That is going to depend on the circumstances. Are they broke to ride, and do you ride them/work them regularly? We operate a horse boarding business and horses are routinely single penned in 10' X 40-50' runs. Usually, but not always, the horses are ridden/exercised at least once a week by their owners. We don't generally have any problems unless a horse in thrown into a run, fed grain and alfalfa hay, and never exercised or ridden.