Arguments against Iron Horseshoes
The use of iron horseshoes forfeits the natural elasticity of the horn capsule. The stiffness of the iron makes it absolutely impossible for the hoof to adjust to the ground the way a bare hoof does. Therefore the joints of the horse's leg are exposed to the roughness of the terrain without any means of shock protection. Without the elasticity of the bare hoof, the full impact of the hoof hitting the ground is transmitted to the hinged joints every time the horse puts its foot down.
Using iron horseshoes weakens the supply of blood to the living structures inside the hoof. Iron horseshoes only allow for a strictly limited flexibility of the horn capsule, although the full unhampered functionality of the so-called hoof mechanism is necessary for optimum blood circulation. This applies to a lesser degree to the widening of the hoof in the heel area, which is still possible to a limited extent if the iron horseshoe is correctly nailed on. More important here is the fact that the natural horn capsule is also twisted vertically when hitting the ground as described above - this kind of deformation is impossible with iron horseshoes.
Iron horseshoes deprive the horse of its sense of touch. A horse with bare hooves walks more carefully. It feels the ground and adjusts its gait accordingly in order to maintain the function of the legs. The iron horseshoe makes it impossible for the horse to feel the ground, the hooves are desensitised. This way the horse is "tempted" to use its legs more recklessly. Iron horseshoes will provoke misalignments in the legs. For instance, shoeing with irons usually produces a flat position resulting from the disproportionally higher amount of horn abrasion in the heel area. It also contributes to a backwards broken hoof-pastern axis (hyperextension).
Due to the inability of controlling the horn wear and the mechanical forces that work on the toe walls, shoeing makes a crooked hoof even more crooked. To correct the appearing misalignments (lopsidedness, hyperextension) repositioning is a must whenever the horse gets reshod. The joints, tendons, and ligaments suffer as much from this abrupt repositioning as they suffered before because of the slow and consistent lopsiding of the limbs. In the latter case there is a slight chance of adapting to the newly developed situation, which is not given with this kind of abrupt repositioning. For this reason horses usually have to "break in" their new shoes.
Iron horseshoes also increase the severity and the occurrence of horses accidentally injuring each other. A lot of horses are sentenced to solitary confinement because of their iron shoes.