cycle embedded in the gluconeogenic
pathway from propionate), complete
oxidation of glucose yields only 65% of
the energy that would have been available
if the dietary carbohydrates were digested
and metabolized directly.
This may appear to be a high price.
Most carbohydrates that ruminants eat
during their lifetime, however, are totally
unavailable by the direct route. The micro-
bial ecosystem gives ruminants the advan-
tage of being able to thrive on feeds that
would not sustain other animals, including
humans. There is lots of good news in this
good news/bad news story. Of the solar
energy captured by the earth’s biomass,
only 5% potentially is available for human
food directly, leaving lots of feed for
ruminants.
Attempts to improve the efficiency of
ruminant metabolism by providing a post-
ruminal source of starch have been
unsuccessful. Unlike the case in direct
absorbers, starch entering the small
intestine of ruminants does not disappear
rapidly. Typically, only half the starch that
enters the duodenum disappears during
transit through the small intestine (Owens
et al., 1986; for discussion see Hill et al.,
1991). Huntington (1997) concluded that
enzymatic capacity, both luminal and
membrane, is the limiting factor in dis-
appearance of starch. Another factor to
consider is that after weaning, ruminants
stop expressing SGLT1 (Shirazi-Beechey et
al., 1995). Expression of SGLT1 has been
induced in adult ruminants by infusing the
intestine with 30 mMglucose for 4 days. It
has not been determined whether practical
conditions can generate the combination of
starch supply and intestinal enzymatic
activity necessary to achieve the threshold
concentration of free glucose for induction
of SGLT1. There is a wide range of indivi-
dual variability, making it probable that
rapid changes could be made by genetic
selection. Intestinal starch disappearance
ranged from 10 to 93% in beef steers
(Harmon, 1992) and intestinal glucose
absorption ranged from 10 to 100% in
dairy steers (Russell and Schmidt, 1984;
and unpublished).
Hindgut fermentation
Fermentation in the hindgut is similar to
that described for the ruminal ecosystem in
most species incorporating the process into
their digestive strategy. Most species use
hindgut fermentation, at least to some
extent. Even in species, such as humans,
where the fermentation is an insignificant
source of energy, the VFAs produced are
important in maintaining a healthy
intestinal epithelium (Cummings et al.,
1995). Fermentation is combined with
direct absorption, but the host animal has
the first access to available substrate. Feed
consumed by the animal is processed
initially as described for direct absorbers.
Much of the starches and non-structural
carbohydrates is digested and absorbed
prior to exposure to fermentation.
Absorption of VFAs differs slightly from
that described for the rumen, with varia-
tions depending on the location in the
hindgut (von Engelhardt, 1995).
Ruminants expose digesta to hindgut
fermentation as well as to pre-gastric
fermentation. Most of the hindgut
fermentation in ruminants occurs in the
caecum. The caecum is a blind pouch at
the junction of the small and large intestine
and functions like a small rumen with a
steady flow of material in and out. Hindgut
fermentation in ruminants provides <10%
of the VFAs relative to ruminal production
(Bergman, 1990).
Equids and swine are colon
fermenters; most of the fermentation
occurs in the colon. The caecum,
although quite large, operates function-
ally as an extension of the proximal
colon. In these animals, the large, distinct
haustrations of the colon retain the
digesta for fermentation. There is good
mixing of material within each haustra-
tion but minimum mixing of contents
among the haustrations. From a func-
tional viewpoint, this is analogous to
many small, ‘closed’ (exchange of
material only with epithelium) rumens in
series passing down the tract. A distinct
and important difference between colonic
and ruminal fermentation is that aceto-
genesis largely replaces methanogenesis142 R.W. Russell and S.A. Gahr