It can be fun to watch when a horse drops down, and with an expression of pure joy, rolls over and over in the mud -- unless you've just given him a bath. While horses can certainly find all kinds of ways to outwit you, rolling in the mud isn't something they do just to get dirty.
When a horse gets sweaty, he starts to itch. Since he doesn't have fingernails or claws to relieve that itch, his best option is to roll around in the mud and dirt. This is why horses who have just been worked or bathed will often head right for a nice muddy patch of ground and roll back and forth. Rolling in mud helps get rid of the tickling sensation of sweat or water.
Rolling is part of a horse's natural grooming habits. A good roll and wiggle in the mud can help remove loose hair and dead skin. To a horse, mud is an appealing body scrub and conditioner. It can also help pull out any loose leaves, stems, burrs or other foliage trapped in his hair.
Insects and Sun
A nice coat of mud gives a horse protection against insects and the sun. The mud helps keep away flies, gnats, mosquitoes and other irritating bugs. It also provides an extra barrier against the sun, which can help reduce sunburn on horses with pale skin.
On a summer day, a mud bath can give some relief from the heat. Even if it's not terribly hot outside, a horse still shedding his winter coat in the spring, or who has been running or working hard might find relief in the mud.
Fun and Socializing
Part of the reason a horse rolls in the mud may be for the sheer enjoyment of it. According to the University of Pennsylvania, there appears to be both an entertainment and social component to rolling. Horses will often roll in the same spot and in the same pecking order each time. Among breeding males, the dominant male rolls last.
Back and Joint Health
Although he may not consciously realize it, a horse rolls in the mud to keep his back and joints healthy. Rolling helps stretch his back and muscles and exercises his joints when he gets up and down. Trainer Cheryl Sutor notes that horses have fewer spine problems when allowed to roll on a regular basis.