Understanding Digestion in the Horse, A Comparative Approach

Understanding Digestion in the Horse,
A Comparative Approach
6 Equinews/ Volume 10, Issue 3
Have you ever considered the gastrointestinal tract of the mouse-chasing tabby that skulks
around your stable? Ever thought about how market hogs derive energy from their diets of corn
and soybeans? Does a three-ton elephant really have anything in common with your horse,
digestive or otherwise?
In today’s highly specialized world, animal nutritionists tend to consider themselves specialists in
either ruminant or nonruminant nutrition. Ruminant nutritionists appreciate the significance fermentation
plays in meeting the nutritional components of animals with multiple stomachs, but they
tend to think of most nonruminants as “simple-gutted.” Nonruminant nutritionists, conversely, often
dismiss the importance of microbial fermentation to the health and well-being of the animal. These
are critical oversights, because fermentation plays a key role in the nutritional ecology of almost
every species of animal including horses.
Therefore, a basic understanding of the function fermentation plays in a wide range of species is
critical when considering its importance in the horse. An appreciation of the continuity of microbial
fermentation across several species allows information gathered in one species to be used for the
benefit of other species.
Digestion of Carbohydrates
In order to understand the significance of fermentation in animal digestion, a brief explanation of
carbohydrate digestion is needed. The digestion of starch occurs primarily though the work of
enzymes in the small intestine. In terms of equine feed management, sources of starch are usually
cereal grains such as oats, barley, and corn. The final product of starch digestion is chiefly glucose.
Though most starch digestion occurs in the small intestine through enzymatic action, minimal fermentation
of starch occurs in the stomach and portions of the large intestine (the cecum and the
colon). The end products of starch fermentation in the large intestine are volatile fatty acids (VFAs)
and lactic acid.
In contrast to starch, plant fiber is digested entirely by fermentation, which results in the
production of VFAs. Fermentation of plant fiber occurs in the hindgut of the horse. Not all
animals are anatomically similar to horses. Others possess distinctive digestive tracts that
determine where fermentation takes place.
Four Basic Digestive Systems
Animals can be divided into three basic groups according to the structure of their gastrointestinal
anatomy and its ability to ferment feedstuffs. First, animals can be classified as
either pregastric fermenters or hindgut fermenters based on the primary location of microbial fermentation
in relation to the stomach.

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

uminant – an animal with a
four-chambered stomach that
regurgitates partially digested
food (called cud) for further
chewing and reswallowing
nonruminant – an animal with
a single-chambered stomach
fermentation – the energyyielding
metabolic breakdown
of nutrients in a no-oxygen
environment
herbivore – an animal that
eats primarily plants
omnivore – an animal that
eats plants and other animals
carnivore – an animal that
eats other animals
sacculated –characterized by
a series of saclike pouches
or expansions
unsacculated – smooth
membrane with no bunching,
ridges, or pouches
digesta – feedstuffs in
various phases of digestion
as they flow through the
gastrointestinal tract
foregut – a collective term
for the stomach and
small intestine
hindgut – the large intestine,
including the cecum and colon
microflora – bacterial colonies
found in the large intestine
8 Equinews/ Volume 10, Issue 3
Table 1. Fermentive capacity expressed as percentage of total digestive tract. 1
Reticulum and
Species Rumen (Stomach) Cecum Colon and Rectum Total Fermentive
Cow 64 5 5-8 73
Sheep 71 8 4 83
Horse — 15 54 69
Pig — 15 54 69
Guinea pig — 71 9 80
Rabbit — 43 8 51
Human – – 17 17
Cat – – 16 16
Dog — 1 13 14
1 Parra, R. 1978. Comparison of foregut and hindgut fermentation in herbivores. In: Ecology of Arboreal Folivores. G.G.
Montgomery, Ed. Smithsonian Institute Press, Washington, D.C.
Animals larger than 2200 pounds must therefore employ a digestive system that is different than
the ruminant to allow for rapid digesta transit, which in turn supports optimal microbial fermentation.
Elephants and rhinoceroses are hindgut fermenters with digesta transit times that are much
faster than ruminants. These massive mammals have adopted the dietary strategy of ingesting
large quantities of dry matter and passing it through the digestive system fairly quickly. Any loss of
digestive efficiency is offset by increased intake. In general, the larger the hindgut fermenter, the
more rapid digesta transit.
A notable exception to the relationship between body size and transit rate in nonruminants is the
giant panda. These animals are actually carnivores that have evolved to survive on a diet of bamboo.
They have simple, short digestive tracts with little volume to accommodate microbial
fermentation, yet they live in the wild as herbivores. Researchers determined the rate of passage
and digestibility in giant pandas. These data, along with digestibility data from elephants and horses,
are shown in Table 2. The giant pandas were fed bamboo and gruel diets, while the elephants
and horses were fed grass hay.
The horses and elephants in these studies ate 1.5% of their body weight per day in hay, while the
giant pandas consumed 4.3% of their body weight. The pandas have adopted a dietary strategy of
Table 2. Comparative digestion and passage between giant pandas, horses, and elephants.
Giant pandas Horses Elephants
Body weight (kg) 119 500 2714
Intake (% of body weight) 4.3 1.5 1.5
Mean retention (hours) 8 30 24
Digestibility (%)
Dry matter 20 50 41
Protein 90 55 50
Cellulose 84 84 2
Hemicellulose 27 53 44

