Easley also believes that the influence of management factors on the wear patterns of horse teeth has been overlooked in standard aging guides. Horses on pasture, for example, are constantly using their incisors to crop off grasses. The abrasiveness of forage varies with the amount of silicates (silica-based minerals) it contains, and grazers' incisors wear faster or slower depending upon forage type and the amount of their grazing. Additionally, horses who live in areas with sandy soil may wear their incisors more rapidly and give the appearance of being older than horses of the same chronological age who live on other soils.
Stall-kept horses, who use their front teeth primarily to pull hay from a net or rack, usually show less wear of the incisors than pastured horses. Feeding from a hayrack rather than from the ground also contributes to a more youthful mouth, Easley says, because the horse does not lower his head and stretch his neck when eating. This means that the normal change of the incisors' profile angle to a more forward slant is slowed.
In horses who suffer from abnormality in the way their teeth come together (malocclusion) or deformities such as overbite ("parrot mouth"), corrective dental care can play havoc with aging techniques by filing away all of the markings associated with age. But if malocclusions, broken teeth or other dental abnormalities are left untended, they produce irregular wear that can also confuse aging: For instance, a horse may give the appearance of being six years old on one side of his mouth and nine on the other. Mineral deficiencies are another cause of abnormal patterns in the teeth and can lead to errors in aging. Finally, horses who crib or run their teeth along stall bars may smooth the normal pattern of their incisors prematurely, making themselves appear older than they really are.
For almost a century, Galvayne's system of matching dental features to specific ages provided experts with a measure of certainty. We now know that this system is more of a guideline than a blueprint.
Discovering that the master plan is inaccurate, even by a few degrees, can cause the builder-or the horseman--some unease. But it doesn't mean that the plan must be discarded. After all, it is not really surprising that individual horses grow, shed and replace their teeth on varying schedules, or that tooth wear varies among horses, depending on breed and environment. As long as you make allowances for some dental "dishonesty" when you look to a horse's mouth for an accounting of his years, you'll come away with some useful information.
"This is the only way we have of aging horses," says Easley, "and while it may not be a very accurate method, it can still usually tell us if a horse is young, middle-aged or old, and it is still a valid thing to do."
Charting dental change
The permanent teeth of horses are called hypsodont, meaning that they have short roots (about three-quarters of an inch) and long crowns (as much as 41/2 inches in unworn teeth). Contained mostly within the gums at first, the crowns erupt at a rate of about one-eighth inch each year. This is enough to accommodate average wear and allow the upper and lower incisors to maintain contact--despite a lifetime of nipping and chewing--unless the lifetime exceeds 30 years, in which case the horse can actually use up his teeth. Sydney Galvayne based his famous aging system on the effect ordinary wear has in exposing successive levels of incisor material in horses after age five (the age when a horse has all his permanent teeth). The following alterations in appearance are the key indicators of increasing age.
- The cement-lined enamel cup (infundibulum) on the biting (occiusal) surface of each incisor wears away in early maturity. The concave cups disappear on the central lower incisors at about age six, on the intermediate incisors at age seven, and on the corner incisors at age eight. Because the cups on the upper incisors are deeper, they take longer to disappear, but they are generally gone by the time a horse is 10 or 11. After the bottom of the back edge of the cup disappears (about age 15), the surface of each incisor is marked by a small, round enamel spot.
- Yellow-brown dental stars appear between ages eight and 10, as the upper portion of the tooth is worn away. They are actually the pulp cavity, which has been filled in with secondary dentin as the tooth has matured. These stars" start out as dark lines in the center of the tooth, then change to more star-like ovals at about age 13, and finally become round at about age 15. In elderly horses, the occlusal surface is worn smooth and patternless.
- As the horse ages, the shape of the incisors' occlusal surfaces alters from narrow oval to roundish to triangular, eventually becoming longer from front to back than side to side.
- The profile of the upper and lower arcades progresses from nearly upright in young horses to a forward slant in the elderly.