OK, here`s the basic version of Adventures in Digestion, sorry if it`s mostly stuff you know. Or, if I "pontificate"! :-> Better too thorough than not thorough enough...
As you know, food is broken down through chewing and chemical action (enzymes and hydrochloric acid) in the mouth and stomach, in order to make the nutrients more available for absorption. After food leaves the horse`s stomach, it passes into the small intestine, followed by the cecum and the rest of the hindgut. (The foregut consists of the mouth, pharynx, esophagus, stomach and small intestine. The hindgut consists of the cecum, large colon, small colon, rectum and anus.) The small intestine absorbs the majority (though not all) of the soluble nutrients, such as sugars, starches, proteins, pectin and fats. What`s left over at this point is mostly fiber from the plant cell walls which are made up of structural carbohydrates (hemi-cellulose and cellulose), lignin, silica, maybe some protein that`s tied up in the cell walls and the inorganic material such as minerals. All of this ingesta moves from the small intestine into the cecum, which is a big, closed ended bag holding around 7-8 gallons of food, depending on the size of the horse.
At this point the carbohydrates in the fiber is tied up in a form that isn`t particularly easy to utilize. In the cecum, there is a population of microbes ("bugs") that ferment and break down the fiber so that the nutrients in it can be absorbed. The structural carbohydrates from plant cell walls that are made available through this fermentation are absorbed from the cecum in the form of volatile fatty acids, as are hopefully whatever is left of the soluble carbohydrates that didn`t get picked up in the small intestine. These VFA`s are then utilized for energy production. The microbes also produce B vitamins. There is some thought that other vitamins and amino acids may also be produced and absorbed in the hindgut, but so far no one really knows and for the most part, protein absorption is thought to be pretty low. All this microbial action also produces quite a bit of heat, which is why feeding more hay on a cold day will keep the horse warmer than will feeding more grain, which is primarily soluble carbohydrate (and therefore absorbed in the small intestine, without producing heat of digestion).
After leaving the cecum, the ingesta then goes through the large colon, where vitamins and water are absorbed while yet more fermentation and energy (VFA) absorption goes on (not as much, though). Eventually, the ingesta goes through the small colon and rectum and what`s left over is what your friend leaves for you on his stall floor.
One of the things to remember about equine digestion (or any other animal for that matter) is that`s it`s not 100% efficient. There`s always going to be nutrients left over in the manure---enough, as a matter of fact, to raise a pig to close to market weight without having to supplment the pig. I know, gross. This is the kind of weird stuff you get in Animal Science. It used to be (may still be) a common practice in the UK and Europe to let pigs "follow" grain-fed cattle and horses after they`d been on a pasture for awhile.
Anyway, to finally get to probiotics. The whole point of feeding probiotics is to try and raise the feed efficiency as much as possible. There are other management practices that will help do this as well (ie feed a consistent ration, in small, frequent meals, processing some of the hard grains, keeping the teeth floated, etc.) but the idea behind probiotics is to keep the microbial population at an optimum level. The more bugs that are present in the cecum when a load of fiber comes through, the more "workers" there are available to break down the fiber and extract as much nutrition as possible before the ingesta travels on down the road. The rate of passage through the digestive tract is controlled by hormones and blood supply, etc., things other than the microbes themselves. Therefore, if a supply of food leaves the cecum and colon before the microbes have had a chance to fully digest it, then it just doesn`t get digested, and those nutrients still remaining are just wasted and excreted in the manure. This is why keeping an optimum population of bugs in the cecum contributes to feed efficiency.
Hmmmm. It would be really neat to see what the difference in heat production is between a horse with a full load of microbes on board vs. a horse with a low population of bugs, huh? Hey, Gayle, got a metabolic chamber we can borrow???? (And maybe some funding?)
A few years ago, Michael Glade at Northwestern did a bunch of work on probiotics (specifically a yeast culture called Yea-Sacc 1040) in horses and also found out that growth was better in weanlings and yearlings fed probiotics, had less incidences of bone disorders, horses at work held their weight better and lactating mares had a higher butterfat content in their milk. The same stuff has been used for years in the dairy and pork industry for those reasons. Nifty stuff, huh?
Anyway, I`ve used several different brands on the market, as sometimes our supply around here isn`t consistent and I`ve had good results with pretty much all of them. Someone at Kentucky Research told me once that Forco is a different product and doesn`t act the same way, but I don`t know the details. Whenever I can, I use the Yea-Sacc, just cuz that was the strain that was utilized in all the research, and you know how I love statistics.
Anyway, the bottom line on probiotics is that it does not replace good nutrition, it just helps the horse get everything he can out of it when he is fed a good diet. As everyone has been saying for the past week or so, the fancy stuff should be used as a supplement to, not a replacement for, just plain ol` good horsemanship.
I hope I got all this right, it`s still a little early in the day and I`m kinda winging it on some of this. If any of the vets or physiologists lurking in the bushes can correct me on boo-boos, I`d appreciate it.
Susan Evans firstname.lastname@example.org