For centuries, horsemen have tried to breed for speed and endurance and found that the greatest genetic potential can yield disappointment as easily as reward. Consider the indomitable Secretariat, who sired a string of mostly unremarkable racers, or the supremely talented John Henry, who sprang from an unheralded sire and dam. These are not isolated incidents in the breeding world. Yet, we can’t lay all of a horse’s ability at the feet of trainers, managers, and other equine caretakers, so some of it must be genetic.

Gus Cothran, PhD, a research professor with the University of Kentucky’s Department of Veterinary Science, says, “I believe there is a heritable component to speed and endurance. But it’s not just a simple trait; the qualities are controlled by a lot of different genes.”

Each of those genes, he says, has a heritability factor of its own (heritability is a measurement of how completely a trait is passed down through the genes). For instance, studies have shown that heart girth is 31% heritable in Arabians (The Genetics of the Horse, CAB International, 2000). In fact, Cothran adds, “While I don’t know that anyone has looked at speed and endurance individually, when you look at racing performance as a general term, it has a heritability of 30-35%, which is pretty typical of complex traits.” (See “Further Reading” at the end of this article for examples of studies that proved this heritability average.)

The remaining 65-70% of a horse’s abilities, of course, would be attributed to non-inherited factors such as nutrition, training, and health care (see “Beyond Bloodlines: Environmental Factors” on page 70).

Not everyone agrees with this ratio, and among the dissenters is Dick Galley, DVM, a Willow Park, Texas, veterinarian who specializes in equine athletes and has spent 26 years working on Thoroughbred and Quarter Horse racetracks.

“In my opinion, the predisposition for these traits is very heritable,” he says. “Any time you selectively breed athletes for specific traits over many generations, the families tend to be more adept at those things. However, a trainer can influence ultimate ability through proper or improper training. But I would reverse those percentages (to 70% heritable, 30% environmental). The reason for that is that you are breeding horses genetically selected for speed to other horses genetically selected for speed, thus you get offspring bred for speed. That is the reason why the Thorougbred industry is such a viable and profitable business. If you had only 30% heritability, then people wouldn’t seek out speed horses and would just breed to the horse down the street.”

Even Cothran admits that not much controlled research has been conducted on the heritability of speed and endurance. “As genetic researchers, we’ve gotten the feeling from breeders that it’s part of the fun, trying different crosses to see what they get. They don’t necessarily want to know,” he says. “However, I do think there are opportunities to look at some characteristics in more detail, and I think we’ll learn more about the genes as we go along.”

Good Conformation Traits

For most breeders, heritability ratios matter less than the main point: characteristics that contribute to speed and endurance can be inherited. So the important question becomes: “What traits do you select for if you want to breed a faster horse or one with greater staying power?”

“No matter what segment of the industry you’re in, conformation is a paramount consideration,” says Galley. “Anything less than what we consider ideal conformation is detrimental.”

Nancy Loving, DVM, a Colorado-based equine practitioner who has been involved in the sport of endurance racing as a competitor and judge for 17 years, wholeheartedly agrees. “The significance of conformation cannot be understated,” she insists. “Certain body physiques will excel over others simply because the horse’s stride length is longer, its gait is more efficient, and it moves with less effort and less expenditure of energy than another individual.”

Such a stride is one thing that gives a horse class, says Ed Anthony, pedigree consultant for Three Chimneys Farm in Midway, Ky., whose job includes advising mare owners on the most suitable stallion crosses. “Horses with it include AP Indy and Fusaichi Pegasus. They have a smooth, fluid stride that just gobbles up the ground. They don’t appear to be going fast — not the cliché of a speed horse setting blazing early fractions — but they cover an enormous amount of ground effortlessly.”

In addition, Loving notes, good conformation “ensures the horse’s athletic lon-gevity by minimizing concussion and strain on the joints, tendons, and ligaments.”

In other words, says Galley, it allows the horse to withstand the rigors of racing and ward off the fatigue factor. Such structural durability also means the horse is less likely to be injured and require lay-up or rehabilitation time, which would disrupt conditioning programs and interfere with consistent performance.

Along with good basic conformation, including strong bones and good-sized joints, the following traits are said to enhance an equine athlete’s potential for speed and stamina:

Good Feet — Big, strong feet with strong hoof walls and well-developed cups to the soles are preferred. Strong hooves provide structural stability that contributes to gait efficiency. They also minimize the development of hoof lameness that could interfere with performance. “A horse that is born with weak feet cannot be improved with feed supplements or diet,” says Loving.

Straight Limb Conformation — “Horses with angular limb deformities suffer greater stress on one side of a joint than another, leading to degenerative arthritis,” Loving explains. “Horses that paddle or wing because they toe in or out are likely to incur interference injuries as their muscles fatigue over the miles. Excessive excursion of any limb will hasten the onset of fatigue.” All of this, of course, can inhibit athletic ability and shorten a horse’s competitive career.

