Little research has been conducted on how long a horse may be hauled before having detrimental effects. Horses should be offered water at least every five hours and have access to hay during long trips. The horse should not travel more than 18 hours without being unloaded. Exercise of the horse during this period will help prevent colic, founder and lower leg edema. It helps to know your horse. Some horses refuse to urinate in a trailer so that it may be necessary to unload every five to eight hours.