Oats have been the grain most often fed to horses. Of the common grains for horses, it is the most nutritionally balanced and best for the horse's overall health. Compared to other grains, whole grain oats are high in fiber (10.5% to 12%), lower in starch and are more easily digested.
These benefits reduce the risk of colic and laminitis (founder), making oats the safest grain to feed horses. In addition, the presence of 5% fat provides both nutritional value and additional energy in a safe form, as opposed to pure carbohydrates which can increase the risk of laminitis.
Oats can sometimes make horses much more energetic. This has resulted in the expression 'he is feeling his oats' to describe someone who is much more energetic than usual. When most horses were worked hard (e.g. plowing fields, pulling carriages) this additional energy was considered an advantage. However, now that many horses are used for slow pleasure riding, an energetic horse which wants to run is often considered a disadvantage. With some horses, the extra energy provided by oats is quite pronounced, especially if they are fed large amounts and have little opportunity to burn off the extra energy (e.g. kept in box or small paddock much of the day), which can result in them insisting on running even when their riders command otherwise. Such cases have resulted in the myth that oats makes horses a little crazy and resulted in the use of other grains instead, although the same problem can occur (but is less likely) with other grains.