Despite being big and strong, your horse has a fairly delicate digestive system. A number of factors can cause digestive problems, such as colic, which can prove fatal in serious cases. The Humane Society of the United States recommends that you wait an hour to ride your horse after he eats.
Horse Digestion 101
Horses are grazing animals, which means they're suited to eating small amounts all throughout the day. They have small stomachs, which are only capable of holding a little food at any one time, but have nearly 100 feet of intestine and colon which is where most of their digestion takes place. Their food only stays in their stomachs for 15 minutes, before beginning to move through the rest of their digestive tract. This limited stomach capacity means horses are susceptible to gas buildups, which can cause more serious problems.
Although colic is a generic term for any abdominal pain, it usually refers to excess gas in your horse's stomach, which -- in the most severe cases -- can lead to ruptures in the digestive tract and death. While your horse is being exercised, your horse's body will divert blood away from his digestive system, which means he digests his food more slowly. This slowing digestive system increases his chance of getting colic. If your horse shows any signs of having colic -- such as loss of appetite, lip curling or persistent rolling -- call your veterinarian right away.
Riding After Feeding
Riding your horse directly after he's eaten isn't advisable. Not only could slowing down his digestive system when it's full lead to a bout of colic, when your horse's stomach is full, his lungs have less space to work. This can make exercise seem more strenuous to him. As such, you should wait at least an hour after you've fed your horse before you ride him. If you're going to be working him especially hard, it's best to wait for three hours before you exercise him.
Feeding After Riding
Just like you need to wait after feeding your horse before riding him, you should also wait after riding your horse before feeding him. Again, you should leave it roughly an hour before you give him anything to eat. You can tell he's safe to be fed once his breathing has returned to a normal rate and his body is roughly at its usual temperature, so that he no longer feels excessively hot or sweaty.