By Rob Arnott, DVM
Horses have been around for many, many years. Some experts have determined from fossil evidence that horses have existed for over 55 million years. These early horses were probably browsers that ate so, leafy vegetation and groundcover in the prehistoric woodlands. The horse evolved over time, and the current form and type of dentition in the horse is believed to have evolved about 15 to 20 million years ago. This evolution allowed adaptation that was better suited to grazing. The changes included a longer cheek teeth row and a deeper skull and jaw to better accommodate the hypsodont (high-crowned) tooth. The hypsodont tooth (a tooth that continues to erupt from the jaw over a very prolonged number of years) is better equipped to handle the increased wear that occurs when grinding the more abrasive grass (which may have trace
amounts of grit from the surrounding soil surface and plant roots).
The horse as we know it today has a hypsodont tooth and an anisognathic jaw conformation; 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, long period of time.
The hypsodont tooth erupts 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 prehend or 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).
The chewing cycle is a repetition of a cyclical movement of the rhythmical contraction of the muscles that control the opening and closing of the jaw. There are three parts to the chewing cycle; the dropping or lowering of the mandible and it’s sliding sideways in relation to the maxilla, the closing of the mandible against the maxilla and the grinding of the mandible across the maxilla. The steps are called the opening, closing and power stroke phases of chewing. 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 mastication (grinding) requires significant motion of the mandible and maxilla in relation to each other. Studies that have examined how the different types of feed aect 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).