Probiotics and Digestive Aids: Microbes to the Rescue

Deep within your horse's gastrointestinal tract, there's a whole civilization in miniature. Microorganisms, billions of them, have taken up residence in his cecum. But there's no cause for alarm; these thousands of different species of bacteria, protozoa, yeasts, and fungi not only belong there, the majority of them actually are beneficial to the horse.

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Probiotics are easy to give to a horse and are effective, and virtually harmless when administered correctly.

The horse's gastrointestinal tract is not unique in this regard. Almost all animal species, including humans, have their share of intestinal "microflora" (a catchall term describing the normal microscopic inhabitants of an environment). The types of microbes can vary from species to species, but their presence and function are universal. They exist in a symbiotic relationship, helping their host animal extract and absorb the full nutrient value from his food. Some organisms are specialists in helping to digest cellulose (tightly linked sugar molecules contained in tough, fibrous foods such as hay, which are broken down into simpler glucose units). Others have a talent for handling the lactose sugars of milk, or for taking the byproducts of cellulose digestion and constructing from those bits and pieces the vitamins for the horse to use. Still others are responsible for maintaining the airless environment of the gut, removing oxygen so that their anaerobic microbial kin can continue to function normally.

While the horse receives the bulk of the nutrients as his food is broken down, he's not the only one who benefits; the microbes take their share and thus maintain their populations. Their presence is essential to the horse, who could not digest fiber (the bulk of his diet) without them.

Horses aren't born with healthy populations of gut microflora. They must acquire them from the environment. The process begins in the birth canal, and continues throughout a horse's youth, with the populations of different microorganisms fluctuating as he grows.

While he's still nursing, he doesn't need fiber-digesting bacteria because he's functioning on his dam's milk. But, as he begins to show interest in fibrous foods like grass, he needs to introduce to his gut the necessary allies to help him digest forage. Coprophagy, the act of eating the manure of adult horses, might seem a distasteful habit, but it's the perfectly natural way that a foal inoculates his system with the microbes he'll need for the rest of his life. By the time he is an adult, he'll have established a fairly stable population of intestinal "bugs," made up of thousands of different species, but with 30 to 40 dominating.

That's not to say that the gastrointestinal tract is all that welcoming a place for microorganisms. In fact, it's relatively hostile, lacking in oxygen and tremendously acidic. Furthermore, because the system is dynamic--food, in liquid form, is almost constantly washing through there--a microbe has to be pretty tenacious not to get washed out with the tide. Some secrete filaments or "glue" to help them stay put; others just seem to swim slowly and constantly against the current in a spiral fashion.

It's just as well for the horse that the GI tract is as unpleasant a place as it is; if it were a more hospitable environment, his gut quickly would be overwhelmed by the burgeoning microbial population. As it is, only select organisms can survive there, and the populations of beneficial microbes are an important barrier to colonization by their disease-causing cousins, usually known as pathogens.

The "good guys" tend to monopolize the available nutrients, and they also produce a chemical soup in which only very specialized microbes can survive. Some species even produce substances that act as antibiotics against other species; some Lactobacillus strains, for example, exude acetic acid, lactic acid, and hydrogen peroxide, which can inhibit the growth of gas-forming bacteria such as E. coli (which can cause disease when they're present in large numbers). Unless invading forces appear in overwhelming numbers, the health status of the gut usually is carefully maintained and monitored by the beneficial microflora.

Microbes At Work

The anaerobic microorganisms that do the work of digestion tend to inhabit the vast fermentation vat of the horse's cecum, the first part of the large intestine. There, they fulfill the same function for the horse as in ruminant, multi-stomached species such as the cow. While cattle digest woody fiber by softening it in the rumen, then regurgitating it for a second chewing session ("chewing cud") before re-swallowing it so the gut microflora can have a second go at extracting fats, proteins, and carbohydrates, the horse manages the same process by digesting the simpler molecules first, in his stomach, before tackling the tougher fibrous particles in the fermentation vat of the cecum and large colon.

In the middle of the large colon is a "recycling center," a region of autonomous nerves that detect food that has not been fully digested and force it back to the cecum, a process that is analogous to the cud chewing method in cows.

