Historically, oats have been the first choice of feeds. Oats are medium in energy, require little or no processing and have more protein than most grains. However, they are variable in energy content. You should avoid oats with a light weight per bushel because of their low energy and high fiber content. The best oats usually come from the north central states such as Minnesota, North and South Dakota and northern Iowa.
Corn is fine for feeding horses, but is highly concentrated in energy.
You must take care not to overfeed it. Wheat is especially dangerous because it causes colic by impacting in the gastrointestinal tract. It is often recommended for horses. Committee on Animal Nutrition, National Research Council, 1989. Feed balanced rations.