Good Topline — A strong back and loins give the horse the ability to strongly engage his haunches. This gives a racehorse a powerful “motor” in his hindquarters and helps an endurance horse drive down the trail or push up a mountain.

Adequate Pectorals — A big chest “gives an opportunity for good lung excursion and provides housing for a big heart muscle,” explains Loving. Large lung and heart capacity generally mean large aerobic capacity — thus maximum endurance and performance at maximum exertion.

Cardiac Ability — A big heart is another plus for an equine athlete. (This is not to be confused with a desire to win, which horsemen also refer to as “heart,” a trait discussed in “The Heritability of Heart” on page 64.) “I do believe that a big physical heart can be bred for,” says Anthony. “And a lot of people even scan for that at the yearling sales, although I think it’s an inexact science at that age, since a horse’s heart can grow so much between the time he’s a yearling and the time he turns two or three.”

Good Shoulder Angle — An open shoulder angle (between the scapula and the humerus) is important for a long stride.

Cannon Bones — Cannon bones should be relatively short for strength under stress, while the radius and tibia should provide length to the legs.

Horses designed to run short distances, like Quarter Horses bred for the 440-yard dash or Thoroughbreds bred for five- and six-furlong events, also must be built for an explosive start and the ability to sprint at top speed from wire to wire. For this, they need powerful hindquarters, big gaskins, strong forearms and shoulders, short-coupled bod-ies, and wide, muscular chests, agree Anthony and Ken Carson. Carson is a former pedigree consultant for Three Chimneys and current general manager of Valor Farms in Pilot Point, Texas, which raises Thoroughbred and Quarter Horse racers. In all, it’s a look that Anthony describes as “almost chunky.” As an example, Carson points to Hadif, a Thoroughbred stallion standing at Valor. “You really can’t tell the difference between him and a Quarter Horse — he’s very muscular,” says Carson.

Conversely, says Anthony, a “two-turn” horse — one bred to cover the classic distances, such as the Triple Crown races — has a rangier build. “He’ll be leggy, with longer mus-cling; he won’t appear to be as muscular as a sprinter,” says Anthony. “And he’ll cover more ground just standing, because of his longer body.”

Of course, the best horses often seem to have both muscling and the ability to run classic distances. “Secretariat was big and heavily muscled,” says Carson, “but he was as good at six furlongs as he was at a mile and a half.”

Hand-in-hand with quantity of muscle comes muscle fiber type. Gal-ley notes that racehorses — sprinters, in particular — rely on fast-twitch muscle fibers, while Loving points out that endurance horses need more slow-twitch fibers.

Endurance racers, she continues, also seem to benefit from a thinner skin, which helps the horse dissipate heat accumulated during pro-tracted exercise. And, says Loving, a lighter coat color can be an advantage. “I’ve noticed that, many times, horses with darker coats have a little more trouble dealing with heat and humidity stress than a gray horse, and certainly coat color is inherited,” she notes.

Breeding Poor Performance

It’s important to remember that bad conformation points are just as heritable as good ones — in your pursuit of athletic strengths, don’t overlook a horse’s weaknesses.

“One problem we see with racehorses, especially in the Thoroughbred industry, is that people have bred winner to winner with little regard to conformation, so they’ve bred in a lot of defects,” says Galley. Anthony agrees, saying, “There is some concern in breeding speed to speed. Not only might you get a horse that cannot run far at all, but unsoundness also becomes an issue.”

Among the most common flaws are “calf” knees (back at the knee), which Galley says predispose the horse to carpal chips — the most common racehorse injury. Other defects include overly long and sloping pasterns, which predispose the horse to chips in the fetlocks and can lead to sesamoid problems and problems with the flexor tendons. The long toe/low heel conformation was once erroneously believed to increase stride, but it has proven not to do so in modern studies, says Galley. (In fact, it has proven to be detrimental to the long-term soundness of racehorses.)

One mistake breeders make is to simply judge a horse on paper — based on pedigree, performance, and perhaps a picture in a magazine. “A lot of people breed catalog page to catalog page,” says Carson. “Too many people never actually look at the stallion.” Anthony agrees, saying, “Speed is important, and it wins a lot of races. But it’s dangerous to breed for just one thing. This is a big balancing act: speed, stamina, and soundness are all equally important.”

Metabolism And Mind-Set

Both Quarter Horses and Thoroughbred racehorses, says Loving, also need bodies that run efficiently during anaerobic exercise. On the other hand, long-distance racers, like endurance horses, need practically the opposite: the ability to move at a steady pace with minimal fatigue for hours on end — efficiency when doing prolonged periods of aerobic work. “Horses can gallop at 24 mph or more,” Loving explains, “but in endurance racing, winning speeds for a 100-mile course may average just nine to 11 mph.”

