An Evolutionary Success Story
By Shelagh Niblock, PAS
Horses have evolved very successfully as consumers of forage and other plant material. Many of their physical characteristics such as body size and capacity have evolved around accommodating their large hindgut where populations of beneficial microbes are maintained, allowing the utilization of complex carbohydrates like cellulose as a source of energy. The teeth of horses have also evolved to allow horses to consume a steady diet of plant material.
Horse owners should know that good horse husbandry includes at least a yearly dental examination by a veterinarian for all the horses in the barn. This health care protocol is equally important for horses of all ages. Young horses need regular dental exams as their teeth change with growing. Performance horses need regular dental work as dental problems may cause performance issues. And seniors, perhaps most of all, need regular assessment of their teeth to ensure they are able to chew properly.
The dental examination may lead to floating of the teeth to smooth rough edges or address other abnormalities which might be causing discomfort for the horse. If the horse’s teeth have been neglected, there may be a need for a more rigorous program of dental work. Some horses need dental maintenance more often than others, and horses with problem teeth may need dental work as frequently as every three to six months.
Why do horses need to have their teeth floated?
Horses left to graze at least 16 hours a day wear their teeth down more evenly and will likely develop fewer dental irregularities than stabled horses with mixed diets. Photo: Shutterstock/Eleon Images
Horses have hypsodont teeth with a large crown or portion of tooth above the gumline and a very large root below the gumline. Hypsodont teeth are constantly erupting and growing out of the gum. Continuous chewing of coarse plant material results in the enamel surfaces of the teeth being worn down. With open-ended roots, hypsodont teeth have a complex system of enamel, as well as tissues called cementum and dentin. Their structure and continuous growth over the life of the horse ensures that even though the forage diet of horses wears their teeth down, the horse can maintain a functional tooth.
Research shows that horses will graze at least 16 hours per day if allowed and will tend to wear their teeth down evenly without developing uneven ridges, waves, or tooth hooks. On the other hand, stabled horses with mixed diets of forage and concentrates are far more likely to develop irregularities in their teeth and consequently need regular dental care.
Humans and animals like dogs and cats have brachydont teeth, which do not continue to erupt out of the jaw throughout the life of the animal, and closed roots as well as a smaller root system and crown.
Enamel, cementum, and dentin
The crowns of equine teeth are covered by a hard mineral-rich enamel. The roots are covered by a calcified tissue called cementum. The cementum is the source of the collagen fibres which attach the tooth to the gum or gingiva of the teeth. Cementum is joined to the enamel at the cemento-enamel junction. The next layer in the equine tooth is the dentin, which is a calcified collagen tissue produced by cells called odontoblasts. The dentin extends right through the root and up into the tooth crown. Finally, equine teeth have an inner core of pulp, which is highly vascularized, extends throughout the entire crown/root structure, and is attached to the lymphatic system of the horse.
Organization of the horse’s teeth
The teeth of the horse are arranged in a pattern that includes 12 incisors — six on the top and six on the bottom — as well as a mature potential of 12 molars on each of the top and bottom arcades. The molars or cheek teeth are comprised of three premolars and three molars for a total of six molars in each quadrant. All four quadrants and all of the teeth within them are numbered for identification according to the modified triadan system. Teeth are usually identified by their quadrant and a number corresponding to where they are in the quadrant. The quadrants are numbered one to four starting with the upper right side of the mouth, progressing to the upper left side, then to the lower left side, and eventually, the lower right side.
Some, though not all, horses have auxiliary teeth called wolf teeth and most horses also have four canine teeth, which are situated in the spaces between the incisors and the premolars. These teeth appear to have no function and may be vestigial teeth from prehistoric times.
Effective chewing for horses is achieved through the grinding action of the cheek teeth of both the mandibular arcade (lower jaw) and the maxillary arcade (upper jaw). The grinding action of the occlusal (biting or contact) surfaces serves to tear fibre pieces apart, allowing them to be mixed with saliva before swallowing. Stabled horses fed a combination of forages and concentrates spend far less time chewing than horses that are grazing all the time. Their teeth do not get worn down as fast allowing for the development of abnormalities, which can cause discomfort for the horse. To maintain effective chewing for the stabled horse the cheek teeth must be smoothed out by floating or removing any sharp edges or hooks. In a normal set of equine teeth the chewing surfaces are somewhat angled and not completely flat. Veterinarians are careful when doing a dental float to maintain effective angles. Horses with neglected teeth cannot always have them corrected within one dental treatment. It sometimes takes several treatments to correct long standing dental issues.
Removal of teeth can also cause problems. All the molars in both the upper and lower arcades are situated in very close proximity with each other. Removal of teeth can result in the migration of some molars into the spaces left, potentially resulting in future problems with the grinding surfaces. Loss of a molar in the cheek teeth arcades can also permit food to be trapped between the teeth and gum. This is a condition called diastema and can result in periodontal disease in horses if not attended to very carefully.
