establish a standing osmotic gradient in the
epithelial interstitium that increases water
transport from the lumen to the interstitium.
There is no debate on this part of the
hypothesis. What is not established is the
proportion of water entering the interstitium
by transcellular versus paracellular routes.
The Pappenheimer hypothesis suggests that
the intracellular Na+ induced by active
transport triggers a contraction of the
cytoskeleton that causes a rearrangement of
the tight junction proteins and making the
junction more porous. This is a point of
contention, but has good support.
Cytochalasin D, which also stimulates con-
traction of the cytoskeleton of enterocytes,
increases tight-junction permeability to an
extent similar to that observed concomitant
with active glucose absorption. In large
animals, the proportion of glucose absorbed
via the paracellular route may be much
greater than carrier-mediated absorption. It
cannot be construed, however, that the para-
cellular route is more important because the
paracellular route will not operate in the
absence of an active transport system.
Generally, most of the -linked glucose
polymers, lactose and sucrose (except in
neonates) are digested and absorbed during
the transit time of the digesta through the
small intestine. Starches that are resistant
to the digestive process are not common in
feedstuffs fed to swine. Digestion-resistant
starches are those that form crystalline
starch granules. Sorghum harvested as high
moisture, then dried and fed without
further processing, contains some resistant
starch. The quantity of resistant starch in
legume seed is significant. The resistant
starch and fibre (fructans and cell wall
carbohydrates) will pass into the caecum
and large intestine to provide substrate for
the enteric microorganisms. Fermentation
by the microorganisms does not provide an
important source of energy for growing and
finishing swine, but hindgut fermentation
can provide a substantial energy source for
the breeding herd. Independently of the
magnitude of energy supply to the host,
hindgut fermentation is an important
process. Details of hindgut fermentation
will be discussed in a subsequent section.
Poultry
Much of the previous discussion on swine
is also applicable to poultry, but there are
important differences in poultry (Duke,
1986a,b). The beak replaces the lips and
teeth. Feed is consumed, one peck at a
time; hence there is very little residence
time per peck in the oral cavity. Lacking a
soft palate, poultry raise their heads to
swallow in order to prevent ingesta from
filling the nasal cavity. Dry, non-pelleted
feeds often are moistened prior to ingesting
with saliva or water carried in the beak and
dumped on the feed. This practice causes
practical problems in maintaining feed
intake and freshness of the feed. Most
poultry feeds are pelleted. Pellets more
closely resembling the natural diet of
seeds, insects, worms, berries and carrion
are easier for the birds to ingest and the
pellets are not moistened before ingestion.
The lumen of the oesophagus is very
large relative to the size of the animal in
order to accommodate the lack of physical
breakdown of ingesta in the oral cavity. The
oesophagus is divided into three distinct
sections: the pre-crop oesophagus; the crop;
and the post-crop oesophagus. The crop is a
distensible pouch in the oesophagus that
stores the ingesta during meals. Between
meals, the crop meters ingesta to the post-
crop oesophagus and gastric digestion.
Salivary and oesophageal secretions, mostly
mucus, are added to the ingesta during its
retention in the crop. The saliva of poultry
lacks amylase, although that of some wild
birds, e.g. sparrows, contains amylase.
Amylase is present in the crop contents of
poultry. The sources are primarily bacteria
ingested with the feed and the feed itself.
As much as 25% of the ingested starch may
be converted to sugars in the crop, and
10% of the starch disappears during
storage in the crop. Disappearance is due to
microbial fermentation and absorption of
glucose through the crop wall.
As material is metered from the
crop through the post-crop oesophagus,
it traverses the proventriculus. The
proventriculus is the secretory portion of
the stomach releasing HCl, pepsinogen,
intrinsic factor and chitinase. Chitinase130 R.W. Russell and S.A. Gahr