Horses were designed for constant grazing. They have small stomachs, and most do much better on diets of small, frequent meals. This type of diet can be a challenge to maintain for many of today's horses. Horses that live full-time in stalls are at the mercy of whatever feeding schedule is established by the barn manager. One way to closely replicate the feeding system most preferred by horses is to feed free-choice hay as the only feed.
While hay is as close to a natural feed as you can get for many modern day horses, it isn't perfect. Hay is dried grasses, so it doesn't provide the same moisture level as an all-grass diet. For this reason, your number one concern with an all-hay diet should be free access to fresh water. While water is important for all horses, regardless of their diet, a horse on an all-hay diet requires water to move the hay through the digestive tract. If he doesn't drink enough, he may develop impaction colic, where a blockage develops in the intestinal tract, or become dehydrated.
Supplementing a horse who eats an all-hay diet can be the best of both worlds. Your horse gets the benefit of eating lots of hay, which is easier on his digestive tract than grain, and you are certain that he is getting the nutrients he needs. There are two different ways to supplement a horse who eats a hay diet. The first is with a vitamin and mineral supplement. These supplements offer all the nutrients your horse needs to be healthy. You only need to feed about an ounce daily, and many are available in pellets, which you can feed your horse by hand, like a treat. There are different supplements available, depending on whether your horse is on a grass-hay diet or an alfalfa hay diet. Talk to your vet about which is best for your horses.
If you are concerned about the amount of protein your horse is getting from his hay, as well as his nutrient profile, you may want to consider a ration balancer. A ration balancer contains all the vitamins and minerals your horse needs, as well as high levels of protein. They make a good choice for horses that maintain their weight well on a hay-only diet, but have high nutrition demands, such as young horses or easy keepers in heavy work.
The drawback of supplementing your horse's all-hay diet is that it may be unnecessary, and it is an added expense. While it is unlikely that you will cause any health problems by adding a supplement to a hay-only diet, you may be wasting money. Ration balancers can be even more pricey, and again, are not always necessary.
One way to know exactly what your horse is eating and if it is sufficient is through hay testing. Talk to your vet or feed store about getting a nutritional analysis of the hay your horse eats. With these numbers in hand, you will know exactly how much protein, calcium, selenium and other nutrients your horse is consuming. Of course, hay testing is not free, and if you buy hay a few bales at a time from different vendors, it will not be cost efficient, however if you buy all your hay for the year at once, from a single grower, hay analysis can be a valuable tool.