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- When is a horse skeletally mature?
Chris W. Rogers 1,2,* , Erica K. Gee 1 and Keren E. Dittmer 1
Background
The age at which horses achieve skeletal maturity is a topic of debate, particularly in the context of racing and sport. Many arguments against early training cite concerns about incomplete bone development, but these views often rely on human growth models rather than equine-specific data. Horses are precocial herbivores, meaning they are born with a relatively advanced musculoskeletal system and can engage in athletic activity early in life. This review aimed to compare equine growth patterns with human developmental stages and determine when horses reach skeletal maturity using measures such as growth plate closure, bone length ratios, and body weight gain.
Methods
Literature Review:
-Structured search in Google Scholar and Web of Science using keywords related to horse growth, bone development, and maturity.
-Inclusion restricted to peer-reviewed studies.
Key Metrics Evaluated:
-Growth plate (physis) closure in different skeletal regions.
-Body weight and wither height development.
-Back length and limb-to-height ratios.
-Comparison with human growth phases (infantile, childhood, pubertal growth spurts).
Results
Growth Plate Closure:
-Most distal limb growth plates close by 10–12 months.
-Proximal limb and vertebral growth plates remain active slightly longer but close by ~24 months.
-The distal radius, one of the last long bone growth plates to close, fuses between 25–31 months.
Body Weight and Height:
-At birth, foals are 10% of their mature weight and 63% of their mature height.
-By 6 months, they reach 43% of mature weight.
-By 2 years, they achieve 96% of their final body weight and 98% of their final height.
Limb and Back Growth:
-The back length-to-wither height ratio stabilizes by 18 months, indicating completion of axial skeletal growth.
-The limb-to-height ratio stabilizes at ~60% by 1 year, showing early completion of appendicular skeletal growth.
Comparison with Human Growth:
-The horse’s rapid early growth period is equivalent to infant and childhood growth in humans but occurs within the first 6 months.
-Horses reach puberty by 10–11 months, aligning with their natural breeding cycle.
-By 2 years, horses have reached skeletal maturity, similar to humans at ~18 years old.
Limitations
Data were primarily collected in Thoroughbreds, though similar growth patterns are observed in Warmbloods, Standardbreds, and other breeds.
Lack of direct DXA bone mass measurements, which are commonly used in human studies to assess peak bone density.
Influence of environment and management (nutrition, exercise) on growth rates was not extensively analyzed.
Conclusions
Horses achieve skeletal maturity by approximately 2 years of age, with growth plate closure, body weight, and height development nearly complete. These findings support the idea that horses are evolutionarily adapted to athletic activity at a young age and that early training (such as in 2-year-old racehorses) aligns with their biological development. This evidence challenges misconceptions about delayed skeletal maturity and underscores the importance of scientific data in guiding equine training practices.
Representation of the equivalent anatomy of human and equines with the reported age (months) at closure ofspecific growth plates (physis) for the horse based on weighted average estimates presented in Table 1. Image modified from Centered Riding by Sally Swift.
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