activity in poultry is much less than in
insectivorous birds, but adequate to hydro-
lyse approximately one-third of purified
shrimp chitin added to normal diets (Vonk
and Western, 1984). Its lumen is approxi-
mately the same size as the oesophagus. The
ingesta is mixed thoroughly with gastric
secretions in the ventriculus (gizzard). The
ventriculus is the site where both physical
and chemical digestion is initiated as a
major effort. Each batch of digesta entering
the ventriculus is churned into a pulpy
consistency by muscular contractions.
The digesta passes into the small
intestine and is processed similarly to what
was discussed for swine. Poultry, however,
lack lactase. No chitinobiase is found in
either the small intestine or the pancreas.
The function of chitinase, therefore, is
mainly to solubilize chitin in order to
enhance digestion of other nutrients in the
feedstuff, rather than to obtain energy from
chitin itself. Relative to body size, the
length of the small intestine is short. There
is some question of the completeness of
starch digestion and absorption in the
small intestine. The proximal caeca has
SGLT1 activity similar to that expressed in
mid-small intestine (Planas et al., 1986;
Vinardell et al., 1986). The extent of
glucose absorption from the caeca in com-
mercial production systems is unknown.
Other
Hindgut fermenters and carnivores will also
process digesta similarly to direct absorbers.
They too are direct absorbers but have
alternative routes for obtaining a substantial
portion of their carbohydrate carbon. Very
strict carnivores have a reduced capacity for
digesting carbohydrate, and excess carbo-
hydrate in their diet may impede their
ability to digest protein.
FermentersPre-gastric fermentation
Ruminant animals rely on pre-gastric
fermentation for initial processing of
ingesta. Ruminants actually harvest
nutrients to supply a complex microbial
ecosystem inhabiting their forestomach.
The ruminant obtains its nutrients from the
wastes and effluent from the fermentation.
There are 20–25 billion bacteria and
200,000–2 million protozoa ml^1 of
ruminal fluids. The population of fungi is
more variable and the role of fungi defined
less accurately. The rumen of an adult
dairy cow has a capacity of 210 l. The
number of microorganisms inhabiting the
rumen of a single cow exceeds the world’s
human population by 1 million-fold! The
fundamentals of this complex system were
established by the classic work of Hungate
(1966), and this has been updated recently
(Hobson and Stewart, 1997). More general
reviews are available (Church, 1988; Van
Soest, 1994).
To ruminate means to chew again,
hence the name ruminant. The process of
rumination is essential for efficient meta-
bolism of fibrous feeds. Ruminants are
meal-feeding animals, eating twice per day
each morning and evening. The ruminal
ecosystem is most efficient if fermentation
is continuous at a constant rate.
Microorganisms do not have the ability to
store nutrients during times of abundance
for use when nutrients are limited; they
grow by dividing and their metabolism is
most efficient during the exponential
growth phase. Cycling of nutrient avail-
ability causes surges in microbial growth
followed by periods of maintenance meta-
bolism with little cell division. Rumination
converts the meal-feeding habits of the
ruminants into a continuous feeding
regimen for the microorganisms. Ruminants
consume large quantities of coarse feeds
rapidly. As the feeds are ingested, not all of
the material is available immediately as a
substrate for the microorganisms. The
waxy cuticle on the surface of plants
impedes microbial digestion. The micro-
organisms attack the broken ends and
places where there are nicks in the cuticle.
The nearly continual process of rumination
continues to provide ‘new substrate’ to the
microorganisms long after the meal has
been consumed. The digestion by micro-
organisms causes loci of structural weak-
ness in the plant material that are fracturedGlucose Availability and Associated Metabolism 131