Understanding Digestion in the Horse, A Comparative Approach

Pregastric fermenters are then subdivided into ruminants or nonruminants. Common ruminants
are cattle, sheep, goats, deer, antelope, and camels. These animals have highly
developed digestive tracts that use fermentation to degrade feedstuffs. Large, multicompartment
stomachs selectively sort and retain plant fiber for extended periods of time.
Digesta then moves to the animal’s “true stomach,” hence the adjective “pregastric.”
Nonruminants in this category include hamsters, voles, kangaroos, and hippopotamuses.
Hindgut fermenters are also split into two classifications according to whether they
depend primarily on the cecum or colon for microbial digestion. Cecal fermenters include rabbits,
guinea pigs, chinchillas, and rats.
Large nonruminant herbivores such as horses, rhinoceroses, gorillas, and elephants depend more
on the colon for microbial fermentation. Omnivores such as pigs and man have sacculated colons
where a good deal of digestion takes place. Carnivores such as cats and dogs have little or no cecal
capacity and an unsacculated colon.
Adaptations for Microbial Fermentation
In order for microbial fermentation to be useful, animals must have digestive systems that can
retain digesta and microorganisms for a long period of time while simultaneously maintaining an
environment suitable for fermentation of plant material. The degree in which a particular species
is able to use fermentation will depend primarily on three factors: (1) the total volume available
for fermentation in the digestive tract, (2) the retention time of ingested material, and (3) the
makeup of the microbial population inhabiting the hindgut.
Volume available for fermentation. The importance of microbial fermentation as a means of digestion
in various species can be demonstrated by the proportion of the digestive tract devoted to
fermentation. Ruminants typically allocate the largest proportion of their digestive tracts to fermentation.
Nearly 75% of the bovine digestive tract, for instance, is suitable for supporting microbial
fermentation. The vast majority of this fermentation capacity is in the reticulum and rumen, two
compartments of the stomach. Nonruminant herbivores such as horses tend to dedicate a smaller
proportion of their total digestive capacity to fermentation. Both ruminant and nonruminant grazers
such as horses and cows usually have more developed digestive tracts than selective herbivores like
rabbits and hamsters.
Omnivores vary greatly in their fermentation capacity. For example, pigs have a voluminous
hindgut accounting for about 48% of their total digestive capacity, but humans devote only
approximately 17% of their tracts to microbial fermentation. As mentioned previously, carnivores
usually have unsacculated colons that represent a small proportion of total digestive capacity.
Table 1 compares the fermentive capacities of specific organs in nine species. Ruminants (cattle
and sheep) use more of their digestive tract for fermentation than horses.
Retention time. The extent to which plant material is fermented depends on how long it is in contact
with the microbes. Longer retention results in more complete digestion, but there is a limit to
the total amount of time the material can be subjected to fermentation before energy production
becomes compromised. Herbivores such as horses depend to a large degree on volatile fatty acids
(VFAs) as a source of dietary energy. These VFAs are by-products of microbial fermentation. If digesta
is retained too long in the fermentive organs, VFAs will be degraded by certain anaerobic
microorganisms, thus depriving horses of energy.
As ruminants become larger, mean retention time increases. Buffaloes, which at maturity have a
body weight of about 2200 pounds (1000 kg), have retention times of between 90 and 100 hours.
Retention times longer than this would make animals susceptible to the aforementioned degradation.
Equinews/Volume 10, Issue 3 7