All horses should have a dental examination by a veterinarian at least once a year, and may need their teeth floated to smooth rough edges and hooks causing discomfort. Photo: iStock/Harlequin129
Teeth of young horses
Horses are considered to be diphyodontous, meaning the young horse develops a set of deciduous or milk teeth starting within a week of birth. By two years of age, a young horse will have 12 incisors (three in each quadrant) and 12 premolars (three in each quadrant) and may be growing the first of their permanent molars. Between the ages of about two-and-a-half years and four-and-a-half years, they will get the balance of their molars, and their canine and wolf teeth if they are going to get them. They will also lose their deciduous teeth, both premolars and incisors, called caps at this point as they are replaced by permanent adult teeth. Sometimes it may be necessary for your veterinarian to remove caps on a young horse, as they don’t always get displaced by the adult teeth replacing them.
Related: The Well-Fed Foot
Aging horses by their teeth
Because of the normal wear on equine teeth as well as normal tooth eruption in hypsodont teeth, it is possible to age horses with some degree of accuracy by examining their teeth. Examination of the incisor teeth will reveal wear patterns on the enamel, cementum, and dentin, which can help establish where the horse is in its life.
A horse that is under eight years old will have a marking on the occlusal surface of the incisor teeth in the lower jaw called the infundibulum or cup. As the horse ages, this wears away due to chewing and eventually the cup is no longer visible. As the occlusal surfaces of the incisors wear away over time, eventually all that is visible is a small hole called the dental star. This is actually part of the pulp cavity in the tooth and usually becomes visible after eight years of age. Horses under 11 have lower incisors with an oval shape, but in horses over 11, the incisors develop a more triangular shape, and eventually as they wear they become more rectangular.
The Galvayne’s Groove is a groove that appears on the third incisor of each quadrant, further helping to age horses. Galvayne’s Groove begins to descend from the gumline at about age ten, is halfway down the tooth around age 15, and is all the way down the tooth by age 20. At 25 years it is half gone, and at 30 years it is completely gone. Aging horses by their teeth over the age of approximately 28 is next to impossible as there is very little tooth left to examine.
Galvayne’s Groove appears on the third incisor in each quadrant, first appearing around age ten. Photo: Canstock/Kiep
Chewing and feed choices
Horse owners know that a barn full of horses munching on hay is the foundation of a contented barn. Chewing is not only important for adequate nutrition, it’s also important for the mental health of horses. Forage quality in terms of nutrient density and digestibility has improved significantly in recent decades due to genetic selection in forage species and the agronomic practices of forage growers, and it has become possible to feed even performance horses a diet that consists of mostly good hay. The selection of the best hay to meet the needs of your horses will bring them health benefits, including the need to chew. Consider choosing hay based on not only the analytes of crude protein (CP), water soluble carbohydrate (WSC), and digestible energy (DE), but also those of the fibre values of acid detergent fibre (ADF) and neutral detergent fibre (NDF) as well. Lower ADF/NDF hays (under 35 percent ADF on a dry matter basis) will tend to have higher energy and digestibility as well as require less chewing. Remember that while each mouthful will require less chewing the horse may consume more mouthfuls, which can facilitate increased energy intake for the performance horse, senior horse, or growing horse.
Alternatively, you could select hays with a higher ADF/NDF (over 35 percent ADF on a dry matter basis on your lab report) if you want to limit intake and encourage chewing for the more sedentary pleasure horse. Maximizing your horse’s ability to chew will encourage better dental and mental health for them. (For more information about choosing forages based on the ADF/NDF content, read The Chew Factor.)
Research findings and anecdotal evidence support the hypothesis that horses grazed full-time may need less maintenance of their teeth than stabled horses eating mixed diets of forage and concentrates. The reality though is that our lifestyle often demands that our horses be stabled, so a horse owner must be diligent in attending to their horse’s dental health. Once a year dental exams may be enough for most horses but if your senior horse is quidding, which is dropping mouthfuls of food out of his mouth, or is having a hard time maintaining body weight, then it may be necessary to invest in increased dental examinations.
Choose hay that best supports the nutritional needs of your horse but remember to include ADF and NDF in your decision-making. Take note of how well your horse is eating, how much time it takes for him to consume a meal of hay, and if he is “sorting” or leaving any behind. And finally, ask your veterinarian to show you what’s going on inside your horse’s mouth when doing his yearly dental exam. Consider taking a picture for your records, then use it to gain a better understanding of your horse’s teeth and how they affect his health and well-being.
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Main Photo: AdobeStock/Douglas Vigon EyeEm