Understanding Digestion in the Horse, A Comparative Approach

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