Never look a gift horse in the mouth
By Dr. Cory Reng, DVM., Assistant Professor, Nebraska College of Technical Agriculture
This is as good advice now as it was a hundred years ago, but what does it mean? It basically means if a horse is free, expect it to be old.
Almost all horse folks know that it is possible to determine a horse’s age by looking at its teeth. It is very rarely necessary, however, because most horses have a paper trail that will confirm the age of the horse. Horses are also retained by one owner for long periods of time in today’s world. There are occasions, however, that being well versed in horse aging may be important.
The most important principle in aging horses by their teeth is that you will only be able to estimate the age. Sydney Galvayne’s book, Horse Dentition, written in the 1880s, suggests that horse aging was an exact science. Since then, we have learned that aging horses is an art that, while based in science, is far from exact.
The most accurate fundamental of aging horses is when teeth erupt. A foal’s incisor will come in starting at the midline. The center four teeth will come in during the first week of life. The next four will be in by 6 weeks of age, and the corner incisors will erupt by 6 months of age. Usually the cheek teeth will be erupted at birth, or within the first week.
Of course, the age of a foal is pretty obvious by other methods. So what is really important is when the permanent incisors come in. The center incisors come in at about 21/2 years of age. These are followed by the eruption of the next ones at 31/2, and the corners erupt at 41/2 years. When the permanent teeth come in, the baby teeth, or caps, are shed. This can startle people when a young horse’s incisors fall out. These incisors then grow in to the height of other teeth. It usually takes 1 year for the incisors to grow in far enough to start wearing against the opposing incisor. Therefore, the incisors come into wear at 31/2, 41/2 and 51/2 years, respectively.
Another important key to age is the eruption of canines. These come in between 41/2 and 6 years of age and will continue to grow for some time. They do not wear much because they are unopposed. These teeth occur in stallions and geldings, but only rarely in mares. The cheek teeth also erupt on a general time frame. These are difficult to see, however, and cheek teeth eruption is not generally used for aging, although it is fairly accurate.
Now for the difficult part of horse aging. After teeth have erupted and are in wear, aging is done by the amount of predictable wear over the years. A longitudinal section of an incisor, along with a series of cross sections, shows the components of the incisor used to determine age. The infundibulum, also referred to as the cup, is an important structure. It is an empty space surrounded by an enamel ring. The other important structure is the dental pulp, which will be filled in by secondary dentin as the tooth wears. This darker dentin is also called the star. The incisors shape is also important to understand. The tooth’s original surface when it erupts is oval; the incisor gradually becomes triangular and then square.
By looking at these parts of the incisor together, you can get a good estimate of the horse’s age. Usually you start with looking at the lower incisors. Upper incisors usually appear a year or two younger. Upper teeth are often very helpful in deciding the range of age likely. Here are some examples.
They keys are a) shape of the incisor, b) presence of star or cup, c) shape of star or cup. The shape of the incisor will start as oval and become triangular at around 13 years. Then the teeth will slowly become rectangular by around 20.
Cups are deep holes in the teeth when they first erupt. They have an enamel coating. Because enamel is harder than the rest of the tooth, that enamel will be the very last part of the tooth to wear. First the cup will lose its ability to hold anything (the sides have worn away) and last the enamel will go. The cups become level with the tooth (and can’t hold anything) starting on the center incisor at 6. Then the next follows at 7, and finally the corner’s cup is gone by 8 years. The enamel will remain for some time.
The stars start to appear around 8. Remember they are the pulp of the tooth that fills in with secondary dentin as the tooth grows out and continues to erupt and wear. Stars are basically the same material as the rest of the tooth, but will be a darker color, stained by the plant pigments. These stars start as a long oval to the lip side of the cup. Initially they go almost the full width of the tooth. As the enamel remnant of the cup wears away, the star becomes more round and more central in each incisor. The stars are usually fairly central by the time the incisor becomes triangular.
Of course, everyone wants to age horses by Galvayne’s groove. This is one of the least accurate measures of a horse’s age, but when taken in whole with other age indicators, it can be used to narrow down your estimate.
Galvayne’s groove usually appears at the gum line of the corner incisor at around 10 years of age. It is half way down the tooth at 15 years. It will be all the way down the tooth by 20 years, and will disappear from the gum line first and go all the way down the tooth by 25+.
The 7-year and 11-year hooks are a function of the changing angle of how the incisors meet. Young horse’s teeth meet at a very steep angle when young (around 90 degrees), but will camp out as the horse gets older (close to 45 degrees). When the teeth meet (or fail to meet) at certain ages, the back of the corner incisor fails to wear, leaving a hook. The teeth go back into wear as the angles continue to change, wearing the hooks back off. Therefore the hooks only appear near 7 and 11 years of age.
Of course, all of these are just guidelines. Every horse is an individual and will get his or her teeth in when they come in. With good documentation, aging horses by their teeth is less and less necessary. It is fun. however, to maintain a lost art.