Why do horses get put down if they break a leg?
Racehorse injuries
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Skeletal fractures account for 87% of fatal injuries. When a horse's leg hits the ground at racing speed on a straightaway, it bears a load that is three times its weight (with the exception of harness racing). When negotiating a turn, centrifugal force increases the load to between 5 and 10 times body weight. When a horse hits the ground, the repetitive impact produces microscopic cracks and crevices inside bone so tiny they are undetectable by standard X-rays. If the horse isn't given enough time for healthy bone tissue to repair the damage with a process called remodeling, the cumulative stress can progress silently to the point where overload causes bones to break.[34]  The American Association of Equine Practitioners (AAEP) recommends a period of rest of at least 10 days between races for all horses to provide an opportunity to refresh and diminish the volume of persistent cyclic loading.[35]