Agriculture Reference
In-Depth Information
be more likely when dietary crude protein comes largely
from nonprotein nitrogen (NPN).
Volatile fatty acids, being weak acids (pK = 4.75
of both isomers of lactic acid (+) and (
) that accentuates
the rumen pH decline resulting in ruminal acidosis. Lactic
acid absorption rate from the rumen is only one-tenth that
of VFA. The liver metabolizes the L (+) lactic acid isomer
more rapidly than the D (
−
4.87),
they almost entirely dissociated in the reticulo-rumen at
pH 6.6. About one-half of the VFA are absorbed through
reticulo-rumen epithelium in their nondissociated form by
passive diffusion with absorption rates being correlated
positively with length of the VFA and negatively corre-
lated with the rumen pH. The remaining VFA are absorbed
as anions by facilitated diffusion in exchange for bicarbon-
ate ions. Epithelial cells of the rumen contain carbonic
anhydrase to promote production of carbonic acid from
CO
2
and water. Carbonic acid in turn is dissociated into
bicarbonate and H ions. These hydrogen ions convert VFA
anions into acids for passive diffusion through the rumen
epithelium. Transfer of hydrogen from carbonic acid (a
weaker acid) to form VFA (stronger acids) lowers the pH
and promotes further VFA absorption. Bicarbonate ions
released into the rumen during absorption will neutralize
more than half of the VFA in the rumen. The remaining
VFA are neutralized largely by salivary alkali.
During absorption, some VFA are metabolized. Most of
the butyrate is converted to
−
) isomer. L lactic acid also is
produced by muscle tissue during anaerobic exercise, but
D lactic acid is primarily a product of bacterial metabolism
found in fermented feeds including silages at up to 10%
of dry matter.
Ammonia is produced in the rumen from deamination
of dietary proteins, or hydrolysis of dietary NPN or urea
from saliva, or diffusing into the rumen from blood. Given
an adequate supply of energy from fermented carbohy-
drates, ammonia is used for synthesis of microbial protein.
Excesses of ammonia are absorbed through the rumen
wall, readily removed from portal blood, and detoxifi ed by
the liver by conversion to urea. Ammonia that escapes
liver metabolism (for example, absorbed by the lymphatic
system) can prove toxic for ruminants.
Gas production in the reticulo-rumen peaks some 2-4
hours after feeding. The major rumen gases include carbon
dioxide (60%), methane (30-40%), and nitrogen, with
small amounts of hydrogen sulfi de, hydrogen, and oxygen.
Carbon dioxide is produced either by decarboxylation
during fermentation, or by neutralization by acids of bicar-
bonate ions that enter the rumen via saliva or exchange
across the rumen wall during VFA absorption. Methane is
produced from reduction of CO
2
and formate. Loss of
methane from the rumen accounts for about 6% of ingested
energy. Methane is a contributor to global warming, so
reducing methane production by ruminants and by anaero-
bic bacteria in wetlands, and by release from petroleum are
of worldwide concern. Hydrogen sulfi de, being toxic to
rumen microbes and animals, is produced from ruminal
reduction of sulfates in the diet or derived from amino
acids that contain sulfur. Hydrogen, present in only small
amounts in the rumen, can increase when animals are
abruptly switched to a high concentrate diet and fermenta-
tion is abnormal. Oxygen enters the rumen together with
ingested feed and water and by diffusion into the rumen
from blood; it is quickly removed by facultative ruminal
bacteria.
Practically no peptides or amino acids are fl ushed onto
the abomasum or small intestine because they are rapidly
catabolized by ruminal bacteria. However, the feed and
microbial proteins that pass out of the reticulo-rumen
undergo lysis by abomasal lysozyme. Microbial proteins,
lipids, vitamins, and starch are digested in the intestines
and make a substantial contribution to the nutrient supply
of the host.
−
- OH -
BUT) with the remainder being similarly metabolized by
the liver. Thus, absorbed butyric acid enters the circulation
as
β
- hydroxybutyrate (
β
-OH-BUT for metabolism by most ruminant tissues as
a source of energy as well as fat, being the four-carbon
primer used for synthesis of short and medium chain fatty
acids unique to the ruminant's milk fat.
Almost one-third of the propionic acid absorbed is
metabolized by the rumen wall to lactic acid. Propionate
is completely converted by the liver to oxalacetate and that
enters the Kreb's cycle. Lactate is converted to glucose for
storage as glycogen by the liver or released as glucose into
the circulating portal blood. Propionate and valerate are
the only VFA used for gluconeogenesis.
The majority of acetate enters circulating portal blood
unchanged except for a small amount that is metabolized
to CO
2
by the rumen epithelium. The most abundant VFA
in the blood, acetate, is readily used by tissues to form
Acetyl-Co-A that yields energy via the Kreb's cycle or
forms fatty acids. Mammary tissues use acetate and
β
- OH -
BUT to form the short and medium chain length fatty acids
found in milk.
Lactic acid produced by amylolytic bacteria is present
as a transient acid in low concentration, and can be used
by secondary rumen bacteria to produce propionate.
However, the low rumen pH associated with high concen-
trate diets may inhibit propionate producing bacteria more
than amylolytic bacteria; this can result in an accumulation
β