There's more going on in the digestive tract than just the breakdown of large particles into smaller particles. As bacteria, yeasts, and protozoa die and decompose, they yield additional proteins, energy, and vitamins that the horse's system can use. And, as we noted earlier, the byproducts of the complex chemical reactions that occur during digestion can yield new and useful compounds, such as B vitamins, when correctly manipulated by the right microorganisms. Every available nutrient is extracted, and the waste products are flushed out of the body. Under most circumstances, the efficiency of the gut microflora is nothing short of amazing.

Sometimes, however, the delicate ecosystem of the gut microbes becomes disturbed. Any number of factors can trigger it--stress, such as horses might suffer when being introduced to a new herd, shipping long distances, and enduring a strenuous training program are the chief culprits. Extremes of temperature, stressful competition, a change in diet, breeding, foaling, deworming, or even having routine work such as teeth floating performed by your vet can start a stress cycle in your horse's bowels. (In rats, overcrowding in cages has been enough to change the bacterial populations in their digestive systems, and in humans, even having an argument has been shown to compromise the levels of certain gut bacteria.)

Antibiotics are another major reason for a depleted microbial population in the gut. Many of these broad-spectrum drugs tend to kill off bacteria rather indiscriminately, taking the good with the bad. The aftermath of a week's worth of antibiotic administration often is loose manure, a compromised appetite, and irritable behavior. In severe cases, the horse's gut might even be at risk of shutting down, increasing the likelihood of colic.

Any horse which has reached a venerable age (usually 20 or more) is likely to have a less-than-thriving microbial population. His system has lost the ability to consistently maintain a healthy environment for the microflora.

Of course, we must keep in mind that the diet of the average modern, domesticated horse is a far cry from the way in which he's designed to eat and digest. Large grain meals, delivered only once or twice a day, with limited amounts of fiber, hardly mimic the almost-constant grazing he was made for--and the impact on his population of gut microflora can be significant.

Whatever the reason, the result of a depleted microbial population in the gut is a horse which loses weight and condition. Even if his appetite remains good, he simply doesn't have his allies working for him in his large intestine, and large quantities of essential minerals, vitamins, proteins, and sugars pass through his system without being absorbed. His manure might be loose, or very fibrous (indicating that the hay or forage he ate hasn't been processed very well), and you might see whole grains scattered throughout. He also might be at higher risk of colic.

Fortunately, there's hope for replenishing microbial populations, with the use of feed supplements called "probiotics," or sometimes "direct-fed microbials" (DFMs). The concept behind these products isn't new--some simple probiotics, such as yogurt, have been fed to animals for more than 100 years. But we're just beginning to discover how valuable these supplements can be. Remember, however, that your veterinarian should rule out any illness or disease in your horse prior to any treatments being undertaken.

Improving Intestinal Integrity

The term "probiotic" was coined to mean "for life," and was designed to imply the opposite of "antibiotic" (against life). A probiotic or DFM is simply a culture of viable microbes, and/or their fermentative metabolites, which will help stimulate the growth and health of "good" gut microflora. In turn, this should help the horse combat stress, improve digestion, and fight pathogenic organisms. Improved digestive efficiency (as indicated by less undigested matter in his manure, weight gain, and a generally improved outlook in terms of appearance and attitude) has been reported as a result of the use of probiotics.

As with most supplements, there are a number of different approaches to accomplishing these aims. Some products are administered orally in the form of a paste, others are syringed into the mouth as a liquid, and still others come in powdered form ready to sprinkle on the feed. Some are designed to be fed on a daily basis as a preventative, while others are meant to be used specifically in the aftermath of stress or antibiotic use. Some are targeted for foals, others for mature horses.

Typically, they contain only one, or a few, of the thousands of species of microbes normally present in the horse's gut; none represent the whole spectrum of microorganisms that might be present in a healthy gut ecosystem. The question, then, becomes: Which of these probiotics is most likely to be beneficial for your horse?

It's difficult, of course, to establish whether or not your horse actually has a depleted microbial population. In a laboratory situation, researchers can extract samples of intestinal fluids and calculate an estimated "head count" for various species. In an on-farm situation, that's practically impossible. Instead, you'll have to go by the classic signs of compromised digestive ability. Fortunately, it's difficult to do any harm with probiotic supplements; those organisms that don't take a toehold in the gut will simply be flushed out of the system or digested. So there's little risk involved in giving probiotic supplements on a preventative or "just in case" basis.

