Aging By The Teeth

(Updated December 2023)

Aging the horse by its teeth is an outdated and inaccurate method.  I no longer recommend this because of today’s abundant methods, including accurate papers and microchips.  The percentage of horses with accurate papers or microchips is low, so the teeth method is still widely used.

Aging the horse by the teeth uses the regular wear of the incisors.  The theory is that the incisor teeth will wear uniformly over time and even within the mouth on each side as the horse ages.  In college, I memorized this chart (below), but I soon realized that the wear rate was not uniform over time between equally aged horses or within the individual horse when comparing the left and right sides. Below is the chart I memorized.

The First Five Years

6 days oldThe central incisors appear.
6 weeks oldThe middle incisors appear.
6 months oldThe corner incisors appear.
2 1/2 yearsThe central incisor caps are lost.
3 yearsThe second cheek teeth caps are lost, and the 4th cheek teeth erupt.
3 1/2 yearsThe middle incisor caps are lost.
4 yearsThe second cheek teeth caps are lost, and the 5th cheek teeth erupt.
4 1/2 yearsThe corner incisor caps are lost.
4 – 5 yearsThe 3rd cheek teeth caps are lost, and the 6th cheek teeth erupt.
6 months timeAll permanent incisors take 6 months from eruption to being in wear.

“Full mouth at five years.”

Although the corner incisor caps sometimes shed after five years, this is a reasonably accurate expression.  Most of the incisor observations are close, although any delay in eruption or deviation in the projection of the permanent incisors will affect the accuracy.  An abnormal mouth confirmation will also change the wear and affect the aging, such as parrot mouth, wry mouth, sow mouth, or trauma to the incisors.

This second table (below) looks at the cups (the hollow “cup” formation in the center of the occlusal surface of the lower incisors), the dental star (a dark spot between the tongue side and the cup), the shape of the occlusal surface and a groove (Galvayne’s groove) that runs the length of the upper corner incisor over time.  It is a static groove relative to the tooth itself but appears to move relative to the gum line and the occlusal edge as the tooth erupts and wears away.

After Five Years

6 yearsThe central incisors lose their cups, oval shape, and dental star starting.
7 yearsThe middle incisors lose their cups.
8 yearsThe corner incisors lose their cups.
10 yearsGalvayne’s groove starts at the gum line, dental star full, triangular shape starting.
15 yearsGalvayne’s groove extends from the gum to half the tooth’s length.
20 yearsGalvayne’s groove extends the entire tooth length, all triangle shape.
25 yearsGalvayne’s groove extends from the middle of the tooth to the occlusal edge.
30 yearsGalvayne’s groove disappears, and all have a round shape.

The factors that affect the wear of the incisors are:

  • The type of tooth enamel (Type 1 is soft and is predominantly in the incisors, whereas type 2 is predominantly in the cheek teeth are type 2.  There is type 3, which I call brittle).
  • The amount they chew per day (the range is between 10,000 and 40,000 chews per day)
  • The movement of the chewing (horses predominantly chew in one circular direction, poorly described as clockwise or counterclockwise just as we are left or right-handed dominant)
  • The tongue movement is affected by avoiding the pain created by sharp enamel points of the cheek teeth).  The tongue wears away the incisors and NOT the opposing teeth.

The aging project is pictures of horses with a confirmed date of birth.  With the horse relaxed and without medication, I took four views of the incisors: left side, right side, straight on and open mouth to see the occlusal surfaces of the lower incisors (cups, shape).

Viewing these aging project posts will take time and a good internet connection.  Open one post at a time and wait for all the images to load entirely before opening another post.

“Age is the number of times the sun has been circled, but how old you are is how you feel in your mind and heart.” – Doc T.

My conclusion from the project is to lump horses into five groups of ages. The links below will take you to a post with the images taken during my aging project so you can see the variability in the changes for yourself.

The Eruption Of Cheek Teeth

In aging the horse, the teeth further back in the mouth are often never mentioned. This omission was the case in December 2023 when an astute follower (thank you, Daniel) sought an answer for the eruption of the cheek teeth (and the reason for this updated topic). Fortunately, the expression “Full mouth at five” still applies.

