Beasts of Burden
- Jun 1, 2004
Any horse owner who tours the medieval gallery at a museum must stop at the displays of suits of armor and marvel. Imagine the sheer weight of such an outfit--then imagine trying to maneuver oneself into the saddle wearing it! Visions of metal-sheathed knights being lowered by crude cranes and slings onto their groaning draft horses might in fact be hyperbole since historians say knights usually managed to mount without extreme methods of assistance, but the burden borne by a noble warhorse of the 14th Century must still have been staggering. And it begs the question: How much weight can a horse carry?
The optimum weight a horse can safely carry will vary according to his size, conformation, condition, age, and the duration of the work to be done, as well as the footing and the speed at which work is done.
Those who study equine anatomy and physiology agree that if you really want an animal that's built to be ridden, you'd be better off seeking a camel than a horse. Even a cow would be a more logical choice, from the point of view of its vertebral construction--it has interlocking lumbar spinal articulations which, in theory, make it capable of bearing considerable loads (although its suitability as a riding animal can be questioned in a number of other ways!). The equine spine lacks these strength-lending links, and with its suspension-bridge construction, the horse's back is actually rather poorly designed for weight-bearing.
Much more logical a skill for the average horse is weight-pulling. Without pressure from above on the spine, horses are able to round their toplines and put their prodigious haunches to good use for the task of pulling weight while in harness. Those who work with driving horses observe, quite correctly, that even young, growing equines can be taught to pull weight without damaging their immature musculoskeletal systems, whereas rushing them into work under saddle while they're still developing can put them at risk for all sorts of weight-bearing-related injuries.
But whether it's pulling weight or carrying it, there's an important question here: How much weight is appropriate, and how much is too much?
In the days when horses were essential for travel and combat, this was a crucial consideration. And while fewer of our horses today are in danger of being worked to death, it's still a good question--and one that can dramatically affect the outcome of some horse sports, most notably racing under saddle (where specific weights are assigned as a handicap to put better horses at a disadvantage over more lightly regarded ones, and thus provide a more exciting contest).
Endurance and three-day event riders also pay attention to weight-carrying (although eventers successfully lobbied a few years ago to abolish the 165-pound, or 75-kg, minimum weight rule and the lead weights it sometimes required). Another avenue where weight is important is with police horses, which might work long shifts with a six-foot two-inch, 250-pound (113.4 kg) police officer on board. Still another is the Western horse which must carry not only 40-plus pounds (18.1 kg) of saddle, but possibly a cowboy of the policeman's dimensions.
Where's the Proof?
The answer to the question, of course, is that weight-carrying depends on a variety of physical traits. The optimum weight a horse can safely carry will vary according to his size, conformation, condition, age, and the duration of the work to be done, as well as the footing and the speed at which work is done. It's one thing to carry a heavy load for five minutes, another entirely to shoulder it for six hours--not to mention the obvious difference between lugging a heavy load on a hard, dry surface and in sticky mud.
Back in 1898, Veterinary-Major Frederick Smith addressed the question of weight-carrying in The Journal of Comparative Pathology and Therapeutics, as well as in his own A Manual of Veterinary Physiology published in 1921. In his study, Smith used 136 light and heavy cavalry horses weighing between 840 and 1,333 pounds (381-665 kg). He and a number of cavalry experts estimated each horse's ability to carry weight, then weighed each animal, eventually coming up with a linear relationship. In other words, the bigger and heavier the horse, the more weight he could carry. Such an observation might seem painfully obvious, and the estimations extremely subjective, but somehow Veterinary-Major Smith came to the conclusion from this exercise that "the rule to ascertain the carrying power of a horse is to divide his body weight by 5.757, and, if intended for only moderate work, add to this 28 pounds."
Why bring up a study so patently flawed? Well, because it illustrates the difficulty in estimating a horse's true weight-carrying ability, as well as the paucity of research on the subject. Apparently even Smith had his doubts about his results, because by 1921 he had backed off on his recommendation, instead suggesting 15-20% of the horse's body weight as a general upper limit for the amount of weight he should carry.
Other charts, such as the ones used today by some therapeutic riding facilities, also lean toward the "20% of the horse's weight" rule of thumb. By this estimation, an average 1,000-pound (454-kg) horse would be capable of carrying a 160-180-pound (73-82- kg) human (with another 20-40 pounds, or 9-18 kg, added on for tack). This figure seems reasonable to most horse owners. But we really have little or no indication of whether that indeed is a realistic upper limit.
Those who analyze the form of Thoroughbred racehorses know that a horse's ability to carry weight can be critical to his success. In the case of a racehorse, of course, we're not talking about maximum weight-carrying capacity, but the amount of weight he can carry before his speed is compromised. We've all heard tales about champions--such as the legendary Man o' War--who were handicapped (required to carry more weight) more and more over their careers. Past a certain point, however, a racehorse risks total breakdown if asked to perform with too much weight on his back. The problem is, each horse has an individual weight limit beyond which he can't perform to his natural ability, but there seems to be no way of determining that individual limit without testing the individual horse.
"I very much doubt there is any scientific way to calculate (weight-carrying ability), or if anyone has done so," says equine physiologist Karen Gellman, DVM, PhD, of the Department of Biomedical Sciences in the Section of Anatomy at Cornell University's College of Veterinary Medicine. "Seems to me it is a pretty de facto situation. The animal votes with his feet if the load is unacceptable."