One of the "original" probiotics is yeast culture. Feeding yeasts to animals, either in the form of yeast-fermented mash or as yeast byproducts from breweries or distilleries, is a time-honored practice with a number of benefits. Yeasts actually are microscopic fungi, of which there are about 500 different species. Only a few of these are used commercially, with the best-known being Saccharomyces cerevisiae, sometimes called brewer's or baker's yeast. S. cerevisiae is the yeast that makes bread rise, and it's also the organism used by breweries to make beer, by distilleries to make distilled spirits and industrial alcohol, and by wineries to make wine. Another yeast called Torula yeast (Candida utilis) is important because of its ability to utilize pentose sugars from processed wood pulp in making paper. A third, Kluyveromyces marxianus, is the whey yeast that can utilize lactose (milk sugar) as a substrate. A "new" yeast to enter the feed industry is Phaffia rhodozyma, or Phaffia yeast. It is interesting because it produces a red pigment that is being used in trout and salmon feeds to impart a richer red coloration to the meat.

Yeasts are facultative anaerobes, which means that they can survive and grow whether or not they are in the presence of oxygen. They grow much more efficiently in an aerobic environment, and they produce carbon dioxide and energy from sugars and oxygen (the CO2 is what leavens bread), but also can act fermentatively (as they do when used to make alcoholic beverages) in the absence of air. In an anaerobic environment, yeasts ferment sugar into ethanol and carbon dioxide, but grow very slowly.

In addition to their fermentative and leavening qualities, yeasts are a good source of protein, with an amino acid profile equivalent to soybean (rich in lysine, and very good for a vegetable protein). Yeasts also contain relatively high quantities of B vitamins such as biotin, niacin, pantothenic acid, and thiamin. A horse with normal digestive function generally manufactures ample amounts of these vitamins in his gut, but if he suffers from impaired digestive efficiency, he might become deficient in B vitamins, making yeast an excellent choice for his digestive health.

Historically, yeasts have been regarded as a good source of the mineral selenium, and often have been added to animal feeds on this basis before sodium selenite came into general use. Commercial "high selenium" yeasts are available that might contain up to 2,000 parts per million of selenium.

In terms of feed supplements, it's important to distinguish between "active dry yeast" and "yeast culture." Active dry yeast consists of pure, dried yeast cells, with viability counts ranging from 15 to 25 billion live yeast cells per gram. There are several formats, the most familiar of which is likely the granular powdered quick rise yeast, which often is used in baking. (The non-yeast portion of the product usually consists of rice hulls or distiller's solubles.)

By contrast, yeast culture is the fermented metabolites of a once-live colony of yeasts, as well as the dead yeast itself (valuable for its nutrient content). According to Mike Officer of Diamond V Mills Inc. of Cedar Rapids, Iowa (a company specializing in several kinds of yeast culture products for animals), an active (live) yeast generally is higher in B vitamins than yeast culture, and it is good at taking up oxygen from the environment, which benefits the anaerobic gut microflora. But the metabolites (byproducts of the yeast's life cycle in a medium) of a dead yeast culture largely seem to be responsible for increasing microbial mass in the horse's hindgut and improving fiber digestion by supplying nutrients and enzymes the existing populations can use.

"When you increase the microbial population, the absorption of nutrients is improved," he says. "Also, feeding yeast culture seems to improve phosphorus utilization in older horses, an important factor in maintaining the health of bones and hard tissues. Phytase, an enzyme which helps make phosphorus more bio-available to the horse, is one of the yeast metabolites contained in the yeast culture."

Officer points out that, unlike some other probiotic products, the aim of a yeast culture is not to transfer a live population to the GI tract, but to enhance and stimulate the existing population.

"Yeast cells are not endogenous to the digestive tract of a horse," he says. "If you feed live yeast, it dies once it hits that acid environment."

Yeast cultures are designed to be fed on a preventative, rather than a therapeutic, basis, according to Officer.

"It takes seven to 10 days, as a rule, for the gut microbes to reach an optimum level (after you begin feeding yeast)," he says, "and the populations will drop about seven to 10 days after you remove yeast culture from the diet."

