Carbohydrate digestion is very important in the horse, as the products (mainly glucose) from this digestion provide most of the horse's energy needs (especially if his main intake is forage, as it should be)...
...in fact, the production of VFA's can provide as much as 30% of his total maintenance energy needs!
Since VFA's and the microbes' role is often not understood, that is what I'll be focusing on in this discussion of carbohydrate digestion.
When carbohydrates reach the cecum, they are fermented by the microbes that live there. During the fermentation process, volatile fatty acids (VFA's) are created.
Ironically, the microbes don't create VFA's to use them...they are a waste product to the microbes, who are simply trying to remove harmful (to them) products from their environment to survive.
It is these VFA's that give the horse some of his energy.
There are also microbes in the colon, but their contribution is much smaller, so we're going to focus on the cecal microbes.
The three most common VFA's are acetate, propionate, and butyrate. Acetate is the most important one, as it can be absorbed directly into the bloodstream and used as energy. Studies have shown that during exercise its absorption from the muscle tissue increases.
Propionate and butyrate can also be used as energy, but must first be converted to other products and eventually to glucose.
Acetate is also an important source of fat in a lactating mare's milk. It has been reported that mares consuming high forage diets had a higher fat content in their milk than mares that consumed high concentrate diets.
The production of the VFA's (and their significant contribution to your horse's energy) is one reason that it is so important that forage be the basis of your horse's diet.
The microbes in the cecum are not all one kind...there are some that thrive on products found in forage (cellulose, lignin, etc.) and some that thrive on the products found in grains (simple sugars, starches, etc). When a horse is eating mostly forage, naturally the microbes that use those products will be dominant in the cecum, since they have more food for themselves than the other microbes.
However, when the balance is upset by a horse receiving little forage and a lot of concentrates, the microbes that consume the concentrates rule. These microbes produce a different ratio of VFA's (less acetate and more propionate) which decreases the amount of energy available to the horse from acetate.
However, because the food is quickly digested by the microbes, the rate of VFA production increases. Good, right?
Microbes that digest forages like to work in a high pH environment...drive the pH too low, and they die off. Remember what VFA stands for? Volatile fatty ACIDS -- and more acid = lower pH.
So the more VFA's that are being produced, the less forage-loving microbes are present. The less forage-loving microbes we have, the less the forage actually gets digested...and the less forage digestion we have, the less carbohydrate digestion we have.
When your horse gets less out of his hay, you get more...
...more poop, that is! Since the horse doesn't have the enzymes to digest the lignin in the hay, if it gets past the microbes that are supposed to digest it, it's coming straight out the back end, and bringing other nutrients (that are bound up in it) with it.
When starch microbes dominate there is also less overall production of VFA's...simply for the reason that starch does not last long enough in the tract to sustain the microbes between your horse's meals.
So right after a meal, there are lots and lots of starch microbes producing lots of VFA's...but then as time goes on, these microbes are out of food and start dying off.
Since the forage microbes got killed off during the low pH period with all those VFA's, there's not many of them around to continue producing VFA's...until your horse has another forage meal and they start reproducing again.
Another of the many reasons its vital to have forage in front of your horse as much as possible! Continuous forage in front of him means continuous carbohydrate digestion by the forage microbes and their survival.
With continuous forage, your horse will also be helping the forage microbes...saliva is a buffer that helps keep the pH up high enough. The more hay he eats, the more saliva he produces, so the forage microbes are winning all around!
So, how do the forage microbes survive?
If they're being killed off every time starch is digested, how do they get back in there?
The answer? They aren't killed off every time your horse consumes starch and sugars (after all, even forage like grass and hay has starch and sugar in it!)...
...they are only killed off when he eats such a large starch meal that undigested starch makes it all the way back to the cecum.
The horse can actually digest starch in the stomach and beginning of the small intestine. In the stomach there are microbes that take care of carbohydrate digestion (ie. starch digestion), and in the small intestine there are enzymes.
It is only when a horse is fed large starch meals that the starch cannot be digested fast enough in these areas and "escapes" to the cecum to cause trouble.
What is a large meal? Studies have shown that any more than 2 grams per kg BW start to affect the microbes and pH levels. For a 1000 pound horse, this means no more than 2 pounds of starch per meal. Some feeds on the market are over 50% starch and sugar, meaning they shouldn't be fed at a rate any higher than 4 pounds per meal.
Understanding the microbes in your horse's system as well as their VFA production can be difficult...but it is such an important part of carbohydrate digestion, so hopefully it is more clear to you now.