THE RELEVANCE OF THE FERAL/WILD HORSE HOOF TO HOOF CARE

Best practice in hoof care should evolve from passed on knowledge, new research, clinical practice and practice review.

Knowledge of wild horse and feral horse feet provides useful supportive information but should not form the basis of the foot model for the care of the domestic horse.

The traditional farriery model assumes that the hoof wall is the principle weightbearing component of the hoof capsule. It is responsible for achieving smooth and painless force cycling between the ground and the appendicular skeleton during loading. Leaf-like primary epidermal lamellae project from the inner aspect of the hoof wall and interdigitate with their dermal counterparts attached to the distal phalanx.

This complex anatomical arrangement forms an integral part of the suspensory apparatus of the distal phalanx which serves to suspend the appendicular skeleton within the hoof capsule. To achieve this optimal weightbearing function the hoof wall must be allowed to grow past the sole plane (at least 3-5 mm length according to Pollitt) so that the sole is relieved of direct pressure on a flat hard surface.

When the foot is placed in soft or deep footing, the most distal structure of the hoof capsule (the hoof wall) will continue to bear greater weight than the less prominent structures (the peripheral sole).

Veterinarians and farriers insist that the correct conformation of the equine foot is critical and many recommend a dorsal hoof wall angle of approximately 54 degrees (range 48-55) with a distal phalanx palmar angle of 3-7 degrees for the forefoot of the horse. Specific functions, such as flat racing and dressage, have been shown to benefit from more extreme conformation.

It has previously been assumed by some authors that the feral horse foot represents an ideal model on which to base foot care practices. The adoption of this model by some groups has shifted the focus of hoof trimming away from the traditional farriery model with a tendency towards excessive removal of the bearing border of the distal hoof wall. The external appearance of the typical hard substrate brumby foot, which is often used as the benchmark model, appears aesthetically pleasing, with little visible pathology.

However, this superficial impression was misleading as a more thorough investigation of internal foot structures using radiographic and lamellar histological assessment, has revealed significant pathology. Previous observers and proponents of the “natural” foot model were apparently unaware of this inner pathology but made assumptions and recommendations for domestic foot care, such as promotion of solar loading and excessive bevelling of the distal hoof wall. The practice of using the “natural” foot model as the optimal morphometric model on which to base foot trimming practices may need to be reconsidered carefully.

If the “natural” horse foot is to be used as a guide to model the morphology of the domestic horse foot, first the appropriate model should be identified. The Australian brumby study of feral horses from six environments identified six different foot models. It appears that the hard substrate foot type was selected as the original paradigm for the bare foot horse with aesthetic appearance as the main selection criterion. That study identified extreme activity levels in feral horses over harsh terrain.

These horses were able to survive partly due to the robust hoof structure they had developed since birth. However, this same robust structure had apparently suffered injury and pathology as a consequence of the extreme activity and is not necessarily recommended for the domestic horse functioning within less extreme requirements.

There was significant pathology identified in the foot types most closely resembling the popular “mustang” foot. Only three feet from the 100 left forefeet assessed were free of abnormality. The most surprising finding in the hard substrate, high travel population was a 67% incidence of chronic laminitis, likely of traumatic aetiology.

A 70% incidence of ungual cartilage calcification in the hard substrate, moderate travel population further indicated the possibility of concussive changes as a consequence of overuse on hard substrate. Some of the characteristics of the popular natural foot have proposed advantages, for example a thick hoof wall, thick hard sole and heavily worn distal wall promoting increased sole bearing.  However, these characteristics may in fact be associated with pathology thus making this model inappropriate if applied to domestic horse feet.

There is currently no clear evidence to support the use of the extreme feral horse foot as a model for foot care. However, there appear to be some morphometric parameters (dorsal hoof wall angle, palmar angle of the distal phalanx) that were consistent between all feral horses in the brumby studies and these parameters may be important when considering the natural form of the equine foot.

An example of the questionable use of the extreme natural foot model is the application of the squared toe and the over-exaggeration of distal wall roll, often referred to as the “mustang roll”. The square (or rockered) toe is a feature thought to be a strategic hoof structure aiding early break-over.

The more conservative (and more commonly used) definition of the mustang roll is a soft radius or bevel applied around the entire circumference of the hoof wall. The function is to prevent distal hoof wall cracking, chipping and wall flares. Because these features occur in the “model” feral horse foot they are often applied with the rasp during trimming. The detailed study of feral horse feet determined the squared toe occurred in extreme cases and was excessive in horses travelling long distances over rough, uneven and mountainous terrain, and in horses required to dig for food or water.