In studies conducted by Diamond V Mills, the regular feeding of yeast culture to weanlings resulted in "significant increases" in weight gain and wither height over weanlings not fed yeast supplements. Other studies have demonstrated an 8-10% increase in fiber digestibility in adult horses, and improvements in nitrogen uptake (an indication of better protein digestion) and a decrease in the amount of nitrogen, in the form of ammonia, which is excreted in the urine.

Many commercial feeds already incorporate yeast in their formulations, but you also can top-dress yeast, in powdered form, on your horse's regular feed. Its use might be valuable particularly for older horses, although Officer notes that recent studies of yeast-fed performance horses also demonstrated some surprising improvements in conditioning (including lower heart rates and respiration rates after strenuous exercise, and less lactic acid buildup in the muscles)--which might mean that this supplement finds a place in the program for high-octane equine athletes in the near future.

Other Probiotics

In addition to yeast, there are several other probiotic products on the market that claim to help increase a horse's digestive efficiency and the health of his microbial population. An Australian product called Protexin, for example, contains seven different strains of bacteria plus two yeasts; the four Lactobacillus species in the formula are particularly good at breaking down milk sugars and other carbohydrates, relieving diarrhea or "scours," promoting weight gain in species such as pigs, chickens, and cattle, and stimulating immunity against common gut pathogens such as E. coli and salmonella. Meanwhile, the yeast Candida pintolepesii might "help protect the stomach lining from bacterial colonization," according to the company, while Aspergillus oryzae aids in the digestion of cellulose. The manufacturer claims that Protexin can be used to stimulate appetite, reduce digestive upsets, reduce stress, and aid in the establishment of gut microflora in very young animals such as foals. And depending on the format and dosage you choose, it can be used preventatively, or on a short-term basis to treat specific problems, or at times when increased levels of stress are anticipated (such as before you ship your horse for a long distance).

As useful as Lactobacillus species might be in probiotic formulations, it's now known that they do not make up the bulk of the intestinal organisms in the population, as was once thought. In fact, they're not even in the top 25. Nonetheless, they remain a popular ingredient in probiotic supplements. Streptococcus spp. thermophilus and faecium, and Bacillus subtilus are also likely to be used. Some products also contain vitamins, minerals, and enzymes, and at least one company combines probiotic microbials with herbs that are reputed to act as digestive aids, such as peppermint, ginger, fenugreek, fennel, violet and papaya leaves, kelp, and dandelion.

Which product to choose might be something of a gamble, but all are likely to have some benefit if your horse truly suffers from a compromised microbial gut population. It might take some experimentation with different brands to discover which product will provide the most benefit for your horses.

Not every horse needs the assistance of a probiotic, of course, but it's comforting to know that most of these products are relatively cost-effective, and unlike many supplements, virtually harmless when administered according to the manufacturer's directions. If your equine partner suffers from less-than-ideal digestive health--and your veterinarian has ruled out specific disease processes--probiotics might be exactly what you've been looking for to re-establish the health of the invisible allies in your horse's gut.

When to Use Probiotics

Consider adding a probiotic product to your horse's feed if he

  • is a hard keeper
  • is more than 18 years old
  • has a poor appetite
  • has loose manure or chronic diarrhea
  • has large amounts of undigested material in his manure
  • shows other signs of digestive distress, such as recurring colic
  • is under stress from showing, racing, or shipping
  • is recovering from surgery
  • has been stressed by veterinary treatment such as floating teeth or vaccinating
  • has received recent treatment with antibiotics or anthelmintic (de-worming) drugs
  • has recently undergone a change of feed or environment
  • is struggling to adapt to extremes of  temperature (a heat wave or cold snap)
  • is a new-born foal
  • has lost weight or condition, or looks "unthrifty."

About the Author

Karen Briggs

Karen Briggs is the author of six books, including the recently updated Understanding Equine Nutrition as well as Understanding The Pony, both published by Eclipse Press. She's written a few thousand articles on subjects ranging from guttural pouch infections to how to compost your manure. She is also a Canadian certified riding coach, an equine nutritionist, and works in media relations for the harness racing industry. She lives with her band of off-the-track Thoroughbreds on a farm near Guelph, Ontario, and dabbles in eventing.

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