Grains can be fed to horses, either on their own, or as part of a prepared horse feed. The following table compares grains commonly fed to horses and are based on whole grains. If the grains are hulled (see discussion below on whole grains), the percentage fiber will be lower and the other percentages higher.
When calculating quantities of the various grains, note that they are vary in density. As shown in the table below, when using a measure (such as a quart or liter), one gets almost twice as much corn or wheat as one gets oats or barley, using the same measure. As the amount of digestible energy (the amount of the energy that the horse actually gets, excluding undigested portions) is the same for almost all grains on a weight basis, the fact the corn and wheat weigh almost twice as much per measure means that the horse gets almost twice as much energy per measure with these grains as compared to oats.
If one looks at just the carbohydrate portion, the difference is even more extreme, as wheat has almost three times as much carbohydrate energy as oats. This is because much of the energy in oats is in the form of fat and fiber. For further information, see recommended grain quantities.
Historically, oats has 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 (flounder), 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.
A common alternative to oats is barley. One advantage is that it is less likely to result in excessive energy. However, horses typically prefer the taste of other grains and some horses are reluctant to eat barley due to the taste.
Barley starch is also more difficult to digest and consequently more of it passes through the foregut to the hindgut, which may increase the risk of colic or laminitis.
Barley grains are relatively hard, so they typically need to be crushed (see discussion below).
Corn (which is known as maize in many countries) is high in starch and consequently high in energy. Although not as nutritious as oats or barley, it is a relatively inexpensive source of energy to fatten up a thin horse or maintain the weight of a working horse. Horses like the taste of corn so there is no difficulty in feeding it to them, although the grains will need to be crushed for digestibility. It has the same problem as barley in that the starch typically ends up in the hind gut rather than being digested in the foregut.
Wheat is similar to maize in that it is high in carbohydrate energy. However, aside from energy it has limited nutritional value. In addition, many horses find it more difficult to digest than other grains. Unless there is a strong economic (or other reason) to use wheat, one should use one of the other grains.
Crushed, Rolled or Crimped Grains
The hull of grains can be difficult to digest, especially as many of the grains are swallowed by horses without being chewed sufficiently to break the outside hull of all the individual grains. This can result in some of the grain simply passing through the horse, without being digested. Although this is likely not a problem health wise, it does mean that substantial amounts of food can be wasted.
To avoid this, it is common to break the hulls, typically by putting them through a machine with one or more rollers which crush the grains. The result is commonly called 'crushed', 'rolled' or 'crimped' grain (e.g. 'rolled oats').
The disadvantage of this process is that it reduces the shelf-life of the grains in two ways. One is that when stored, the nutritional value decreases at a faster rate. The other, which is a more serious issue, is that mold or fungus can establish itself quicker on crushed grains than on un-crushed grains, which can cause a number of different illnesses. Consequently, it is best not to store crushed grains for long periods of time. If you feed a lot of crushed grain to your horses, it may be more economical to buy a small grain crusher (from a horse equipment store) so that you can freshly crush yourself rather than buying crushed grain.
With oats, crushing is not necessary as healthy horses will normally digest most of the grain even if it is uncrushed. With barley and corn, crushing is more important, as these are difficult to digest if un-crushed and un-hulled.
One can either buy whole grains (with the hull) or hulled grains (hull has been removed). Whole grains are generally healthier as they are higher in fiber and the hulls may contain additional nutrients lacking in the grain kernel. However, whole grains may need to be crushed (see above discussion) for digestability. In the case of horses with a weaker digestive system (e.g. very young, very old, or sick), one may prefer to use hulled grains for better digestibility.
If one uses straw bedding, one may wish to avoid using the same grain. For example, if one uses wheat straw it may be best not to feed wheat grain or if one uses barley straw it may be best not to feed barley grain.
The first reason for this is that horses sometimes develop allergies, and the risk of developing an allergy is higher with increased exposure. So if a horse is fed wheat and also has wheat bedding (which it not only lies on but also likely eats some of), then the risk of developing a wheat allergy is increased.
Related to this is the issue of what to do if a horse develops an allergy. For example, if it develops an allergy to wheat and one uses wheat bedding, it is advisable to switch to another type of bedding, since horses which eat food they are allergic to can develop a number of health issues (e.g. excessive water retention, poor digestion). Especially in a commercial stable, where alternative bedding types may not be available, this can be an issue.