Ask the Nutritionist: Dr. Getty's Forum for Equine Nutrition
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All the best,
Dr. Getty
Hi Liz,
Well, the answer is, "it depends!" (How's that for clarity?!)
It really depends on the content and size of the meal. If the meal contains cereal grains (oats, barley, corn, etc.) or is high in fat, then it stays in the stomach longer and therefore, you should wait approximately 30 minutes to an hour to exercise your horse. Walking is fine, but more than that can be problematic since food is still in the stomach and trotting or running can possibly lead to colic.
If the meal is high in forage -- grass, hay, hay extender, beet pulp... Then, it doesn't stay in the stomach long and even moves through the small intestine rapidly. It ends up in the hind gut where the bacteria can digest it and derive calories from it. So, a high fiber diet like this is ok for feeding just before exercise.
If the meal is very large, then regardless of whether it is digested early in the gut (carbohydrates, fat, and protein) or in the hind gut (fiber), caution should be taken when exercising the horse. A meal of concentrates (commercial feeds, oats, and other starchy or fatty feeds) should never be more than 4 lbs since the horse's stomach is not large enough to handle more than this safely.
I hope this info is helpful!
All the best,
Dr. Getty 
Where are you from? Bayfield, CO