The purposes of skin and hair are many and equally important. They keep everything inside in and everything outside out. It grows hair and adds fat to keep us warm, and it sweats to keep us cool. It senses movement and helps to feel the succulent blades of grass. It shakes off flies and screams for attention when it is damaged. It remains flexible around joints but firm over muscles and the face.
Skin is clever in that it can take care of itself. When cut, it heals through an amazing and precise process. But when humans and other factors mess with this process, healing becomes troubled in a process almost unique to horses. It also can become allergic to fly bites and a bed of food and a breeding site for flies when damaged. Viruses can affect skin integrity and may cause several neoplasia types of skin diseases (equine sarcoid, aural plaques, warts). Cancer can strike, which can be troublesome (melanoma) or life-threatening (squamous cell carcinoma). It can react with foods that cause sensitivity to light (photosensitization). It can itch to the point of damage. It can also be rubbed to ecstasy (mutual grooming) or torn from fighting or self-inflicted wounding (self-mutilation).
Good bacteria normally cover the skin and excreted oils that keep the skin healthy. Daily washing with soaps and detergents removes these protective features. And why wash a horse when they go right out and roll in the dirt? Soaps should be limited and cleaning done with only water and a scrubber. I have not used soap on my body and hair for years, no one has complained of any body odor, and my skin is very healthy. There is a lot of research on this in humans, but again, none in horses.
There are a lot of topics with images here because the skin and hair are visible. I’ll give brief explanations for each, but I have found two things that help a poor hair coat and skin. The first is to feed high-quality protein filled with all essential amino acids. I have seen improved hair coats in as little as two weeks with the addition of all the amino acids. The second is eliminating all grains, grain byproducts and sugar treats from the diet. More on this under nutrition. You may find that daily baths with soap are no longer needed to keep the horse shining. Saving money AND time is a treat for most horse owners.