Is is okay to let my horse eat as much hay as he wants

My horse is about 20 years old, not overweight, has 1/2 acre to roam freely. Would it be okay to let him have free access to his hay. He doesn't seem to overeat. He will walk away from his food when he is full and go back later to finish. I've heard that a horse doesn't know when to stop eating.

Yes - offering as much hay as the horse wants is called "free choice" hay feeding. I do believe this is best for the horse's health, because in the wild, horses forage constantly for food. Horses don't sleep much, and if they were free, they would spend the rest of their waking hours nibbling on food.

The horse's digestive tract is designed to always have a small amount of food passing through it. The healthy bacterial flora in the horse's digestive tract can start to die off after as little as FOUR hours without eating. This is why feeding probiotics to horses is normally an excellent idea; their digestive tracts are constantly being disturbed by unnatural feeding schedules. When you consider how long stabled horses normally go without eating every day, it is no wonder that colic is a common problem! Three meals a day is great for people, but not so great for horses.

Roughages are so low in nutritional / calorie content, it's pretty difficult for a horse to become overweight from eating too much hay. That would be like a person gaining weight from eating nothing but salad. :) Some people believe too much hay will result in "hay belly", but that is usually caused by other factors, such as age, or lack of excercise. It will certainly not cause him any digestive problem or colic.