We often assume a horse needs shoes without really thinking about why or how that affects a horse's overall health. Yet standard veterinary texts, such as books by James Rooney, DVM, and O.R. Adams, DVM, on equine lameness, refer to shoeing as a "necessary evil." What makes shoeing necessary in some instances is the need for additional traction caused by the weight of the rider, which in turn causes excessive wear to the hoof wall, especially on hard surfaces. What makes shoeing a potential evil is that it restricts the hoof in ways that might not be optimal for its long-term health. The compromise between the requirements of the working horse and the health of the same horse's feet might be to leave him unshod for a few weeks out of the year.
A recent study by Robert M. Bowker, VMD, PhD, Professor at the College of Veterinary Medicine at Michigan State University, and Lori A. Bidwell, DVM, of the Rood and Riddle Equine Clinic in Lexington, Ky., helps clarify how allowing a horse to go barefoot for at least a small portion of the year could, in fact, help promote soundness. Bowker's training is in veterinary medicine and neurobiology. He also teaches first-year veterinary anatomy, morphology (the study of anatomical form), and how to do a neurologic exam on various animals. This familiarity with the anatomy of a variety of species gives him a unique perspective from which to study the equine hoof.
Bowker examined a sample of 125 barefooted horses (which had never been shod). These horses were mainly Quarter Horses (the study was funded by the American Quarter Horse Association), Thoroughbreds, Arabians, warmbloods, and their crosses that had been turned out on relatively rough gravel and compacted sand terrain, most of which were ridden a few to several times per week. In addition, 10 show horses, used to being shod and working an average competition schedule, were turned out barefoot on similar terrain during the fall, winter, and spring. Plaster casts were made of the bottom of the hooves of their right forelegs for measurement and evaluation purposes for nearly five months. What evolved from this study was that if horses were allowed to go barefoot, their feet eventually began to gain characteristics that most believe correspond to a healthier foot.
Three weeks after shoe removal, the front feet of these show horses showed definite signs of changing conformation. Their feet tended to widen with a more "shallow cupping" of the soles. The central sulci became shallower or more open, rather than having deep crevices at the heel area, and calluses began to form on the soles at the toe, indicating greater wear and weight bearing at that site. There was a reduction in the distance between the apex of the frog and the toe at the dorsal hoof wall, as the breakover distance was shortened naturally in these horses by the way in which they moved over the terrain.
At six to nine weeks after shoe removal with normal wear and no trimming, the entire frog area became larger, and the width of the feet increased as well. The heels of the frogs (back part of the frogs) usually began to make contact with the ground at that time, which resulted in a gradual enlargement of the frog and parts of the sole surface. This increased weight-bearing surface of the foot distributes the weight of the horse over a greater area, which reduces the load or stress on the entire weight-bearing area.
During the same time frame, with normal wear and no trimming, the imprints of the feet on imprint boards and plaster of Paris moldings clearly showed that the bars and frog had begun supporting the horse's weight.
Having dissected thousands of feet, Bowker recognizes that these horses' adapted barefoot hoof characteristics are the same as those found in sound hooves with no internal problems. Conversely, his goal is to learn which external hoof characteristics might indicate internal hoof problems and potential lameness.
His findings show that in healthier feet, the bars on the bottom of the foot make contact with the ground and exhibit a wider angle than feet in worse shape. The paracunal sulci (next to the frog) of these healthy hooves are usually packed with dirt, not manure. It seems that an optimal angle for the bars might be critical to a healthy foot--a wider angle (about 60°) is better than bar angles that are more vertical or upright, such as those seen in contracted feet. Healthy feet seem to exhibit an equilateral (equal-sided) triangle in the distances between the heels and the apex of the frog.
Bowker's information corresponds with studies by Gene Ovnicek, Registered Journeyman Farrier (RJF), and others on wild mustang feet. There has been some controversy over what the data on wild mustangs has to do with shod domestic horses, since mustangs are not subject to domestic conditions. In part, this discussion was due to the idea that wild mustangs were actually a separate species and therefore related to the domestic horse, but different genetically. As it turns out, these herds are actually a melting pot of many breeds, so the designation of "feral horses" is more accurate. What that means is that any differences in the feet of feral horses compared to domestic horses is due primarily to environment rather than genetics. Once this is clarified, it becomes evident that environment makes a substantial contribution to hoof health, or lack thereof.
