Nailing an iron shoe on to the hoof negates many of the adaptive, circulatory and shock absorbing mechanisms that the horse has evolved. It's the equivalent of placing an iron ring round your car tyres.
Peripheral loading - the horse carries its weight in the walls of the hoof - not evenly distributed across walls, sole, bars, frog, heel butresses.
Increased concussion from iron hitting hard surfaces
Increased interference
Greater likelihood of tripping and stumbling
Increased risk of severe hoof damage when they are pulled off in the field or during work leading to wall damage, abscesses etc.
Iincreased risk of injury to other horses and humans
Reduced circulation to the hooves and legs caused by interference wih the hoof mechanism
Tendency towards causing the caudal hoof to contract
Trapping infection under the shoe, toe and quarter clips undermining the hoof integrity beneath them
Compromised integrity of the hoof capsule through nailing
Shoeing inevitably carries a risk of nail-bind, misdirected nails, concussion to the hoof
Shoes can camouflage underlying lower leg health problems.
A: There are many ways in which shoes can restrict the natural function of the hoof. The major impact of the shoe is to dramatically restrict the hoof mechanism and interfere with its natural shock absorbing capability.
When the horse steps on to his hoof, the hoof wall flexes wider at the bottom; when he lifts it off the ground, it returns to its narrower "closed" shape. This spread-and-squeeze acts like a pump, circulating blood around the hoof with each step. The circulatory system in the hoof also acts like a hydraulic damper to absorb shock.There are several natural shock absorbing mechanisms in the hoof - for example there is a moisture gradient across the hoof wall that acts like a hydraulic damper, dissipating large amounts of the energy generated by landings.
With a shoe on, the hoof is "cast", the hoof can't flex, so the pump doesn't work. Insufficient blood and nutrients are pulled into the foot to build and maintain strong tissues. So, the quality of sole, wall, and frog can become poor; injuries are slow to heal; and the white line deteriorates over time. Clips generate point loadings that cause proliferation of the tiny laminae that act like velcro to hold hoof and pedal bone together, so instead of fewer strong laminar commections there are are large numbers of smaller and weaker commections.
In the tough and elastic barefoot hoof, the flexing of the weighted hoof can absorb the concussion of the horse weight as it lands. But a metal shoe will hold the foot inflexible, cancelling out 75% of its ability to absorb shock. Instead, the concussion goes on up the leg and damages joints and tendons that are not designed to take so much shock.
Left: The diagrams illustrate the force transmission up the leg with peripheral loading. all the weight of the horse is carried on a fraction of the available ground loading surface. Add to this the concussive effects of a metal to hard surface interface and these "peripheral loading devices" seem much less attractive. The hoof is designed to carry the weight of the horse across the whole surface of the foot - walls, frog, bars and sole.
Try this - choose a flat surface and hit it with the edge of your hand like a karate chop. Now hit it with the whole flat of your hand. Which one do you prefer?
Horses will grow the size and shape of foot that they need to support them in the environment that they are in, the work that they do, the surfaces they walk on, and to cope with any conformational faults that they have. Iron shoes deny them the opportunity to do this by forcing the foot to conform to a standard shape.
This is much better!
Left: probably the worst underrun heel I have seen on a lovely trotter in Ohio. This is a deliberate shoeing approach to improve the "reach" of the horse. Whren you are on the scurry behind these horses, you can see why the long hoof is an advantage. However there must be immense strain on the deep flexor tendons and lever forces on the laminae at the toe.