When is a horse skeletally mature? Researchers examine the science

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Horses and humans follow similar phases of growth, but in horses they are heavily restricted to the first two years of life, researchers say.
Image by Rebecca Scholz

Horses enter skeletal maturity by the time they are two years old, the authors of a just-published review have concluded.

There is little variation in the age of maturity across different horse breeds, they reported in the open-access journal Animals.

The authors found that the horses’ timeline for skeletal maturity aligned with the racing industry practice of training and racing two-year-olds.

Researchers Chris Rogers, Erica Gee and Keren Dittmer, with New Zealand’s Massey University, said there is often debate within the lay literature, and social media in particular, about the age at which horses should be started and introduced to racing or sport.

“To optimize the welfare and longevity of horses in racing and sport, it is important to match exercise with musculoskeletal development and the ability of the musculoskeletal system to respond to loading,” they said.

“The justification for not exercising horses at a certain age is often in contrast to the scientific literature and framed, with incorrect generalizations, with human growth.”

The review team, citing 55 peer-reviewed papers, described growth and bone development in the horse within the framework used in medical and human literature.

A schematic representation of the three periods of growth in human and equine development and the relative age at attainment of skeletal maturity. Graphic: Rogers et al. https://doi.org/10.3390/ani11123402
A schematic representation of the three periods of growth in human and equine development and the relative age at attainment of skeletal maturity. Graphic: Rogers et al. https://doi.org/10.3390/ani11123402

Measures of bone growth-plate closure and body growth show that the horse completes the equivalent of rapid infant growth by weaning, at four to six months of age.

At about 11 months, the horse completes the equivalent of the childhood phase of growth and enters puberty. Puberty in horses can occur from as early as eight months, but typically occurs two or three months later.

At two years old, the horse has achieved most measures of maturity used within the human literature, they said. These include the plateauing of vertical height, the closure of bone growth plates, and adult ratios of back-length to wither height, and limb length to wither height.

“In the horse, these development periods are completed very early in life,” they said.

The published science supports the hypothesis that the horse evolved to be a precocious cursorial grazer — that is, with limbs adapted for running.

The horse, they concluded, is capable of athletic activity, and use in sport, relatively early in life.

The species, they noted, evolved as an herbivore to exploit an open plain/grassland environment. It evolved to escape predation through running from an early age.

Foals are born at about 10% of their mature weight and have a musculoskeletal system advanced enough to permit standing and suckling within the first hour of birth.

Within their first week, foals are able to regularly cover more than 7km a day with their mother.

The authors noted a recent review that demonstrated that the horses’ musculoskeletal system, and the long bones of the limbs, in particular, respond positively to loading provided by locomotor play.

The evidence indicates that not only are foals born with an advanced musculoskeletal system, but that the developmental potential of the musculoskeletal system is highly receptive to exercise, even before weaning.

“Both horses and humans demonstrate similar patterns of growth,” they said, “but in the horse, these phases of growth are heavily restricted to the first two years of life.

“This condensing of the growth phases and early attainment of maturity reflects the ecological niche to which the horse evolved to exploit as a cursorial herbivore.”

The review team said the available scientific literature on bone growth and maturity, in combination with study findings on training and exercising young horses, reflect the hypothesis that evolutionary programming means the horse is capable of positive musculoskeletal responses to exercise early in life.

“Current industry practices of racing and training two-year-old horses are in alignment with the horses’ developmental potential and evolutionary programming.”

Rogers, C.W.; Gee, E.K.; Dittmer, K.E. Growth and Bone Development in the Horse: When Is a Horse Skeletally Mature? Animals 2021, 11, 3402. https://doi.org/10.3390/ani11123402

The review, published under Creative Commons License, can be read here.

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2 thoughts on “When is a horse skeletally mature? Researchers examine the science

  • November 30, 2021 at 9:25 am
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    Wonder how much the racing industry paid for it? There are all kinds of research that indicate otherwise. Only agreement is that the two yr. old growth plates at knees are closed.

    Reply
  • December 1, 2021 at 1:13 am
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    Perhaps the horses height is at a majority of maturity, but the joints are not. I have been rehabilitating horses for years and many are those put under full work at two or early three. More research needs to be proffered to show the problems caused by this ill advised concept.

    Reply

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