What's a "Good" Foot?
Studies have sought to discover exactly what factors promote the optimal health of the equine hoof. Since problem feet in the wild are generally weeded out by natural selection, the remaining herd forms a perfect sample for observing the characteristics of healthy feet.
There is a considerable overlap in the characteristics of feral horses' feet and the feet of healthy domestic horses allowed to roam barefoot for a few weeks. The overlap between feral hooves' characteristics and those of shod domestic horses is not as great, yet the trends are undeniably similar.
Both studies demonstrated that only when the feet are overgrown is the hoof wall used as the primary weight-bearing surface. In both instances, as long as the horse is allowed to move freely over varied terrain, the hoof wall is filed down in response to the environment with the bars, frog, and sole (especially the callus) bearing a considerable percentage of the horse's weight.
This is an important distinction because of how these weight-bearing dynamics place stress on different parts of the hoof. Bowker found that when a bare hoof is placed on a hard surface, only a small percentage of the foot surface actually bears weight (6-7%), while in the same feet placed on a hard rubber surface, the percentage of the surface area bearing weight increases to about 25%.
The important point here is that as the weight-bearing area of the hoof increases from harder to softer surfaces, the load on the tissues decreases rather dramatically. While the area for support increases as the horse moves from concrete to hard rubber, the actual load in the tissues will be reduced by more than six times. While this difference in load with different surfaces might seem relevant to a discussion of hoof characteristics, consider this: Reducing load by increasing the area of the hoof contacting the ground decreases load on those weight-bearing structures--whether it's because of a softer surface or because of hoof conformation.
Additionally, Bowker's findings showed that when the horse puts weight on a hoof, the hoof wall flexes outward 2-4 millimeters depending on the hardness of the footing. This leaves us with something to think about--if we are artificially impeding this movement by nailing shoes onto the hoof wall, how does that impact the structures of the foot?
Rooney's revised edition of his classic text The Lame Horse also refers to this dilemma. He states that if you draw a chalk line around the foot of a shod horse standing on hard ground, then do the same thing 15 minutes after the shoe has been pulled, you will find that the foot has expanded beyond the original line. The shoe restricts the normal expansion of the hoof.
In addition, the hoof wall has a degree of "fluidity" to it that enables it to move depending upon the surface that the hoof is placed upon. This "fluidness" depends on the moisture content of the hoof wall as well as the qualities of the hoof wall itself. The increased surface area of contact when the hoof is placed upon hard rubber, Bowker believes, is due to both the movement of the hoof wall and the rubber--it is more than sinking into the rubber. All of this confirms the importance of good footing and hoof wall quality to a horse's soundness.
The Sensitive Hoof
Proprioception is the capacity of the nervous system to sense where the body and the limbs are in space, and to evaluate weight-bearing surfaces as well as touch. There are different receptors or nerve endings for different kinds of stimuli, and Bowker's research has focused on receptors that relate specifically to locomotion. This subject has been well mapped out in cats and laboratory rats, but up to this point, little has been done to understand how horses assess the ground beneath them.
The same receptors that stimulate cutaneous (skin) sensations in other species are present in the equine hoof. The hoof is a sensitive and responsive organ, much like (though not quite as sensitive as) the tips of our fingers. This might help explain why their feet (and gaits) respond differently to different surfaces.
The other implication of this is that by altering the bottom of horses' hooves, we might be interfering more than we realize with their perception of the environment. Control of the muscular contractions that stimulate movement depends on information from the receptors about the terrain the horse's feet are encountering, which helps the brain decide how much and how quickly muscles should contract in order to achieve the desired movement. The information from the receptors might be inaccurate if the hoof (their interface with the environment) has been altered.
According to Bowker's research, environment is critical in the formation of the internal structures of the equine hoof. The lateral cartilage and the digital cushion respond to their environment and use. In feral horses, the digital cushion is smaller and the lateral cartilage is larger.