The roll is a consequence of constant chipping and abrasion as the hoof makes contact with rocks and abrasive footing from all directions as the horse moves across the terrain. As the foot hits a rock and a fragment of hoof wall is dislodged energy is dissipated, rather than propagated through the hoof to the underlying soft tissues and skeleton. The wearing process that creates the roll may be a protective feature of the hoof wall but the actual architectural feature of the roll itself may have no protective function in the horse’s foot.

Therefore the practice of creating the squared toe and excessive roll artificially with the rasp may be of no benefit to the horse with a “normal” foot. These horses are often living on the edge of survival. The application of the exaggerated form of this feature to the managed domestic horse living in a paddock environment should be reconsidered. In specific situations, such as when a lamellar wedge is present, a more extreme trim may be applied to reduce the dorsal wall forces at break over.

However, in healthy bare foot horses, a small bevel of the distal hoof wall appears to be effective in preventing hoof wall cracking and chipping and at the same time does not affect the break over location, and allows the outer most hoof wall tubules to bear weight as they are designed.

At the trot the peak force through the hoof wall is up to 1.5 times the horses’ body weight. The back half of the horse foot is designed to deform and absorb shock so it has elastic structures such as the frog, heel bulbs, ungual cartilages and digital cushion. The front half of the foot is designed to be rigid to allow it to perform its major weight bearing function. The outer hoof wall tubules, like the front half of the foot, are rigid, also designed to bear weight.

The variable water content, and thus elasticity, of the hoof wall tubules (wet on the inside and almost dry on the outside of the hoof wall) provide a natural dampening mechanism to reduce the impact of the forces transmitted to the skeleton. Removing the rigid outside hoof wall tubules from immediate ground contact by excessively bevelling the hoof wall will place the inner moist hoof wall tubules in a position of more direct weight bearing, a function which they are not bioengineered to perform on their own.

Another example of the questionable use of the assumptions made from the natural foot model is the practice of attempting to create the same hoof wall thickness around the circumference of the hoof wall. Clearly, the feral horse hoof wall is thickest at the midline dorsal wall (toe) and gradually thins towards the quarters and heels.

The selective removal of “thick” sections of hoof wall may be counterproductive as it is likely to remove outside wall hoof tubules, particularly at the toe and break over locations, from immediate weight bearing. The outside wall tubules are designed to bear weight and the hoof wall must be robust in these areas to support the hoof capsule as it deforms with load bearing.

One challenge currently facing horse owners is the creation of an environment most conducive for horse health. This may include the provision of a surface under foot that balances hoof growth and wear and promotes musculo-skeletal preparedness for the environment in which the horse is used.

Horse owners have a choice of substrates for housing horses and there are active feeding systems available that modify the daily distance travelled. The feral horse studies identified the negative long term implications of substrate and movement on foot health.  Care needs to be taken in choosing one environment over another because of the possible harmful consequences.

Although feral horses living and travelling on hard substrate appear to have robust feet, modified by the environment and able to withstand locomotion over hard substrates, the current study suggests that there are some negative consequences associated with this lifestyle. In light of this observation, further research is required to fully understand the impact of high travel distances and various footings on the health and well being of domestic horses in managed care.

Feral horses in Australia, like many places throughout the world, survive in conditions not entirely natural for horses. These environments at times provide abundant food sources which may be inappropriate for horses. At other times feral horses experience sustained droughts and food and water shortages. The separation between food and water affects travel distance and the choice of terrain over which horses travel. This in turn affects the form and structure of horse feet.

Feral horse feet often appear aesthetically beautiful, robust and pathology free, and appear at first glance to be an ideal model for horse feet. While some features of feral horse feet appear to give guidance for trimming practices, others have been misinterpreted and over-emphasised. In light of recent more detailed study of feral horse feet there is sufficient argument for the model to be reconsidered.

While the veterinarian and hoof care provider is often blamed for the demise of the horse, it has been the industrial revolution and the changing role of the horse from a beast of burden to a recreational and companion animal, coupled with the shortage of available rural land for horse husbandry in many locations throughout the world, that is the major contributor to the perceived problems of the modern horse.

The detailed study of feral horse feet has shown that the feral cousins of the modern domestic horse are also vulnerable to foot pathology despite being free from the confines and influence of human intervention.