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Stalling Young Horses Alters Normal Bone GrowthBy Kentucky Equine Research Staff · November 26, 2014

A major concern with young performance horses is the high incidence of skeletal injury. Young, growing horses transferred from pasture to stalls prior to yearling sales or commencement of training may be predisposed to injury. A decrease in bone density of the third metacarpal has been demonstrated in young racehorses soon after the onset of training. However, it was unknown whether the decrease in bone density was the result of bone remodeling caused by increased strain rates on the bone associated with training, or bone modeling resulting from decreased strain rates associated with a change in housing from pasture to stalls at the start of training.

Transferring young horses from pasture to stalls has been shown to result in decreased osteocalcin concentrations, indicating a slowdown in the rate of bone formation due to decreased physical activity. Studies of other species have demonstrated similar decreases in bone strength in response to confinement rearing. Though horses on pasture may not run excessively, only a few fast strides per day may be necessary to prevent bone loss associated with limited physical activity available to horses housed in stalls. Consequently, pastured horses may have a skeletal structure that is better prepared for training and competition. If so, the common practice of housing yearling horses in stalls prior to yearling sales or commencement of training causes concern about the effects of stalling on bone growth.

At a nutrition conference sponsored by Kentucky Equine Research, a report was made of a study conducted at Michigan State University. The study was designed to determine if bone development is negatively affected when yearlings are taken from pasture to be housed in stalls and allowed limited exercise. In addition, the consequential effects of the change in housing on bone modeling/remodeling at the onset of training were determined.

Sixteen Arabian yearlings, with an average age of 18.6 months, were pair-matched by age and randomly placed into two groups. One group was housed in box stalls while the second group was kept on pasture. Radiographs of each horse’s left front leg were taken every 28 days to measure bone density. Blood and urine samples were also collected and analyzed on a regular schedule to look for more indications of skeletal status and mineral excretion.

After an 84-day pre-training period, six horses from each group were randomly selected to complete a 56-day training period. Analysis of the radiographs showed that stall-housed horses had a decrease in bone density from day 0 to day 28, while pasture-reared horses had greater bone density. Urinary analysis indicated that some bone resorption occurred in stalled horses during the first four weeks of stalling, but this indicator returned to baseline after day 28.

It appears that housing yearlings in stalls may negatively affect normal bone growth experienced by yearlings allowed to remain on pasture. Although it was not tested in this study, free access to exercise may have provided sufficient loading on the legs of pastured horses to promote normal bone growth. Results suggest that housing yearling and two-year-old horses in stalls without access to forced or free exercise impairs normal bone growth, compared with horses maintained on pasture. Initial training did not appear to alleviate the negative effects of stalling on bone formation.