The cheek teeth form behind the interdental space, the area in horses’ mouths where only the canine teeth form in male horses. They divide into two groups: the pre-molars and the molars. In the young horse, only the pre-molars exist, and these come in deciduous and permanent forms.

The evolutionary purpose for half of the mouth being filled at birth only with teeth that shed is to offer a temporary chewing surface for the foal to eat the ground plants, allowing them to survive. Developing the complex and layered cheek teeth that will last their whole life of chewing takes time. Staggering the eruption times of the permanent cheek teeth gives a relatively stable chewing surface from a time before the permanent teeth can develop. Staggering also prevents a long period where a gap occurs between teeth. Without this system of deciduous teeth and staggered eruption, the horse would have too few cheek teeth to allow it to survive.

The Pre-molars And The Molars

Deciduous pre-molars are the first three cheek teeth on both sides, top and bottom. The total is 12 teeth. They appear in the foal either at birth or soon after. Like the deciduous incisors, these early “milk teeth” are ejected in a sequence and replaced with permanent cheek teeth over time. The definition of a “pre-molar” is a cheek tooth that first forms as a deciduous tooth and is eventually replaced with a permanent cheek tooth. The opposite is a “molar,” which develops without a deciduous tooth.

Foals eject their deciduous pre-molars in a relatively timed sequence. Unfortunately, this is often irregular between breeds, within the same breed, and often within each horse as you compare these teeth within the same mouth. The chart below is a guide.

Permanent Pre-molar Eruption

2 – 2 1/2 yearsFirst Permanent Pre-molars erupt
2 1/2 to 3 1/2 yearsSecond Permanent Pre-molars erupt
3 to 4 yearsThird Permanent Pre-molars erupt

Like the pre-molar cheek teeth, there are also three molars on each side, top and bottom. The molars erupt without a deciduous precursor and with a timeframe that is also variable. The charts below are a guide.

Permanent Molar Eruption

About 3 yearsFirst Molars erupt
About 3 1/2 yearsSecond Molars erupt
Between 4 and 5 yearsThird Molars erupt

Cheek Tooth Eruption Over Time:

Age# of cheek teeth# of capped cheek teeth# of permanent cheek teeth
2 years12120
3 years1688
4 years200-416-20
5 years24024

Three things become clear from looking at the eruption of cheek teeth over time:

  1. Between 2 and 5 years of age, every horse has dynamic changes in the mouth. Therefore, horses seen between 2 and 5 years with erupting new teeth need more frequent floating (every 3 to 4 months). This frequency is standard in the young racehorse.
  2. Between 3 and 5 years of age, permanent teeth are in front of (rostral) and behind (caudal) deciduous teeth (caps). In some horses, this arrangement can squeeze the eruption of the third permanent pre-molar, causing it to rotate on the vertical axis (twist), deviate to the side (lean medially towards the cheek or laterally towards the oral cavity), or both twist and lean. Often, this will self-correct, but the deviation becomes permanent in some, leading to excessive sharp edges and trapping of food, decay, or gum damage. This squeezing can also affect the second and fourth cheek teeth, but the third is the most common.
  3. Horses floated before five years have an incomplete mouth, with the last cheek teeth remaining under the gum. The last (most caudal) cheek teeth are often the first to become sharp due to the ever-present tongue activity there, adversely affecting the bit response. Many non-racehorse owners are surprised when their recently floated 4 1/2-year-old horse soon shows a poor bit response. These unerupted teeth were untouched by the floating blade, and like all recently erupted cheek teeth, they are very sharp when they appear.

Most people will not use the cheek teeth to age the horse, yet it is essential to horse owners investing time and money into training that they recognize the activity within the mouth of their young horses under five years of age.

Canines And Wolf Teeth

Canine teeth, found in the interdental space, are usually in male horses (stallions and geldings) but can occasionally be found in female horses. There are between zero and 4 teeth, with eruption times varying from birth to 18 years, so they are not useful for aging horses.

Wolf teeth are officially a pre-molar being removed in the evolution of horses (vestigial). There are between zero and 4 found in horses and are, therefore, unreliable for aging horses.

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