Gellman adds, "Different breeds seem to be more or less suited for different loading situations, as well. Donkeys and mules are well-known to carry or pull enormous weights relative to their size. Ponies are similar; when I was in Ireland, full-grown men were riding and jumping on little Connemara ponies, with no apparent ill consequences."
An even larger size differential can be seen in Exmoor and in the Shetland Islands, where ponies barely 11 hands high shoulder adult humans without batting an eyelash.
"I would think (weight-carrying ability under saddle) is very individual," Gellman says, "dependent on the animal's strength, condition, willingness to please, and so forth."
Gellman stresses that the vagaries of the "live weight" of a rider comprise an important factor in a horse's ability to lug that rider around. "A dead weight load is probably easier for a horse to carry than a bad rider, since the load won't dynamically unbalance the horse," she says.
She points to a study by Claire Farley, PhD, professor in the Department of Integrative Biology at the University of California, in which the speed at which ponies would naturally choose to break from a trot to a canter, under saddle, was determined by forces on their musculoskeletal systems rather than metabolic efficiency. When the riders provided no request for the transition, the ponies tended to switch to a canter at a velocity slower than that at which it was metabolically advantageous for them to canter. Farley then put weights on the ponies and found they chose to break to a canter even earlier, implying that increased weight (or force) triggered the transition.
Conformation and Weight-Bearing
But not everyone agrees that dead weight is preferable to live. Before minimum weight requirements were abolished from three-day eventing, most three-day event riders agreed strongly that lead weights positioned over their horse's withers represented a much less forgiving burden than the live weight of a rider. A well-balanced rider can often ease the load of his or her own weight, just by being skillful about centering that weight over the horse's center of gravity (just behind the withers in most circumstances), to move seamlessly with the horse.
Conformation analyst Deb Bennett, PhD, of the Equine Studies Institute, notes that the primary requirement of a riding horse is to bear a rider's weight on the freespan of his back without strain, and that certain conformational qualities can make that easier for the animal. In her well-regarded book Principles of Conformation Analysis, she offers the following wish-list for weight-carrying ability:
- An excellent loin coupling--broad, short, smooth, and strong, yet flexible for coiling. The circumference about the loin and groin should be about the same as the heart-girth;
- A short to medium-length back;
- A neck set high on the shoulder, with a shallow vertebral curve at the base of the neck;
- Moderately high withers, with a peak that lies well behind the horse's elbows;
- A pelvis that constitutes at least 30% of the body length and slopes from 18-22 degrees; and
- A total body weight of less than 1,450 pounds (658 kg).
Bennett suggests that weight-carrying ability, as demonstrated by endurance horses and military trials, is primarily a function of body breadth, especially over the top of the loins, rather than a function of height. "Few tall horses--over 16 hands--are broad enough while at the same time staying within the ideal weight limit," she writes. "Potential soundness goes way down as weight exceeds 1,450 pounds (658 kg). Consistently, winners of the old military-style endurance rides were men mounted on broad-backed horses that were just tall enough to fit them."
Interestingly, Bennett makes a case against massive size being the best criterion for weight-carrying capacity. "Draft horses (are not) particularly adapted to bearing weight; huge size does not confer weight-carrying ability. Although they have increased the animal's weight through selective breeding, draft-horse producers have been unable to obtain a proportional increase in bone," she writes. "No large draft horse possesses eight inches (20 cm) of bone (bone means the circumference of the forelimb measured just below the animal's carpus, or knee) per 1,000 pounds (454 kg) of weight; by this standard, a 2,000-pound (907-kg) drafter would have to have a 16-inch (40-cm) cannon/tendon circumference. Statistics instead show that modern draft horses average only five inches (12.7) of bone per 1,000 pounds, or 10 inches (25.4 cm) of bone on a 2,000-pound horse. This is fine, so long as the adult animal is never expected to work at suspended gaits, but does most of his work at a walk.
"However, since suspension--thrusting the body clear of the ground--is a necessary quality in riding horses that trot or canter, they cannot be massive and be expected to remain sound."
Her opinions are borne out by historians, who tell us that the mounts of medieval knights were not the towering Shire-type beasties we envision, but stocky, short-ish equines averaging about 15 to 15.2 hands.
The type of horse Bennett has described in her wish-list might indeed be the sturdiest, soundest animal in terms of weight-carrying ability, and that's an important consideration if you're a man who's six feet five inches tall, weighs upwards of 250 pounds (113.4 kg), and intends to ride the length and breadth of South America, averaging 50 miles (80.5) a day, without stopping. It might even be your most important criterion if you're a 150-pound (68-kg) woman who intends to foxhunt two or three times a week in all sorts of terrain, and expects to be out for five to six hours at every meet.
But fortunately for the vast variety of horse types and breeds out there, humans have all sorts of uses for horses, not all of which require superior weight-carrying ability. Become a skillful and well-balanced rider and show consideration for your horse's limitations, and with any luck your horse will never have to seriously test his weight-carrying ability--or, as Gellman says, "vote with his feet."
About the Author
Karen Briggs is the author of six books, including the recently updated Understanding Equine Nutrition as well as Understanding The Pony, both published by Eclipse Press. She's written a few thousand articles on subjects ranging from guttural pouch infections to how to compost your manure. She is also a Canadian certified riding coach, an equine nutritionist, and works in media relations for the harness racing industry. She lives with her band of off-the-track Thoroughbreds on a farm near Guelph, Ontario, and dabbles in eventing.