Horses have evolved to handle a wide variety of vegetation in their diet, but with a few key differences between the feral horse and the domesticated horse. A horse ranging freely in search of food consumes nutrients such as carbohydrates and fats in a very diluted form because grasses and other plants are at least 75 percent water. Wild horses also get a lot more exercise than domesticated horses, which is important to good gut function. We don't really know exactly why, but research bears this out.
The dilution factor is important because most gut upsets in the horse are related to the large intestine. The horse's large intestine is essentially a fermentation vat—like the four stomachs of a cow. Nutrients are first broken down by the bacteria and protozoa that live in the hind gut. The byproducts of that breakdown are actually what the horse then absorbs and turns into proteins, fats, and carbohydrates that the body can use.
Each specific type of organism (and there are hundreds) will need a specific type of food to survive. Some are better at using complex fibers. Some like simple sugars. Others will thrive on high protein. Still others will be intermediate forms and will further break down the products produced by other types of organisms. When the organisms receive a constant flow of foods to ferment in a high volume of fluid, they can adapt to changes much easier.
When a large "load" of fermentable food hits the hind gut all at one time, if it is different from what the organisms are accustomed to handling, the result can be a rapid increase in the "bugs" that prefer that type of nutrient. This, in turn, can change the chemistry in the intestine and cause other forms of bugs to die off. Consequences for the horse range from poor utilization of feed (or some portions of it) to gas, mild distention, diarrhea, or full-blown colic.
If the large intestine becomes too acidic, as it can if unusually high amounts of sugar, starch, or complex plant sugars enter it, the lining can actually be damaged and bacterial toxins absorbed into the body. This can make the horse very ill. It can even cause laminitis.
Most people know you shouldn't start, increase, or change grains rapidly. This is one of the most dangerous things you can do in terms of risking intestinal upset. However, what you may not realize is that changes in hay can be bad, too.
The large intestine is the major site for breakdown of hay. Even if you always feed the same type of hay, such as timothy, Bermuda grass, or alfalfa, not all hay is created equal. Different cuttings, under varying growth conditions and even different strains of the same type of forage, can vary by 100% or more in the level of rapidly fermentable nutrients they contain. Changing hay types risks dietary differences in both the levels and relative proportions of fermentables your horse's system must adapt to.
Even rapid changes in pasture plants can cause problems for your horse if their composition changes too much. This is especially true in the spring and fall when grasses are growing (or regrowing) at a rapid rate. Young growths of grass are lower in slowly fermented fiber types and can have wide swings in the amount of simple carbohydrate they contain. Very high protein levels in young, growing pastures can also cause gut upset.
These guidelines can help you prevent feeding-related gut upsets:
• Introduce grain feeding gradually, no more than 1 pound per feeding.
• Allow three days between each increase in grain to enable organisms to adapt.
• Don't feed more than 4 pounds of grain at one time.
• Make changes in hay gradually, replacing from 10% to 25% of the old hay with the new variety; increase every three days.
• Accustom horse to lush pastures gradually, especially if grass is growing rapidly (spring and some fall conditions).
• Keep hay available for horses on young growths of pasture grass to provide complex and slowly fermented fiber which the grasses may be lacking.
Eleanor M. Kellon, VMD, of Equine Nutritional Solutions in Ephrata, Pennsylvania, is an authority in equine nutrition and expert in the field of equine nutraceuticals. Her most recent book is Horse Journal Guide to Equine Supplements and Nutraceuticals (Globe Pequot Press).