Horses have teeth that continue to erupt from the jaw over a very prolonged number of years. These make them better equipped to handle the increased wear that occurs when grinding the more abrasive grass, which may often have trace amounts of grit from the surrounding soil surface and plant roots.
The horse as we know it today has has a jaw conformation where the upper jaw, the maxilla, is wider than the lower jaw or the mandible. This arrangement allows horses to maximize their chewing efficiency, prolong the effective life of their chewing equipment (premolars and molars), and hopefully, remain adequately fed for a long period of time.
The teeth erupt on average about three to four millimeters per year to compensate for the wear from the daily grinding action of the food processing. The average permanent (adult) premolar or molar (grinding or “cheek” teeth) have a reserve crown of approximately four inches (100 mm); under ideal conditions one could estimate that the happy, healthy horse should have teeth that should not wear out for 25 to 30 years.
In order for the horse to obtain food, it must first grasp the food. If they are grazing, as they lower their head to the ground surface the maxilla (upper jaw) slides slightly backward as the mandible (lower jaw) slides forward. As the head comes into position at the ground surface, the incisors (front) teeth should be aligned to cut or shear off the grass pasture. This allows the horse to graze or cut the pasture very close to the ground surface without disturbing a significant amount of surrounding grit, dirt and debris. Under normal circumstances, the horse will cut the grass off at ground level, rather than pulling the plant out by the roots.
The lips, tongue, cheeks and hard palate all serve a role in moving the food along the "conveyor belt" into the oral cavity for further processing. The lips act as a sorting/selection tool to find, test and pull food into the mouth. The tongue acts as an auger to work the food back in the mouth, where the bolus is pushed out onto the grinding surface of the cheek teeth (premolars and molars).
Some horses will consistently chew or process their food in one direction; others will process or chew their food in both directions. The important point to remember is that grinding requires significant motion of the mandible and maxilla in relation to each other. Studies that have examined how the different types of feed affect how a horse chews its food have shown that a much larger range of motion is required to grind hay than a concentrated feed source (i.e. grain).
As the food is ground, it is moved across the surface of the tooth, out into the oral cavity; the cheek contains the feed and pushes it back onto the surface of the cheek teeth where it is crushed again. The ridges on the roof of the mouth in the hard palate aid in the direction of the food, passing it further back into the mouth where the tongue pushes it out onto the tooth surface for additional grinding. This process is repeated multiple times until a thoroughly chewed bolus arrives in the back of the mouth for swallowing. Any changes in this finely tuned, delicately balanced and “machined” process can greatly affect the horses ability to find, collect and process its food to nourish itself. Our domestication and current housing and management of the horse also can have a tremendous impact on the horses ability to maintain a balanced and healthy food processing machine (mouth).
Information from the American Association of Equine Practitioners