Hi Robyn!
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.
See ya,
Susan Evans