Balancing the Microbes in the Horse's Digestive TractBy Dr. Kathleen Crandell · January 3, 2012
The importance of maintaining a balanced microbial population in the equine digestive tract is often underscored in popular-press articles and primers on horse nutrition, but what does “balanced” mean? What types of microbes are living in the digestive tract? How many are there, and what is their purpose? What can be done to keep them balanced?
A delicate balance exists among the different types of microbes that reside in the gastrointestinal tract. Humans cannot live on forage because we lack these microbes that break down forage into usable bits. The products of forage digestion are volatile fatty acids, which horses use for energy. Humans have some microbes that should stay in balance as well, although they pale in number and function compared to those of herbivores. We are being inundated with products that address that function in humans, and nutritionists emphasize the importance of that balance in the human digestive tract in overall health and well-being. Like humans, horses cannot perform their best if digestive microbes are out of balance.
In the horse, many types of bacteria are present in the gastrointestinal tract, and each type is efficient at digesting certain nutritional components. The microbial population is complex and consists of bacteria, protozoa, and fungi/yeasts. The number of microbes is too high to fathom, but one drop of cecal fluid from a horse could potentially have one billion bacteria.
In general, there are five types of microbes:
- cellulolytic bacteria: digest fiber
- proteolytic bacteria: break down protein
- lactic acid-producing bacteria: digest starch
- protozoa: produce volatile fatty acids
- fungi/yeasts: break down fiber
- other bacteria: produce B-vitamins
These microbes are actually found throughout the entire digestive tract, but because most favor a pH-neutral environment, nearly all inhabit the cecum and colon.
Due to specificity to different substrates, the population of individual microbes will vary by the type of food that is present in the digestive tract. Cellulolytic bacteria make up the majority of the bacterial population in the cecum and the colon because that is where the fibrous parts of the diet (cellulose and hemicellulose) reside for 12 to 48 hours. The interesting thing about microbial populations is that their makeup (percentages of each of the different types of microbes) can actually be variable from one horse to the next. That might explain why some horses can eat anything while others have digestive upset with the smallest change in diet.
The goal of microbial balance involves keeping the numbers of cellulolytic high and the lactic-acid producers low. Further, as cellulolytic bacteria produce volatile fatty acids, proliferation of pathogenic bacteria, like E. coli or salmonella, cannot occur. If the lactic acid-producing bacteria population starts growing, they produce enough lactic acid to influence the pH of the gut, making it less hospitable for cellulolytic bacteria and more agreeable for the pathogenic bacteria. This is when problems may arise. How tolerant an animal is to changes in the pH before problems begin can be highly individual.
Balance is all about population control. When there is an overload of one kind of substrate that one type of bacteria digests, then there is a population explosion of that type of bacteria. This affects the populations of the other types of bacteria, since they have less substrate to digest and the by-product of the digestion of the booming population may make the environment less habitable for the rest of the microbials. Shifts in the bacterial populations can cause digestive upset.
An example would be when a horse normally consumes a high-forage diet and the amount of grain is increased suddenly. The number of cellulolytic bacteria will be high, but now there is more starch from the grain, which the lactic acid-producing bacteria will work on. The lactic acid-producing bacteria start reproducing at an exponential rate to keep up with the amount of starch present. Increases in these bacteria will result in large amounts of lactic acid being produced and dropping the pH of the environment. Soon the pH will be too low for the cellulolytic bacteria to reproduce, so they start to die. As cellulolytic bacteria die off, fiber cannot be digested efficiently. The overall result has a negative effect on the microbial environment, which may adversely affect how the horse feels.
How can you tell if there is not a good balance in the digestive tract? Signs can be subtle or obvious and can vary in which way they show themselves among horses. Subtle signs might be a grumpy, cinchy, irritable, or depressed horse. More obvious signs would be gassiness, loose stools, diarrhea, chronic mild colic, or, in a worst-case scenario, laminitis.
How to keep it in balance? A normal high-forage diet (lots of pasture and/or hay) will supply the microbes with exactly what they are looking for to maintain balance, particularly the cellulolytic bacteria, protozoa, and fungi.
Here are three ways to maintain microbial balance:
- Keeping a steady supply of forage available to the horse will assure the good populations of microbes have plenty of substrate to break down.
- Making changes in forage or concentrate type and quantity gradually will give the microbial populations time to catch up with the new substrates.
- Avoid feeding large amounts of concentrate or grain at one time by dividing it into smaller, more frequent meals so that it does not overwhelm the cecum and colon. For horses on high-grain diets, help keep the pH from getting too low with a hindgut buffer.