It has been known for a very long time that oats and horses just seem to get along well. Of all of the cereal grains we can feed to horses, oats is the only grain that can be fed safely without being cooked. Why? Horses are able to digest a large portion of the starch that oats contains in their small intestine. Which means only small amounts of starch will be deposited into the hindgut.
If we feed corn or barley uncooked, only about 25% of the starch they contain is digested in the small intestine. The rest is fermented in the hindgut.
When starch gets into the hindgut, starch fermenting bacteria ferment it very quickly and cause a build-up of acid and create a condition called hindgut acidosis which has all sort of negative consequences for your horse (including changes in behaviour and laminitis). The balance between fibre fermenting and starch fermenting bacteria is also put out of balance… the ‘good’ fibre fermenting bacterial populations are reduced while the less desirable starch fermenting bacteria increase in numbers.
All of this we have known for a while. More recent research however (Harlow, 2015) has also shown that corn starch is potentially more of a problem in the hindgut than oat starch. This researcher incubated corn and oat starch in test tubes with faecal material from horses and found (briefly) that corn starch caused a significantly greater increase in starch fermenting bacteria than the oat starch and gave rise to higher lactic acid production.
So oats seems to have a double safety catch built in for our horses. First, most of its starch appears to be digested in the small intestine. Second, if some starch does end up in the hindgut it appears less likely to upset the hindgut bacterial populations present there, which should mean it allows your horse to maintain a healthy population of the ‘good’ fibre fermenting bacteria.
So if you need to feed a grain and either don’t have access to or don’t like ‘cooked’ grains for some reason, go for oats, they are the safest choice for your horse.