Why do horses wear shoes?

Horses have been central to human transportation and agriculture for centuries. These symbols of power and speed require hoof care and new shoes every four to six weeks to stay on the job. But why?

Horses (Equus caballus) that are domesticated for human use and selectively bred for performance wear shoes because their feet are delicate and therefore need protection, said Dr. Fernanda Camargo, a veterinarian and equine extension agent at the University of Kentucky. "Shoes provide protection to some areas of the foot of the horse," Camargo told Live Science in an email. "They prevent the hooves from wearing out too much, and thus becoming sensitive."

The exterior of the hoof, known as the wall, is made of a horn-like material that grows continuously and has to be trimmed, just like a person's fingernails, according to University of Missouri Extension. "Shoes also help the foot maintain its proper shape," Camargo said.

However, rough terrain, such as sand and rocks, can cause the exterior to wear away, exposing the sensitive inner hoof. Then, the horse experiences pain and may be unable to walk. Historically, such impairments would have prevented horses from being used on the battlefields or during the harvest, so shoes were added to reinforce the hoof wall, Camargo said.

It's estimated that horses have been wearing shoes of some kind since they were domesticated about 6,000 years ago, Camargo said.

Related: Why do we still measure things in horsepower?

Originally, horseshoes were made of leather or plant material. Metal shoes nailed to horses feet were first used around A.D. 500 and became commonplace over the next 500 years, Camargo said. While aluminum and steel shoes nailed to the hoof are still the most common, she said, a variety of other materials — such as rubber, resin and plastic — can also be nailed or glued to the hoof as a shoe.

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While many horses need shoes, not all do; it depends on the type of riding, the terrain and how frequently the horse is ridden. Those ridden on rocky terrain or concrete are more likely to need shoes. Even horses that aren't ridden may require shoes to protect them from the terrain or therapeutic shoes to help manage a foot condition. But "a lot of horses that are just ridden here and there, and are kept on grassy/not hard terrain will do just fine without shoes, with regular farrier visits," Camargo said.

Meanwhile, wild mustangs don't wear shoes and manage to travel over rough terrain because they have very strong feet, Camargo said. But their hooves can still wear down and cause lameness. If this happens it will cost a mustang its life, she said.

Some people wonder whether nailing shoes on the horse's hoof hurts them. There are no blood vessels or nerves in the wall of the hoof, according to University of Missouri Extension, so if the shoe is nailed on properly, it isn't painful. "But improper shoeing can absolutely hurt," Camargo said. If the shoes or nails are placed wrong, the shoes are the wrong shape or size, or if the farrier applies pressure in the wrong areas, they can hurt the horse. And if the hooves are badly trimmed beforehand, it can lead to pain or lameness with or without shoes, she said.