Some aspects of metabolic efficiency are heritable, making it easier for some horses to condition and compete to their fullest genetic potential. Endurance horses demonstrate metabolic efficiency by rapid heart rate recoveries. On the other hand, some horses which are less well-endowed genetically can be trained to develop excellent metabolic capabilities through strategic conditioning practices and consistent training for the intended exertion.

Unlike equine track stars, endurance horses also must be able to make a rapid metabolic recovery after exertion, says Loving. “Many horses win simply by having a faster recovery rate at vet checks. They arrive and almost immediately meet the pulse criteria, get shuttled through the exam, and end up passing other horses whose progress has been delayed because they’ve taken longer to reach the pulse criteria.”

Distance horses also need “the ability to cope with changing routines and environments,” says Loving. “The horse that takes care of itself — that has an ability to eat and drink well before, during, and after the com-petition — will be able to demonstrate peak performance.” This is heritable in that tem-perament and disposition are often heritable.

For one thing, this adaptability helps the horse maintain good body condition, something important for racehorses as well. An endurance horse needs those reserves to pull on during competition, while a racehorse needs the reserves in order to recover and not lose condition after a tough competition. However, excess reserves (body fat) are detrimental to both athletes, explains Loving. “Too much fat negates the heritable abilities to dissipate heat from the body and places an added load on the musculoskeletal structures.”

Pedigree And Performance

Of course, the most brilliant parents can’t guarantee a stellar foal. Even breeding the best to the best, one can only hope for the best, not count on it. You can increase your odds of success, though, by studying the pedigree and performance record of sire and dam, their relatives, and their other offspring.

“I think it’s a huge factor,” says Carson. “I think you can get carried away with looks. Every year, I try to push myself away from solely (numerically) good-looking horses — a mare can look good on paper, if the pedigree and the race record are there.” For example, he mentions the well-known Phipps family’s Thoroughbred broodmare band — dams of top racers year after year. The culls, Carson says, tend to have obvious flaws — pigeon-toed or calf-kneed conformation or a plain appearance. “But the blood is still there,” he says, “and from those culled mares, last year you got King Cugat, by Kingmambo, and Kona Gold by Java Gold.”

Loving recommends that breeders look for certain attributes in a genetic line, such as strong and steady performance in related animals in your chosen sport, or longevity of the related horses in that sport. For instance, in the sport of endurance, were the related competitors mostly “flash in the pan” horses which competed only a few years, or did they have many years and thousands of miles accumulated before having to retire?

Specific to sprinters, says Anthony, you want to see a family heavy on horses which tend to mature early and have won at five or six furlongs against fast horses. He also wants to know how the horses break from the gate. For those short races, “You want one that breaks fast, not one that comes out slow and then picks up speed over the last furlong or two,” he says.

Galley is another strong believer in performance records. “If a horse didn’t perform well, there’s a reason,” he notes. “Either he wasn’t that talented, or he was injured. If he was injured, was it due to poor conformation or an accident?” It’s one thing if a horse with an impeccable pedigree and a family history of staying sound while winning gets in a trailer accident and has to retire. It’s another game entirely, insists Galley, if a horse goes to the track and gets injured because of poor bone structure.

However, Anthony notes that broodmares often don’t have much of a record. “There are a lot of occasions where a filly is unraced,” he explains. “Sometimes, she’s so valuable as a broodmare that the owner doesn’t want to risk injury.” That means her ability — how far and how fast she can run — is a bit of a mystery. “And that’s where pedigree becomes paramount,” he says. “For instance, if she’s a Nijinsky daughter, you’d think she probably has the foundation for stamina.”

Carson believes that the best route to success is breeding like to like — a good sprinter to a good sprinter, and so on. “If you breed a good-running, sound mare to a similar stallion, it stands to reason that you will get a foal that’s similar to its sire and dam,” he says. “But if you breed a bulldog-like mare to a giraffe-like stallion, there are a lot more variables in the picture.”

Of course, as Carson realizes, that formula doesn’t always work. For example, says Anthony, take the Thoroughbred filly Jersey Girl. “Her sire, Belong to Me, was a sheer sprinter, and her dam was also a speed horse. Logically, this filly should’ve gone five furlongs and just run out of steam. But she won the Mother Goose Stakes at 1 1/8 miles, beating top classic-type fillies, including Banshee Breeze. So there are certain cases where a pedigree suggests something that just doesn’t happen.”

The Art And Science

The bottom line, says Anthony, is that “there are no complete rules. It’s an art and a science, with a lot of guesswork and money involved. Nothing is a slam-dunk deal, and we’re all just trying to bump up the percentages.” All the more reason, suggests Galley, that before you make that final mating call, “you seek help from a good horseman — a breeder, a trainer, a veterinarian — someone with lots of experience in breeding and in your specific discipline. Anything to help your chances of success.”

The Heritability Of Heart

Along with speed and stamina, it’s heart—that intangible competitive spirit and deep-seated desire to win—that separates champions from also-rans