The equine digestive tract nurtures both good and bad bacteria and probiotics can tip the balance in favor of the former, thereby promoting good health"Probiotics" has a nice ring to it. It suggests a positive slant on horse health, and certainly the goal of
feeding probiotics is to enhance the horse's well being.
Probiotics, which are also called "direct fed microbials" and "intestinal microbial inoculants," are feed supplements composed of beneficial microbes and their fermentation products, often with added yeasts, molds, enzymes, minerals, or vitamins. They are fed to help the horse maximize the nutrients he extracts from his feed and to maintain a balance between beneficial and pathogenic organisms in his gastrointestinal tract.
Feeding microbes to horses is not a new idea. In the early part of this century, horsemen recognized that introducing certain bacteria into a horse's digestive tract helped sick animals recover faster. The manufacturing process was a bit cruder, however. Microbial soups were produced by mixing manure with water or harvesting the visceral contents of slaughtered animals. Today's probiotics undergo stricter quality control. They are now grown into the third or fourth generation under sterile laboratory conditions, then transferred to carrier products such as brans, hulls, or grain byproducts.
How they work
When the horse eats, feed passes through the stomach into the small intestine and finally journeys to the cecum and colon. The cecum and colon, often referred to as the horse's hindgut, act as a fermentation tank for the horse's feed. Over two dozen beneficial and pathogenic organisms participate in the digestive process, each fulfilling a specific role.
The gut microbes break down fiber to produce volatile fatty acids, vitamins, and amino acids. They also contribute to a healthy pH level so that the horse absorbs nutrients through the gut wall at the optimum rate. The pH level in the gut (pH is a measure of how acidic or basic the environment is) acts as a thermostat for the gastrointestinal (GI) tract of the horse. If the pH gets too high or too low, the beneficial microbes can no longer survive and pathogens get the upper hand.
The balance among the good guys and the bad guys in this microbial stew is maintained through complex interaction. For example, strains of Lactobacillus and Streptococcus produce acidic compounds which have a slightly antibacterial action that inhibits pathogenic Escherichia coli. Others lower intestinal oxidation-reduction potential, which inhibits organisms that need oxygen for their metabolism.
As long as the proper balance between the two populations is maintained, the animal is healthy, can metabolize his feed efficiently, and can grow at a normal rate. When the balance is upset, the horse suffers. Laminitis is an extreme example. A carbohydrate overload from too much grain causes a drop in the pH of the hindgut which kills gut bacteria and damages the mucous lining of the GI tract. The dead organisms produce endotoxins. Because the gut wall is compromised, these pass through easily into the bloodstream to cause problems throughout the body, including the feet. The overall health and well-being of the horse rests on healthy microflora in the hindgut.

Upsetting the balance
There are many stresses that can affect the population of the gut microbes besides a grain overload. Changes in management-including feed-or changes in the environment can unbalance the microflora. The stress of conditioning, transportation, or racing also can. Horses that are sick, especially if they have diarrhea or have been on antibiotics for an extended period, have problems maintaining a healthy microbial colony. Probiotics are also promoted for newborn foals to get the microflora up and growing in their digestive tracts and also to counteract the stress of weaning.
Unfortunately, the balance in the hindgut can be upset far more quickly than it can be restored. An abrupt change in feed can cause initial changes in the hindgut and fecal output within three hours. Within eight hours, there are gross changes in the microflora and a significant difference in pH.
While probiotics can curb symptoms of severe diarrhea within 24 hours, the horse is far from "cured." It takes about three days to see real changes in the microbial balance and three weeks to reestablish it completely. That is also why it is difficult to evaluate the benefits of any changes in feed until the horse has been on the new ration for several weeks. Any change in feed challenges the bacterial population.

Feed for need
There are two schools of thought about feeding probiotics. Some people like to micromanage their horse's supplements, administering probiotics only when they know the animal has been or will be subjected to stress. They may dislike the idea of feeding bacteria-even beneficial ones-to an animal.
Others feel that feeding a daily dose of probiotics to a horse is a form of preventive medicine. Stressful events are not always predictable, they reason, so better safe than sorry.
Some probiotics deliver live microbes and fermentation products; others use dried, dormant, or dead cultures. Manufacturers start their formulations with strains of organisms known to benefit horses. Common choices include bacteria like Lactobacillus acidophilus and Streptococcus faecium and molds like Aspergillus oryzae. Feed tags refer to CFU's (colony forming units) in terms of billions per dose. Many companies add yeast cultures with Saccharomyces cerevisiae. Though not truly probiotic ingredients, many manufacturers also add enzymes, vitamins, and minerals to distinguish their products in the marketplace.
Those "window dressing" ingredients have the potential to get a truly stressed horse into trouble. If a horse is sick enough or stressed enough that a drastic change in pH has caused the intestinal mucosa to slough off, the absorption rate from the GI tract is radically altered. Substances that the horse might ordinarily sort out in the gut and eliminate without passing them through the gut wall might now make their way into the bloodstream in toxic doses.
A classic example is the ferrous fumarate used in one probiotic promoted for newborn foals in the early 1980s. The very young foals with their immature and very permeable digestive tracts, absorbed excess iron and died.
As with all supplements, the best advice is to feed what the horse needs; no more, no less.

How probiotics work

• The equine gastrointestinal tract is normally comprised of beneficial and pathogenic microbes.
  
• Stress-changes in environment, feed, management-can alter the balance of these internal microbes.
  
• Such an imbalance may weaken the horse and lead to serious health problems-i.e.; laminitis.
  
• Probiotics are benfeficial microbes introduced into the system as feed supplements.
  
• These beneficial microbes help maintain a balance between the good and bad bacteria within the gut.
  
• Probiotics also enable an animal to metabolize feed efficiently and maximize its nutritional benefits.