extremely high intake and short retention time. Although the
panda’s diet of bamboo is high in fiber, digestibility of fiber is
quite low. Instead, pandas extract the cell contents from the
bamboo, and the animals depend very little on microbial fermentation.
Protein digestibility was 90% from the bamboo
because most of the protein in bamboo is located within the cell
contents rather than in the cell wall.
Horses and elephants illustrate the general trend in rate of
passage and digestibility in large nonruminant herbivores as it
relates to body size. Horses have a rate of passage equal to about
30 hours, and they digest about 50% of the dry matter in hay.
Elephants, conversely, have a shorter retention time, about 24
hours, and lower dry matter digestibility. Fiber and protein
digestibilities follow the same trend.
Microbial populations in different species. Although animals
vary greatly in their dependence on microbial fermentation, the
populations of microbes that inhabit the organs utilized for fermentation
and the environments within these organs are
remarkably similar.
Despite the fact that pigs, dogs, and ponies vary tremendously
in their dependence on microbial digestion, they all have hindgut
environments conducive to fermentation. While VFA concentrations
are high in the large intestine of each of these species, pigs
and dogs have higher colonic VFA concentrations than ponies.
This shows that species that are normally thought of as monogastrics
have active sites of fermentation in their large intestines.
Pigs are quite capable of utilizing high-fiber diets, though
this fact has been largely ignored as intensive swine management
programs have developed. These days, pigs are often
kept in confinement and fed high-starch diets that contain significant
amounts of corn. In their classic animal husbandry
text, Feeds and Feeding (Nineteenth Edition), Henry and
Morrison state, “Pastures are so important to pork production
that they often make all the difference between profit and
loss. Few facts in swine feeding have been so clearly proven,
both by scientific experiments and in the common experience
of successful farmers, as the high value of pasture or forage
crops for all classes of swine.”
In horses the microflora in the hindgut are susceptible to diets
that veer severely from primarily forage. A close look at starch,
which is abundant in cereal grains such as corn, barley, and oats,
proves that it is a versatile energy source. However, problems arise
when it is overfed. Studies at Kentucky Equine Research (KER)
have shown that pH of the hindgut drops significantly in horses
following a grain meal rich in starch, with the lowest point occurring
between four and eight hours after feeding. Changes in
hindgut pH make horses susceptible to colic and laminitis. For
this reason and others, KER nutritionists generally recommend
grain meals be small, generally not exceeding five pounds.
For horses that must consume large quantities of grain in order
to fuel exercise or maintain body weight, a hindgut buffer such as
KER’s EquiShure is appropriate because it steadies the pH, preventing
sudden downward shifts that could harm microflora.
Interspecies Comparisons
A constant source of debate among animal nutritionists is
whether information from one species can be used to improve the
way other species are fed. Are data gathered from ruminants relevant
to horses? Are data gathered from horses transferable to
pigs? In light of the similarities between microbial fermentation
across many species, the answer would appear to be yes.
One example of this involves yeast culture. Early work with
ruminants showed that yeast culture affected microbial fermentation
in a number of beneficial ways. Initially, nonruminant
nutritionists dismissed this information as unimportant for monogastric
animals primarily because the rumen was deemed an
inappropriate model for rabbits, pigs, or horses. Research with
rabbits and horses, including some conducted at KER, showed
that yeast culture affects the intestinal microbes in these species
in much the same way that rumen microflora are affected.
Transfer of information across species is important for the wellbeing
of all animals. An open-minded approach to feeding often
yields benefits for multiple species.
Microbial fermentation is important for most animals. The
anatomical adaptations that each species has developed depend
primarily on body size and natural diet. The total volume of the
digestive tract devoted to fermentation and the amount of time
digesta spends in these organs varies greatly from species to
species. The types of microflora that inhabit the digestive tracts of
various animals, however, are similar. Thus, it would seem logical
that dietary manipulations that affect one species may also
affect other species if the differences that exist between intestinal
architecture are taken into account.
When researching feeds and feeding practices for horses,
KER nutritionists look beyond research conducted on horses. By
casting a wider net and looking at the research performed in
other species, KER feed formulators maintain their scientific
advantage.
Equinews/Volume 10, Issue 3 9

Reprint Courtesy of
Kentucky Equine Research, Inc.
3910 Delaney Ferry Road
Versailles, KY 40383
Phone: 859-873-1988
Fax: 859-873-3781
Order Department: 888-873-1988
www.ker.com
info@ker.com