Study: Horses Mature Enough for Athletic Endeavors by Two Years Old

Some social media outlets, lay literature, and organizations question the wisdom and safety of training and competing two-year-old horses. In support of these practices, experts from Australia recently reported that horses attain maturity early and that the majority of growth is completed before they are two years old, concluding “current industry practices of racing and training two-year-old horses are in alignment with the horses’ development potential and evolutionary programming.”^*

The researchers compared the growth of horses to that of humans, as the two species reportedly have similar growth patterns:

• The period from birth to weaning is equivalent to the rapid growth period of infants (birth to three years of age). There is rapid longitudinal growth of the foal’s distal (lower) limbs and a remarkable increase in body weight coincident with rapid development of muscle mass.
• The period from weaning to 10-11 months of age is comparable to childhood phase (3-10 years of age), characterized by a steadily decreasing rate of growth until the onset of puberty. For horses, this means a steady decline of average daily weight gain.
• The pubertal growth phase for horses begins around 10-11 months of age. This phase, like that in adolescent humans from 12-18 years of age, is characterized by an initial period of accelerated growth followed by deceleration in growth until the final adult height is reached. At the onset of puberty, horses are approximately 92% and 60% of their adult height and weight, respectively.

The research team also observed the following:

• At birth, foals are approximately 10% of their mature weight. By six months, around the time of weaning, their weight quadruples and is then about 43% of their mature weight. By one and two years of age, horses are 61% and 96% their mature weight, respectively.
• By two years of age, horses reach approximately 98% of their mature height.
• Growth plate closure—when long bones stop growing—is well defined, ranging from eight to 31 months. Those patterns were relatively consistent across breeds, suggesting that growth patterns are similar regardless of breed. It should also be noted that while the growth plates may still be visible on X-rays, suggesting they are still “open,” data support the observation that growth actually ceases several months before they appear “closed” on X-ray.

“The finding that horses achieve 98% of their growth by two years of age supports how horses evolved as herbivores, necessitating avoidance of predation via flight beginning at an early age. Foals are therefore born with a musculoskeletal system advanced enough to allow them to stand and suckle within a mere one hour of birth and cover long distances with its dam within one week of life,” explained Kathleen Crandell, Ph.D., a nutritionist for Kentucky Equine Research.

She added, “In addition to being skeletally mature early in life, we cannot forget that early exercise helps reduce injury and supports development of the musculoskeletal system in young horses. Early exercise therefore improves the welfare and longevity of horses.”

Crandell recommends offering young, athletic horses Triacton, a product scientifically proven to increase bone density and to protect the gastrointestinal tract. “Triacton supports bone quality of young horses entering training during high-risk transitional and early stages of training, and provides digestive buffers for the stomach and hindgut,” Crandell advised.

Interested in other growth studies? Kentucky Equine Research has performed multiple studies over the years. Here’s a sampling.

Thoroughbred Growth and Future Racing Performance

Bodyweights and Growth Rates of Spring- and Autumn-born Thoroughbred Horses Raised on Pasture

Body Weight, Wither Height and Growth Rates in Thoroughbreds Raised in America, England, Australia, New Zealand, and India

*Rogers, C.W., E.K. Gee, and K.E. Dittmer. 2021. Growth and bone development in the horse: When is a horse skeletally mature? Animals (Basel) 11(12):3402.