Nearly Half The Kentucky Derby Field Is Racing Against A Half-Brother And if the horse in the next stall isn’t a half-brother, he’s definitely some kind of cousin.

In every Kentucky Derby, breeding is the silent, invisible force that can play a huge role in deciding the outcome of the race. What’s unusual about the 2018 Kentucky Derby is the sameness of the breeding.

When the race kicks off at about 6:30 p.m. on Saturday, the 20-horse field will feature nine runners who were sired by just three stallions, which means 45 percent of the runners will be competing against what humans might consider a half-brother. And that’s the third-highest percentage in the 94 Kentucky Derbys for which horseracingnation.com has sire data on all the competitors.

Leading the family business this year is the stallion Scat Daddy, who sired four colts in the race: The morning-line favorite Justify (3-1) and the second-favorite Mendelssohn (5-1), as well as long shots Flameaway (30-1) and Combatant (50-1). To see how unusual it is for four half-siblings to enter the same Kentucky Derby, we looked at the sires of every Derby horse on record. Assuming none of his progeny scratch, this year Scat Daddy will tie Chicle as the most prolific derby sire for any one year. (Chicle sired four racers in the 1923 Derby.)

Scat Daddy isn’t the only stallion with multiple offspring in this year’s race. Three competitors were sired by Curlin: Vino Rosso (12-1), Good Magic (12-1) and Solomini (30-1). And just to keep it interesting, Bolt d’Oro (8-1) and Enticed (30-1) are both by the stallion Medaglia d’Oro.

Scat Daddy died unexpectedly in 2015 at age 11, so this crop of 3-year-olds will be the second-to-last of his direct offspring to run for the roses. But Curlin, the two-time horse of the year, has a chance to enter the record books as a stallion. Overall, Curlin has already sired eight Derby contenders and he’s only 14, which means he may be able to keep breeding for another decade or so. According to our research, the record for fathering Derby runners is held by Black Toney, who sired 16, including Derby winners Black Gold and Brokers Tip.

Curlin could break into the ranks of top Derby sires Sires with at least five offspring who raced or are set to race in the Kentucky Derby, 1875* through 2018 RK sire born Age (if alive) Offspring in the derby 1 Black Toney 1911 16 – 2 Sir Gallahad III 1920 13 – 3 A. P. Indy 1989 29 10 – North Star III 1914 10 – Ballot 1904 10 – Broomstick 1901 10 – 7 Tapit 2001 17 9 – Heliopolis 1936 9 – 9 Curlin 2004 14 8 – Giant’s Causeway 1997 8 – Nasrullah 1940 8 – The Porter 1915 8 – 13 Scat Daddy 2004 7 – Malibu Moon 1997 21 7 – Tiznow 1997 21 7 – Distorted Humor 1993 25 7 – Cox’s Ridge 1974 7 – Mr. Prospector 1970 7 – Hail to Reason 1958 7 – Nashua 1952 7 – Bull Dog 1927 7 – Blue Larkspur 1926 7 – Man o’ War 1917 7 – Chicle 1913 7 – Sweep 1907 7 – 26 Mineshaft 1999 19 6 – Pulpit 1994 6 – Unbridled’s Song 1993 6 – Smart Strike 1992 6 – Unbridled 1987 6 – Wild Again 1980 6 – Danzig 1977 6 – Alydar 1975 6 – Halo 1969 6 – Khaled 1943 6 – Pharamond II 1925 6 – 37 Into Mischief 2005 13 5 – Maria’s Mon 1993 5 – El Prado 1989 5 – Dynaformer 1985 5 – Gone West 1984 5 – Gulch 1984 5 – Dixieland Band 1980 5 – Sovereign Dancer 1975 5 – Seattle Slew 1974 5 – Damascus 1964 5 – Graustark 1963 5 – Bold Bidder 1962 5 – Herbager 1956 5 – Ambiorix 1946 5 – Count Fleet 1940 5 – Alibhai 1938 5 – Bull Lea 1935 5 – Royal Minstrel 1925 5 – Sickle 1924 5 – Pompey 1923 5 – Wrack 1909 5 – Fair Play 1905 5 – McGee 1900 5 – Show more rows *Sire data is incomplete for many of the early Derbies. Sources: Horse Racing Nation, Equineline.com

The 2018 Derby is something of a chance to avenge the 2007 Kentucky Derby for both Curlin and Scat Daddy, who had disappointing results in that race. Curlin finished a respectable third, but went on to beat that year’s Derby winner, Street Sense, to win both the Breeders’ Cup and the Preakness Stakes, the second leg of the Triple Crown. Scat Daddy had been a favorite at the betting windows before the 2007 Derby, but he finished a dismal 18th after suffering an injury.

