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Breeding Thoroughbred Racehorses Blends Art and Science / Wednesday, February 4, 2015

Breeding Thoroughbred Racehorses Blends Art and Science: Foaling Series Part I

It’s that time of year again. With each passing day, more and more little ones are running through the fields here in Lexington, Kentucky with their proud moms. From now until June, more than 20,000 Thoroughbred foals will be born in North America. Of the foals born this year, only 20 of them will make it to the starting gate in the 2018 Kentucky Derby. A great deal of work goes into producing a healthy, thriving munchkin. The process begins long before a thoroughbred racehorse is ever born.

Breeding racehorses is as much of a science as it is an art and breeders and horse owners spend countless hours studying pedigrees and analyzing the “best” sires to send their mares to. Some people consider figures such as the “dosage index,” which is a mathematical statistic to determine the projected distances progeny will best perform at. It takes into account several generations of a horse’s pedigree and the performance of the family. For more information on Dosage Index click here. Other people use the TrueNicks system to plan their matings. This system assigns a grade from A++ to F based on the degree of affinity between the sire and broodmare sire. To use TrueNicks to create a hypothetical foal click here.

Some people use their years of experience to help them decide which stallion they believe will produce a quality foal with their mare. Many other factors besides mathematical figures go into selecting a stallion including size of the mare and stallion, stud fee, performance of mare and stallion, previous matings, etc.


For instance, a breeder with a small mare would likely choose a tall, large boned stallion (such as Rock Hard Ten or Tiznow) to breed to in hopes of putting some size into the foal. A breeder may chose to mate his sprint type mare to a sprint type stallion (such as Speightstown or City Zip) in hope of producing a speedster, or may chose to mate to a classic distance type runner (A.P. Indy or Pleasantly Perfect) to increase stamina influence. Sometimes the horses with the highest dosages indexes and most prestigious family lines don’t live up to expectations. Sometimes horses with obscure pedigrees go on to do great things on the racetrack. That is what makes breeding an art. To browse the Bloodhorse Stallion Register click here.


Once a mare owner selects a stallion, they submit the mare to the breeding farm to be accepted. The stud farm can accept or reject the mare after considering her pedigree, race record, and progeny. Some stallions cover over 200 mares a season, and as with anything, some are in higher demand than others. The term “booked full,” means no more mares will be accepted to breed to a particular stallion.


Mares are carefully monitored using ultrasound technology and “teasing” to determine when ovulation is most likely to occur, and the mating should occur as close to the end of the heat cycle as possible. When a mare is in estrus (the phase of the estrous cycle when she is in heat, a period of maximum sexual receptivity), there will be evidence of one or more follicles on her ovaries. Click here to learn about the process of teasing mares and click here to learn how rectal palpation and ultrasound are used to detect ovulation.


A mare’s estrus cycle occurs when the length of daylight is long, and natural estrous (entire cycle) begins in the spring. However, mares can be placed under artificial lights as early as December to prepare them for the beginning of breeding season in February.


The covering process varies depending on the stallion and behavior of the mare. Trained personnel handle both the stallion and mare during the mating process, and many times mares are restrained and protected to prevent injury. An experienced stallion will cover a mare in a few minutes. Other stallions are quirky and may balk at certain types of mares (some stallions will not cover grey mares for instance). Once complete, the mare loads back up and goes home. It’s worth noting that every single cover has to be a live cover. There is no artificial insemination with registered Thoroughbreds.


A pregnancy can be diagnosed using ultrasound about 14 days after ovulation. If a mare does not conceive, she will come back into heat 14-16 days after the end of the last heat period. Sometimes mares return more than once to be bred, but with any luck and skill, it only takes one time. With quality care and some help from Mother Nature, a foal will be born 11 months later.

Be on the lookout for Part 2 of this foaling series next week.

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