Plants provide an abundant source of food for animals.
Plant cell walls contain bundles of long fibers.
Macrofibrils are composed of smaller bundles of microfibrils.
Microfibrils contain strands of micelles which are composed of chains of cellulose matrixes.
Pectin fibers between the microfibrils give the plant flexibility.
Hemicellulose, composed of several different carbohydrate molecules are connected to the microfibrils.
Micelles are made up of a matrix of cellulose chains.
Cellulose is composed of glucose molecules that are held together by strong glycosidic bonds.
Glucose is the most important source of energy for living things.
The glucose in cellulose chains can only be used when the bonds between the molecules are broken.
Humans and other animals do not have the necessary enzymes in their digestive system
needed to break the bonds to release glucose molecules.
Ruminant herbivores like the goat rely on the microbes in the rumen to utilize
the energy from plant sources.
Rumen microbes secret enzymes called beta-glucanses which can break the bonds
between glucose molecules in cellulose.
Microbes absorb free glucose molecules which are metabolized (broken down)
into two pyruvate molecules and released energy.
In anaerobic (oxygen free) conditions, pyruvate is further metabolized in a process called fermentation.
The waste products of fermentation are called volatile fatty acids (VFA's).
VFA's supply nearly three quarters of the goat's energy supply.
The three most common VFA's are Acetic acid, Propionic acid and Butyric acid.
Acetic acid is the main product of microbe fermentation.
Acetic acid is aborbed through the rumen papillae and is transported by the blood to the liver.
Acetic acid is used by the goats system in lipid synthesis.
Propionic acid is absorbed through the papillae and is removed from the blood by the liver
where it serves as a substrate to synthesize glucose.
This is the only source of glucose for the goat.
Butyric acid is changed to a type of ketone body (beta-hydroxybutyric acid) as it
passes through the papillae epithelium and is used for energy production in body tissues.
Some other waste products produced by microbe metabolism are lactic acid,
and carbon dioxide and methane gases.
Cellulose digestion by microbes is a slow process.
The acidic waste products of fermentation are continuously removed
which keeps the rumen from becoming too acidic.
Starch (such as from grain) is also made up of chains of glucose but the bonds
between the molecules are different from the cellulose glycosidic bonds.
Starch is easily fermented by bacteria that digest starch using the enzyme amylase.
A sudden increase in dietary starch rapidly increases the concentration of VFA's,
especially the strong acid lactate. Rumen pH then becomes acidic.
If the pH drops below 5.5 the bacteria and protozoa that digest cellulose will be inhibited or die.
The pH of the rumen will continue to fall, but some starch digesting bacteria
can be resistant to an acidic pH.
Some bacteria are able to use VFA's in their metabolism.
If starches are added to the diet slowly, bacteria which can convert lactic acid to
propionic and acetic acids will increase in number.
The rumen pH will not drop to dangerously low levels but on a moderate starch diet
the rumen can remain slightly more acidic than optimal.
A healthy rumen contains bacteria, protozoa and fungi each which specialize in digesting
different food materials and in turn produce various by-products that are
used by the goat for nutrition.