It may be cheaper to feed a single grain like oats compared to commercial feeds. Check the price. Oats is a high-fiber feed that's important if you limit hay intake. Oats is the safest grain, too. If feeding more than 6 pounds of grain, feed it twice a day.
Some owners feed whole, shelled corn or even corn on the cob.
The latter was a common practice with work horses and mules. But don't feed more than about 3 pounds of whole, shelled corn to a horse daily. Corn on the cob can result in choke. Another hay stretcher could be hay cubes. Usually you can find alfalfa hay cubes, but grass hay cubes are rare.