This great graphic, created by Naomi Tavian (@equinaomi), is the best illustration I’ve seen that illustrates how long it takes for a horse to become fully mature — essentially not until they are six. It makes you think hard about the sports where we ask them to perform at ages two and three, where they are still so unformed.
Part of the issue lies in the fact that horses appear to mature far earlier. A foal can stand within a half hour of its birth.
In 1979 Dr. Harold Hintz reported Windfields Farm’s growth data for 1,992 foals from birth to 22 months of age. The records illustrate how quickly foals grow. Thoroughbreds and other light horse breeds will reach 84% of their mature height at six months of age. Assuming a mature Thoroughbred will be 16 hands, the six-month-old weanling will be approximately 13.2 hands. At 12 months that horse will have reached 94% of its adult height or around 15 hands, and at 22 months it has almost finished growing in height, reaching 97% of its full height at approximately 15.2 hands. (The Principles of Bone Development in Horses).
Weight takes longer. At six months, a foal weighs 46% of it’s mature weight and at a year, weighs 65% of it’s final weight.
Bone density is directly impacted by the ability of the young horse to move.
Research from the Netherlands reported significantly lower (37% ± 4%) bone density of the third tarsal bone of the hock of five-month-old foals that were housed in box stalls compared to pasture-raised and box-raised foals that were sprint-trained from one day of age. However, the authors caution against the sprint training of foals because of possible negative long-term effects. In another study from Michigan State University, researchers maintained 17 weanling Arabian horses in stalls 24 hours/day, on pasture 12 hours/day, or on pasture 24 hours/day for 56 days. Nutrient intakes were standardized.
The 12-hour pasture turnout group and the full-time pasture turnout group had increased bone mineral contents. Cannon bone circumference increased in both the pasture group and the 12-hour pasture groups, but not in the stalled group. The take-home message is to try to keep foals, weanlings and yearlings turned out at least 12 hours a day. (Training and Bone Development)
Since horses often look more mature than their bones can support, it makes it extra important for riders and trainers to keep the rate of growth plate fusion in mind as they increase work and expectations. As shown in the illustration above, and called out below, the process of growth plate fusion starts in the hooves and then moves upward over time. Interestingly, certain dimensions, such as the height of the coffin bone, is determined at birth.
This is a pretty sobering diagram when you think of how much we ask of young horses, who’s bones are still growing and fusing! The last bones to fuse are the vertebral column, which takes at least 5 1/2 years or longer — male horses mature later and taller horses with longer necks also take longer. According to Dr. Deb Bennett, this means that a 17-hand gelding may not be fully matured until they are 8 (source). This is something I definitely felt with my Trakehner gelding. I bought him when he was five and he was at least seven before I felt he had fully matured.
So make sure your young horses get plenty of turnout and don’t be in a rush when training. Taking your time gives your horse the chance to mature from the inside out.