in colonic fermentation (Miller, 1995). For
a comprehensive review of acetogenesis
see Drake (1994). The acetogenic and
methanogenic pathways are parallel,
differing primarily in substrate specificity.
Replacing methanogenesis with aceto-
genesis in the rumen would increase
carbohydrate energy retained as VFAs
from 75% to ~95%. A worthy project for
industrious students.
Colonic fermentation has advantages
over ruminal fermentation when feeds are
either of very high quality or very low
quality, if available in abundance. The
advantage when fed high-quality feeds is
obvious. The host has first access to the
feed, and non-structural carbohydrates can
be digested and absorbed directly by the
host without fermentation losses. The
hindgut microorganisms then provide
additional energy to the host by digesting
structural carbohydrates. When there is an
abundance of poor-quality feeds, colonic
fermenters perform better than ruminants.
Colonic fermenters adapt to poor-quality
feeds by increasing the transit rate through
the tract. Digestibility decreases but
nutrient availability increases by increas-
ing intake. The ruminant strategy of
maximizing digestibility becomes a
liability because passage rate slows, filling
the rumen with undigestible material,
which limits intake.
Poultry are caecal fermenters. The
process in poultry differs from that in
ruminants. The caeca are filled by anti-
peristaltic flow. The contents of the caeca
ferment, followed by emptying of the
caeca. Thus in poultry, it is a ‘batch’
process rather than a continuous flow
process as in ruminants. Because poultry
feeds are digested very well in the small
intestine and urine mixes with the digesta,
we hypothesized that caecal micro-
organisms would have an abundance of
available nitrogen relative to energy.
Supplementing the hindgut micro-
organisms with an energy source in the
form of lactose fed to young turkeys
(poultry lack lactase) increased weight
gains by 50% relative to control turkeys
(Russell, 1999, unpublished).
Coprophagic strategy
Rabbits are hindgut, caecal fermenters and
are the only wanted farm animals (rats in
the granary do not count) with a
coprophagic feeding strategy. Their diges-
tive strategy differs markedly from that of
other farm animals. Like poultry, the
caecal fermentation is a ‘batch-type’
fermentation, but differs in function.
Excellent reviews of rabbit physiology
and nutrition are available (Cheeke, 1987;
McNitt et al., 1996). The initial ingesta is
chewed to a very fine consistency in the
mouth. Upon swallowing, the digesta is
processed as described for direct
absorbers. A significant difference is that
the digesta is acidified to ~pH 1.5 in the
stomach. This is 5- to 10-fold more acidic
than that of most animals. The conse-
quence is that there are very few species
of microorganisms that survive these
conditions to eventually inhabit the
hindgut. As a result, the ecology in the
hindgut is much more fragile.
Rabbits are paradoxical. They require
fibre in their diet but, contrary to popular
belief, they are poor digesters of fibre.
Fibre is essential to maintain motility in
the hindgut. As digesta enters the caecum,
the particulates are separated from the
solubles. Solubles are retained in the
caecum for fermentation and the large
particulates are propelled down the tract,
with solubles and small particulates
captured in the haustrations, and pushed
toward the caecum by antiperistalsis. The
large particulates are evacuated from the
tract as ‘hard’ or ‘day’ faeces. These faeces
are not recycled. The solubles and small
particulates are retained in the caecum for
fermentation. Excretion of the hard faeces
precedes excretion of the ‘soft’ or ‘night’
faeces, called caecotrophs. Approximately
8 h after feeding, the caecal contents
become coated with mucus. The mucus-
coated caecal contents (caecotrophs) are
evacuated via the anus directly into the
mouth and swallowed. The caecotrophs
remain in the stomach for up to 12 h
where a lactate-producing fermentation
occurs until the acid stomach fluids pene-
trate the caecotrophs. The fermentation inGlucose Availability and Associated Metabolism 143