Why have these two returned so prominently, via their progeny, to the track? The answer lies both in talent and modern breeding practices.

When thoroughbred racehorses shift to their second career as studs, it’s never a sure thing that they will produce high-quality foals. But as their progeny start showing promise, the stallions are held in higher regard, so breeders send in better and better mares, combining the most promising bloodlines in hopes of producing more competitive offspring.

“Once a horse shows aptitude and ability, his commercial appeal increases,” said John Sikura, president of Hill ’n’ Dale, the Kentucky breeding farm where Curlin lives.

Both Curlin and Scat Daddy attracted more interest from breeders as their progeny were increasingly successful. Curlin’s stud fee was set at $150,000 for the 2018 season, and Scat Daddy was also commanding six figures before his untimely death.

The fact that Curlin and Scat Daddy have so many horses in the field is also a function of the modern approach to breeding: Keep ’em busy.

Stallions today might “cover,” or breed with, more than 200 mares in a season, whereas the great sires of the past were breeding to a list, or book, of about 50 mares.

“What you’re seeing is the ‘big book’ phenomenon,” said Jamie Hill, co-owner of McMahon and Hill Bloodstock agency.

In 2014, when this year’s crop of Derby contenders was bred, Curlin bred with 152 mares and Scat Daddy covered 202, as reported to the Jockey Club. Of their more than 350 matings, seven progeny have made it to the 2018 Kentucky Derby, which only takes 20 horses every year.

Shared parentage, though, is nothing in the big picture of breeding thoroughbred race horses. Look a few generations back, and they’re practically all related.

Certain names crop up over and over in the pedigrees of this year’s Derby runners. Mr. Prospector, for example, shows up in every 2018 Derby runner’s lineage.

Mr. Prospector got lucky Relationship of Mr. Prospector to every horse in the 2018 Kentucky Derby Mr. Prospector was the horse’s … Horse great-grandfather great-great-grandfather great-great-great-grandfather Combatant ✓ ✓ ✓ Good Magic ✓ ✓ Vino Rosso ✓ ✓ Flameaway ✓ ✓ Justify ✓ ✓ Magnum Moon ✓ ✓ Mendelssohn ✓ ✓ Solomini ✓ Enticed ✓ ✓ Firenze Fire ✓ ✓ Free Drop Billy ✓ ✓ Hofburg ✓ ✓ Noble Indy ✓ ✓ Promises Fulfilled ✓ ✓ Audible ✓ Bravazo ✓ Instilled Regard ✓ Lone Sailor ✓ My Boy Jack ✓ Bolt d’Oro ✓ Source: Equineline.com

“He’s a dominant force. He was a sprinter, and he set a track record,” said Hill, adding that the horse’s widespread influence is even more impressive for the fact that Mr. Prospector was breeding in the era where studs covered 40 to 60 mares a year.

Another major stallion, Northern Dancer, is widely represented. Scat Daddy, for example, had both Northern Dancer and Mr. Prospector in his pedigree. And when Scat Daddy was paired with the mare Stage Magic, whose bloodline also traces back to Mr. Prospector, they produced the 2018 Derby favorite Justify.

While breeders are concerned that too much inbreeding will create freakish horses, the right combination can make for horses that are freakishly brilliant, said Hill, whose family owned the 1977 Triple Crown winner Seattle Slew.

Can Justify get it done? He’s got history working both for and against him. Justify did not run as a 2-year-old, and the last horse to win the Kentucky Derby without running at age 2 was Apollo, more than 130 years ago in 1882. But Justify has been working with Triple Crown-winning trainer Bob Baffert, who has won the Derby four times.

Scat Daddy also sired another morning-line favorite, Mendelssohn, who won the UAE Derby in Dubai by a huge margin — almost 19 lengths. He’s under the care of star trainer Aidan O’Brien.

Among Curlin’s offspring, Vino Rosso won his last race, the Wood Memorial, and is trained by 2017 Derby-winning trainer Todd Pletcher. There’s also reason to like Good Magic, who won the 2017 Breeders’ Cup Juvenile and is trained by rising star Chad Brown.

They’ve all got about two minutes to make daddy proud.