Explain why ruminants, cecal fermenters, and monogastrics have such different
abilities for digesting forages.
Forage is plant material eaten by grazing animals and is composed of mainly plant leaves
and stems. According to Wikipedia, “the term forage [has been used historically to mean]
only plants eaten by the animals directly as pasture, crop residue, or immature cereal
crops, but it has also been used more loosely to include similar plants cut for fodder and
carried to the animals, especially as hay or silage.” Therefore, we can loosely imply that
forage is mainly cellulose, with different dietary vitamins and other nutrients and
proteins. But for ruminant animals, cecal fermenters, and monogastrics, the main
difference in digesting these forages is the cellulose aspect.
The most important difference between ruminants and monogastrics is the complexity of
the ruminant stomach. Monogastrics are very limited in their ability to digest and extract
energy from cellulose in their large intestine. Ruminants and cecal fermenters, though,
are able to extract energy from cellulose very well in the rumen/reticulum of the ruminant
or the large intestine of the cecal fermenter. This is due to the ability of bacterial
fermentation, which takes place in the reticulum of the ruminant and the cecum and large
colon of the cecal fermenters.
In monogastric animals, the function of the stomach is limited to temporary storage and
preliminary mastication for the food into a liquid mass. Little to no absorption of
nutrients takes place in the monogastric stomach. In ruminants, however, the stomach has
an absorption function as well as the regular functions of mastication and acidification by
the digestive juices. The key to the ruminant animal’s effective ability to digest forages is
the presence of symbiotic microorganisms in the digestive tract, especially in the
forestomach. These microbes and their digestive abilities turn the ruminant into a “mobile
fermentation plant” for cellulose.
The symbiotic microbes produce cellulase enzymes, an ability that no mammal can do on
its own. The cellulase then breaks down the cell walls of forages to release fatty acids
into the gut. These fatty acids are then absorbed by the ruminant and contribute to a large
portion of the animal’s overall energy acquired from digestion. Then the food passes into
the intestine and absorption of amino acids, lipids, carbohydrates, and other nutrients, is
possible. This double digestion of the ruminant allows them to use cellulose as an energy
source, which is otherwise denied to other herbivores.
Compare and contrast the end products of protein and carbohydrate digestion
between the pig and the cow. (This is an exercise to determine if you understand
the predominantly enzymatic digestion of the pig compared to the microbial
digestion of the ruminant that is then followed by enzymatic digestion.)
The following are diagrams I created to help me remember the steps in which
carbohydrates and proteins are digesting in the pig and cow. The first figure is the
diagram in which proteins are digested in the pig.