According to Teresa Burns, DVM, a clinical instructor in equine field services at Ohio State University,, rolled hay, or round bales, is sometimes fed as a more economical food for horses, but it’s a bad idea for multiple reasons.
“Inflammatory airway disease or heaves, a form of asthma, can happen in horses that eat round bales,” Burns says. “These large round bales house mold spores and can even lead to botulism. The horses are standing in these bales and inhaling everything that was growing inside these bales, with the center being the worst part.
“Small square bales are better, but it’s faster and cheaper to feed the large bales, which is a big mistake.”
Veterinarians and horse nutritionists agree, that providing plenty of good forage for horses is a major priority in keeping horses healthy. A horse owner who is observant about the hay being fed will double check the hay for any contaminants including mold and/or fungus before feeding it.
Depending on the growing and harvesting conditions of hay, some round bales may be perfectly good, but it's best to make sure any hay given a horse will not be hazardous to its health.