You’ve heard your horse’s intestinal tract referred to as a furnace. Obviously that’s not literally what happens. It is where fermentation takes place. Fermentation reactions produce cheese and your favorite adult beverage but that’s not what is going on either.
One of the attention grabbing factoids they threw at us early in veterinary school was that cows could survive by fermenting nothing but newspaper. They won’t be healthy for very long doing this but the point was that newspaper can be converted to calories to sustain life. This hasn’t been tried with horses but all would agree that we couldn’t survive on hay the way horses do.
Fermentation is the enzymatic breakdown of a substance with the enzymes in this case coming from bacteria, yeast and protozoa. Cellulose, hemicellulose, complex plant carbohydrates, fructans and soluble fibers like pectin and betaglucans are all indigestible by the horse’s digestive enzymes but can be fermented in the large intestine. The organisms use some of the energy contained in these food fractions for themselves and what is left over, the products of fermentation, are readily absorbed into the blood stream and are what the horse uses as energy sources.
The products of fiber fermentation are primarily acetate, butyrate and propionate. Simple sugars, starch and fructans also produce lactate. These fermentation products can account for 60% or more of the calories absorbed by the horse.
Most people have a misguided poor opinion of lactate. It is an important fuel that can be converted to pyruvate and burned for energy, or converted into glucose or glycogen. Acetate is a very efficient energy source, ready as-is to enter the mitochondria to be burned. Acetate also spares glucose which can then be used for other things like replacing glycogen stores. Acetate is the major fermentation product of hay with even larger yields from beet pulp or soybean hulls.
If butyrate reaches the liver it can be converted to fat but most of it is used by the intestinal lining cells. Propionate, also produced in smaller amounts than acetate, can be converted to glucose.
Now if someone asks you why horses don’t get drunk from fermenting their food (don’t laugh, I’ve seen a claim that they can!), you’ll know why.
Eleanor Kellon, VMD