The inner portion of the lateral cartilage, closest to the digital cushion, tends to be fibrocartilage (made up mostly of fibers like normal connective tissue) in "good-footed" horses. The evidence of hundreds of Bowker's dissections indicates that the healthier foot is the one that has thick lateral cartilage tissue with considerable fibrocartilage and a digital cushion composed of fibrocartilage, rather than one with a thin lateral cartilage (0.22 inches thick versus 0.4-0.8 inches thick in healthy hooves) and a fatty, elastic digital cushion. In the younger horse (less than four or five years of age), this part of the foot has not fully developed yet; as far as we know, the digital cushion is still composed mainly of fat and elastic tissue.
"We believe that given the correct stimuli, i.e., exercise, the lateral cartilage and digital cushion will respond," Bowker says.
Environment is a very important contributor to the formation and composition of the internal structures of the equine foot. Bowker's findings suggest that the lateral cartilages and digital cushion respond to various stimuli within the environment--i.e., if the foot is used (and stimulated by the ground), the internal structures will respond to support the weight and function of the foot and horse. Conversely, if the horse becomes a "couch potato," remaining in the stall for much of the day or not being given sufficient foot stimulation, the internal foot does not respond or develop into a good foot. In feral horses the lateral cartilages are much larger and thicker than in these "couch potatoes," and the digital cushion is composed mainly of fibrocartilage.
This means that it is important for horses to be allowed access to turnout, preferably over varied terrain on a regular basis. The stimulus of free contact with the environment at least three to five times a week, in addition to regular work, is what generates more resilient tissues inside the hoof. The gradual nature of the change requires a consistent regimen of activity.
"We believe that the more exercise the better, much as with our own health!" says Bowker.
Bowker's findings are confirmed by Ovnicek's work with feral horses. The most striking difference between the feral horses and domestic horses was visible in the feet of feral yearlings, which exhibited enormous frogs. These horses have no option but to be moving at speed on a variety of terrain within an hour of birth. The ensuing size of the frog and apparent health of the foot indicate the beneficial effect: Most farriers know that foals which are turned out will not have as many foot problems as those foals that remain in the "comfortable" setting of the stall on thick bedding.
"It's similar to humans taking our shoes off and walking over a rocky road," Bowker says. "The first time we do it, it hurts our feet, but if we did it all summer we could do it with ease and not even notice it! The horse isn't much different."
Although shoes are quite necessary for some horses to do what humans ask them to, many don't need them and indeed are better off without them. Also, shod horses have been shown to benefit from a period of turnout without shoes. If your horse is constantly shod, you should work with your farrier (and possibly your veterinarian) to decide if some barefoot time should be part of your horse's future. Even without shoes, the hoof should still be trimmed professionally on a regular basis (i.e., at five- to six- week intervals).
Bowker, et al. Sensory receptors in the equine foot. American Journal of Veterinary Research, 54, 1840-1844, 1993.
Bowker et al. Functional anatomy of the cartilage of the distal phalanx and digital cushion in the equine foot and a hemodynamic flow hypothesis of energy dissipation. American Journal of Veterinary Research. 59, 961-968, 1998.
Bowker et al. Effect of contact stress in bones of the distal interphalangeal joint on microscopic changes in articular cartilage and ligaments. American Journal of Veterinary Research. 62, 414-424, 2001.
Rooney, James R. The Lame Horse: Causes, Symptoms, and Treatment. Millwood: Breakthrough Publications, 1974.
Stashak, Ted S. Adams' Lameness in Horses, 4th edition. Philadelphia: Lea & Febiger, 1987.
About the Author
Gabrielle Pullen, GCFP, NCMT, has been working professionally with horses and riders since 1994, enhancing mobility and performance using a variety of skilled modalities that address neuromuscular function. Her take on rehabilitation includes a broader paradigm than mechanically working on muscles as if they were inanimate objects. She focuses on fine-tuning awareness, building on the refinement of their existing highly sensitive consciousness to improve their movement and behavior.
POLL: Visits from the Vet