In the normal mouth the cheek teeth are held apart by 2 to 3 millimeters when the mouth is closed and the incisors are touching. The surface of the cheek teeth are sloped from side to side, so as the horse chews from side to side, the upper cheek teeth slide up and down the lower cheek teeth. This grinds the feed. If the front teeth are overgrown, the mouth is impaled too far open, and the cheek teeth do not have enough contact time to grind feed efficiently. Eventually the surfaces of the cheek will loose their normal angle as well, and then the mouth is a real mess.
Inefficient chewing is detrimental, sometimes even fatal, in the horse.
There are places in the equine large intestine where the diameter of the bowel decreases dramatically, and the bowel makes an 180 degree turn at the same time. This slows the transit time of feed through the bowel. When feed slows down it starts to ferment, and can cause the pain of colic. Excessive fermentation can also lead to systemic bacterial infections and other illnesses. Choke is a fairly common digestive problem that can be caused by poor dentition.