)
Tapit (A.P. Indy branch)
Tonalist (A.P. Indy branch)

Caro
Uncle Mo (In Excess branch)

Of the 18 stallions who ranked in the top five between 2015 and 2024, seven (39%) descend from the Mr. Prospector sire line, primarily through the Fappiano branch.

Six stallions (33%) descend from the Northern Dancer sire line, primarily through the Storm Cat branch. This group includes Into Mischief, the leading sire by overall progeny earnings each of the last six years.

The Bold Ruler sire line yielded a quartet of leading sires (22%), all from the A.P. Indy branch. They include Tapit (the leading sire of 2015 and 2016) and his sons Constitution and Tonalist.

One stallion (6%) descends from the Caro sire line: Uncle Mo. Caro descends from Nasrullah, the sire of Bold Ruler, so Uncle Mo represents a different branch of Nasrullah’s descendants than the A.P. Indy/Bold Ruler line.

Now let’s examine the leading stallions by progeny earnings on dirt:

Leading sires by progeny earnings on dirt, sorted by year

Leading sires by progeny earnings on dirt, sorted by sire line

Mr. Prospector
Candy Ride (Fappiano branch)
Curlin (Smart Strike branch)
Gun Runner (Fappiano branch)
Munnings (Gone West branch)
Quality Road (Gone West branch)
Street Sense (Street Cry branch)
Unbridled's Song (Fappiano branch)

Bold Ruler
Bernardini (A.P. Indy branch)
Flatter (A.P. Indy branch)
Lucky Pulpit (A.P. Indy branch)
Malibu Moon (A.P. Indy branch)
Mineshaft (A.P. Indy branch)
Tapit (A.P. Indy branch)
Tonalist (A.P. Indy branch)

Northern Dancer
Ghostzapper (Deputy Minister branch)
Into Mischief (Storm Cat branch)
Paynter (Deputy Minister branch)
Pioneerof the Nile (Fappiano branch)
Practical Joke (Storm Cat branch)

Caro
Uncle Mo (In Excess branch)

Here we see a small shift. The Mr. Prospector sire line is still strongly represented with seven out of 20 stallions on the list (35%). But the Bold Ruler line matches that number with seven representatives (35%), all from the A.P. Indy branch.

In contrast, the Northern Dancer sire line has lost a little ground with only five names (25%), while the Caro line is once again represented by one stallion (5%). However, the Northern Dancer line includes Into Mischief, the leading dirt sire each of the last six years.

The leading stallions by progeny earnings on turf paint a very different picture:

Leading sires by progeny earnings on turf, sorted by year

Leading sires by progeny earnings on turf, sorted by sire line

Northern Dancer
Giant's Causeway (Storm Cat branch)
Justify (Storm Cat branch)
Kitten's Joy (Sadler's Wells branch)
Into Mischief (Storm Cat branch)
Medaglia d'Oro (Sadler's Wells branch)
Not This Time (Storm Cat branch)
Scat Daddy (Storm Cat branch)
War Front (Danzig branch)

Mr. Prospector
City Zip (Carson City branch)
English Channel (Smart Strike branch)
Twirling Candy (Fappiano branch)

Halo
More Than Ready (Southern Halo branch)

Bold Ruler
Tapit (A.P. Indy branch)

Caro
Uncle Mo (In Excess branch)

Only 14 stallions achieved a top-five ranking between 2015 and 2024. A whopping eight of these (57%) came from the Northern Dancer sire line.

The Mr. Prospector line managed to contribute only three stallions (21%) to the list, though they include four-time leader English Channel as well as Twirling Candy, who led the rankings in 2024.

The Bold Ruler sire line turned up only one representative (7%), the same as the Halo (7%) and Caro (7%) lines.

The percentages shift yet again when viewing the leading stallions by progeny earnings on synthetic:

Leading sires by progeny earnings on synthetic, sorted by year

Leading sires by progeny earnings on synthetic, sorted by sire line

Northern Dancer
Bucchero (Storm Cat branch)
Ghostzapper (Deputy Minister branch)
Giant's Causeway (Storm Cat branch)
Hard Spun (Danzig branch)
Kitten's Joy (Sadler's Wells branch)
Old Forester (Storm Cat branch)
Mendelssohn (Storm Cat branch)
Reload (Danzig branch)

Mr. Prospector
Court Vision (Gulch branch)
Curlin (Smart Strike branch)
English Channel (Smart Strike branch)
Munnings (Gone West branch)
Niigon (Fappiano branch)
Speightstown (Gone West branch)
Where's the Ring (Seeking the Gold branch)

Icecapade
Milwaukee Brew (Wild Again branch)

Halo
Silent Name (Sunday Silence branch)

Caro
Souper Speedy (In Excess branch)

Here we find the Northern Dancer and Mr. Prospector sire lines closely matched, respectively yielding eight stallions (44%) and seven stallions (39%) from a total of 18.

The Bold Ruler line is strikingly absent (0%). In its place, we find three other sire lines represented by one stallion apiece: the Icecapade line (6%), the Halo line (6%), and the Caro line (6%), with the latter represented this time by three-time leading synthetic sire Souper Speedy rather than Uncle Mo.

One note: Icecapade is a son of Nearctic, the sire of Northern Dancer, so Milwaukee Brew represents a different branch of Nearctic’s descendants than the Northern Dancer line.

Conclusions

The Northern Dancer and Mr. Prospector sire lines have enjoyed resounding success for many reasons, one of which is surface versatility. Multiple stallions from both lines can be found among the leading sires by progeny earnings on dirt, turf, and synthetic from 2015-24.

True, the Northern Dancer line is particularly strong on turf (producing 57% of leading turf sires from 2015-2024 compared to 44% on synthetic and 25% on dirt), and the Mr. Prospector line shines on dirt and synthetic (yielding 35% and 39% of leading sires compared to 21% on turf). But as stallions like Into Mischief and English Channel demonstrate, it’s possible for elite dirt sires to hail from the Northern Dancer line, and for top-tier turf sires to come from the Mr. Prospector line.

In contrast, the Bold Ruler sire line—specifically the dominant A.P. Indy branch—is all about dirt prowess. Whereas the A.P. Indy branch yielded 35% of leading dirt sires from 2015-2024, it produced only 7% of leading turf sires and 0% of leading synthetic sires.

These statistics can be used when analyzing unraced horses, or horses trying a particular surface for the first time. Look at pedigrees, examine sire lines, and see which horses trace back to Northern Dancer, Mr. Prospector, or A.P. Indy/Bold Ruler. Surface preference clues helpful for betting are waiting to be found.