uptake that is extruded on the basolateral
membrane by the Na+/K+ ATPase. The
stoichiometry has not been determined.
The acetate anion is a significant source of
energy for the epithelial cells and is a
source of CO 2 for intracellular bicarbonate.
Transport of the anions across the baso-
lateral membranes is less certain, but intra-
cellular anion/basolateral extracellular
bicarbonate exchange is likely.
The strategy of ruminants is to retain
materials in the rumen for extensive
fermentation. The mechanisms causing this
are not defined clearly (Mathison et al.,
1995). Physical characteristics of the feed
have an influence, but are not the only
factor. There is a ‘threshold’ size for parti-
culate passage through the reticulo-omasal
orifice. The rate at which feeds break into
particles smaller than the threshold is
important, but many particles less than
threshold size are retained in the rumen.
Particulates to be passed into the omasum
appear to concentrate in the honeycomb
pattern of epithelia folds in the reticulum
and migrate toward the orifice during
reticular contractions. Liquid passage rate
and omasal ‘fill’ influence the rate at which
these particulates are transferred into the
omasum. Generally, slowly digested feeds
remain in the rumen longer than rapidly
digested feeds, but there are exceptions.
Very fibrous feeds with minimal available
protein, i.e. straw, may be digested and
passed from the rumen so slowly that they
reduce nutrient intake below requirements.
On the other hand, extremely nutritious
forage, i.e. spring pasture, which is
digested rapidly, may pass from the rumen
too rapidly for significant digestion. As
a consequence, animal performance
declines. The decline in performance can
be avoided by providing a small amount of
coarse roughage to slow the rate of passage.
Although many farmers have known this
for generations, scientists are beginning to
understand the reasons. Grains, which are
digested rapidly, have a relatively long
retention time because they are less
stimulatory of rumen motility than forages,
and the rapid generation of fermentation
acids is inhibitory to motility.
The relationship between a ruminant
and its ruminal microbial ecosystem is
symbiotic. The ruminant provides sub-
strate, a warm anaerobic space (with some
assistance by facultative anaerobes
attached to the rumen epithelium), buffers
to help maintain a desirable pH and
systems for removal of microbial wastes
(eructation to eliminate CO 2 and methane;
passage to eliminate undigestable residue;
absorption to eliminate VFAs and
ammonia). The VFAs are a waste product
of microbial metabolism, but are an
important fuel for ruminants, providing
60–80% of total energy. The micro-
organisms also are an important source of
protein for the ruminant, providing up to
2.5 kg of microbial protein per day to the
ruminant (lactating dairy cow; Clark et al.,
1992) as the microorganisms pass from the
stomach to be digested by the ruminant.
Much of the microbial protein is derived
from non-protein N from the feed and
metabolic waste N from the ruminant’s
metabolism. Microorganisms have the
ability to synthesize B vitamins and with
rare exception do so to the extent that there
is no need to supplement the ruminant’s
diet with B vitamins.
There are costs associated with this
relationship. Generally, 25% of the
energy of the carbohydrates fermented is
lost, i.e. the VFAs retain 75% of the
energy. Most (90% or more) of the carbo-
hydrate carbon that becomes available for
metabolism by the ruminant is in the form
of VFAs, thus very little glucose is avail-
able for absorption and the micro-
organisms ferment carbohydrates that
potentially would be available to the
ruminant directly (starches) as well as or
rather than the structural carbohydrates
that are unavailable directly. Ruminants,
therefore, are very dependent on gluco-
neogenesis for maintaining blood glucose
concentrations. Of the VFAs, only
propionate is a major source of carbon
for gluconeogenesis. Conversion of
propionate to glucose also costs metabolic
energy (4 ATP mol^1 glucose). Combining
the costs of fermentation and gluco-
neogenesis (ignoring the costs of the futileGlucose Availability and Associated Metabolism 141