Physiological implications of domestication on the horse
Very often, certainly in the argument of barefoot versus shod, comparisons re made between the wild and domestic horse. But in reality, are they in fact different species, or at least physiologically different?
When considering the implications of domestication, two main areas come to mind. Hoof morphology and posture. Shoes become a necessary evil of domestication with inevitable physiological implications, but this necessity for shoes itself is a product of the creation of a different physiology.
Starting from birth there are influences on the development of the young horse from their domestic setting. From their feet to their poll, domestication is affecting their development. Gard et al. (2015) studied the differences in the hooves of calves that were allowed to roam with their parents and calves that were barn raised. Results indicated that exercise on alternative terrain increased the volume and surface area of the digital cushion of the feet of dairy calves, which should make them less susceptible to lameness. We can extrapolate that domestic horses will not, for the most part, have as strong and robust hoof capsule anatomy as their wild counter parts. meaning a lower elastic modulus and hardness.
Fig.1 Gard et al. showed that foals allowed to roam with their parents in a natural environment will have larger, stronger more robust haemodynamic systems.
Just as important as the lack of movement, is the lack of varied substrate and therefore stimulation, and the increased levels of hydration the domestic hoof is often subjected to. A certain amount of moisture is documented as being important for the flexibility of the hoof, keratin materials are brittle without water molecules to plasticize the material. However, excessive soaking has been said to cause the hoof to become too flexible affecting its load bearing capabilities and possibly leading to increased plastic (permanent) deformation over the hoofs natural elastic (temporary) deformation under load.
Studies have shown us that both the tensile strength and hardness in the tubular and inter-tubular areas decreased significantly after full hydration. Even more interestingly and perhaps relevant is that the stiffness and hardness of tubules become less than in the inter-tubular areas, which is due to the higher water absorption in the tubular areas, this means these structures lose their reinforcing characteristic.
A study showed that in a fully saturated hoof sample, the tensile strength and hardness decreased by as much as 98%. In reality a hoof would probably not reach that level of saturation, but it illustrates possible implications of domestic boggy fields. To worsen the situation, while wild horses have the benefit of a more stable moisture content, the domestic horse often has huge fluctuations in moisture content affecting the hoof wall matrix. This leads to “crumbly” walls, wall separation and makes room for bacterial invasion.
Fig.2 The disruption of the hoof wall matrix as a result of the un-natural wet dry cycles the domestic hoof experiences. This also opens up the areas to anaerobic bacterial invasion.
The diet of the domestic horse of course also plays a huge role in the robustness of its hoof. In the wild, evolution has allowed the horses biological systems to fit into its natural ecosystem. As the vegetation and seasons the horse traverses ever changes, so to do the vitamins, minerals, and nutrition it exhumes naturally change.
Poor hoof quality, according to many studies, could be an expression of a lack of any one of the following: Crude protein, Sulphur containing amino acids (methionine primarily, cysteine), Essential fatty acids, Zinc, Copper, Selenium, Vitamin E, Biotin, pyridoxine etc. This shows the complexity of nutrition that enables ideal hoof wall growth and composition. Domestication immediately inhibits the natural relationship the horse has with its food and therefore its ability to grow the ideal hoof unless substantial effort is put into understanding and addressing the complexities. This domestic diet also has huge implications on the teeth, which we will outline later in this discussion.
Then of course we have to factor in when humans domesticated the horse their workload and intensity of work increased exponentially and equestrian sports that are simply unsafe to perform without the increased traction of shoes were created. Add to this the disregard for the concept of survival of the fittest and you can suddenly see the necessity of protecting the feet of beasts of burden. Horses who couldn’t cope with escaping predators because of disease caused by poor conformation or other pathological issues were easy prey, but humans do not have survival of the fittest in the forefront of their mind. Economic reason, or sentiment, far outweighs Darwinian principle.
So, with the domestic horse having inherently weaker feet due to the lack of development, living in conditions that create a softer hoof, with poor conformations that predispose to injury and playing sports that require extra grip, we see the man-made physiological need for shoes.
But shoes present with a whole new set of physiological implications. The hoof is a miracle of evolutionary bioengineering. Its structures and mechanisms work as a singularity to absorb the huge concussive forces it can be subjected too. Applying shoes restricts this function. If we look all the way back to snow and Birdsall 1990 they discussed how the shoe was restricting the natural deformation of the hoof. This was measured in 2001 by Roepstorff et al. who showed that the expansion and contraction of the hoof was restricted by the application of a steel open heel shoe, but the application of frog support padding helped to return this closer to the barefoot in terms of expansion but not contraction. How relevant contraction of the foot is to its health needs to be researched. What this does suggest is the negation of the haemodynamic mechanism is directly related to the amount of natural deformation occurring.