Keeping the equine microbiome healthy involves feeding an appropriate diet, and when necessary, adding a supplement that offers a variety of active prebiotic ingredients.

Unlike us, horses can thrive on a diet consisting solely of highly fibrous plant material. The reason? The activity of the bacteria and protozoa inhabiting the equine intestinal tract, collectively known as the equine microbiome, makes it possible.

The stomach and small intestine

The upper portions of the equine digestive tract – the stomach and small intestinal segments called the duodenum, jejunum and ileum – work the same way as ours. The stomach begins the digestion of protein and fat, while the small intestine completes the process, using enzymes released by the pancreas and intestinal cells to digest and absorb fats, amino acids from protein, and simple sugars. Starch is digested to glucose (a sugar) before being absorbed.

The work of the microbiome starts in the stomach and small intestine. Several classes of bacteria have been found to colonize the upper intestinal tract, predominantly Lactobacilli and Streptococci. These ferment sugars, starch and short plant sugars or small fructans, producing lactate and volatile fatty acids, which are then absorbed by the horse and used for energy or to produce fats or glucose in the liver.

Bacterial fermentation in the stomach and small intestine serves several functions. It reduces the load of carbohydrates that must be digested by the horse’s enzymes. It also reduces the rise in blood sugar from eating. Lactate and volatile fatty acids are produced (see sidebar on page xx). Finally, these active bacteria release growth factors into the intestinal fluid, encouraging the further growth and proliferation of beneficial bacteria.

The large intestine

In the large intestine (cecum and colon), the microbiome continues with the same functions as in the small intestine, but a more varied population of organisms makes it possible to ferment more complex plant materials such as long plant sugars, fructans, cellulose, hemicellulose and the soluble fibers pectin and beta-glucan.

Last, but not least, the microbiome provides gentle stimulation to the immune system via the GALT – gut-associated lymphoid tissues. The beneficial microorganisms provide direct protection from organisms that would harm the intestinal tissues (by competing for food via their sheer numbers), and by secreting various antimicrobial substances.

Feeding the microbiome

Obviously, maintaining a healthy population of organisms in the microbiome is critical for the health and digestive function of the horse. As with all living things, the health of the microbiome begins with feeding it.

The foal is born with a sterile intestinal tract. He picks up organisms from the environment, and those that find the temperature, pH, oxygen and fluid levels to their liking – and that are getting the type of food they need – will set up house and flourish. The process of picking up organisms from the world around him continues in the adult horse. Keeping them there is then a matter of diet.

To maintain a good population of appropriate organisms, you have to feed the horse like a horse. The diet is by far the most important source of prebiotics (substances that encourage the proliferation of beneficial organisms). The feral horse eats a diet of predominantly grasses, but will also browse on other plant materials, from bushes to tree bark. The domesticated horse inevitably has less variety in his diet, but you can still mimic this diet by feeding a diet of predominantly hay and/or pasture.

Hard-working horses may not be able to eat enough hay or pasture to maintain their weight. Horses can tolerate grain feeding if most of the grain is digested in the small intestine without spilling over into the large intestine (hind gut) in excessive amounts. A safe amount is generally accepted to be 1 gram of starch per kilogram of body weight, which amounts to 1.25 kg (2.75 lbs) of plain oats (which are only 40% starch) for a 1,100 lb horse.

Many other feedstuffs are given to horses, each with a unique profile of fat, fiber, starch and protein. If you choose foods with a high soluble fiber content and low to moderate starch, you will be supporting the microbiome and still be getting higher calories than from hay or grass. These foods include beet pulp, soybean hulls and flax seeds (whole or defatted). When fed daily, psyllium husk fiber is also a prebiotic, since the organisms quickly adapt to fermenting it.

Many digestive supplements also contain prebiotic ingredients – substances that feed the beneficial organisms directly, or create a favorable environment for them to grow. Check the analysis and ingredients list for items such as fermentation products or extracts, yeast cell wall, live Saccharomyces cerevisiae or boulardii organisms, mannan-oligosaccharides, fructo-oligosaccharides, chicory root, slippery elm bark and marshmallow root. In addition to providing growth factors, fermentation products and extracts have high enzyme activity that assists both the horse and the microbiome in breaking down and utilizing his diet.

Substrates explained

The food used by a microbe is referred to as a substrate. Bacterial processing of the microbe’s substrates is called fermentation. Structurally uncomplicated substrates such as glucose or other simple sugars are absorbed intact and used by a wide variety of organisms. More complex substrates like starch or fiber must be processed first. The organism uses enzymes to break down this type of substrate into smaller substances. This is often a cooperative effort between different organisms with different enzyme capacities. For example, one class of bacteria may break down starch and ferment the glucose that results in lactate. Another class will use the lactate as their food.

Volatile fatty acids

Volatile fatty acids (VFAs), aka short chain fatty acids, are the ultimate end product of bacterial fermentation. The dominant VFAs are acetate, propionate and butyrate. They are small organic compounds whose larger cousins are used to put together triglycerides and the phospholipid membrane around cells.

Most acetate is used directly as an energy source, but could be turned into fats. Propionate that is not used is converted into glucose. Butyrate is an important energy source for the cells lining the intestines. These cells use most of the butyrate; excess can be converted to fats in the liver.

The horse’s digestive process is complicated, but supporting it doesn’t have to be. Feed him an appropriate diet, and when necessary, choose a supplement with a variety of active prebiotic ingredients to keep his microbiome flourishing.