Agriculture Reference
In-Depth Information
Chapter 6 for more detailed descriptions of the functional
anatomy of these organs.
The reticulo-rumen is the primary site for fermentation
by diverse but specialized microbes. Rumen motility origi-
nates in the reticulo-rumen for the processes of rumination
and eructation. Environmental features unique for the
reticulo-rumen that prove useful for action of ruminal
microbes include (1) a reasonably isothermal environment
regulated by the homeothermic metabolism of the animal,
(2) constant infl ux of water and feeds, (3) a relatively
constant pH achieved through absorption of fermentation
acids and salivary buffers, and (4) removal of fermentation
end-products by absorption or passage.
located close to the luminal mucosa of the abomasum and
small intestine. These receptors are excited by both
mechanical and certain chemical stimuli and may be
responsible for chemostatic regulation of intake.
Ingested feed enters the reticulum through the cardia.
Heavy materials (stones, sand) may immediately drop to
the bottom of the reticulum and remain there. Less dense
and fi brous materials fl oat high in the reticulum and cranial
sac of the rumen forming a raft of entangled particles.
Reticular and rumen contractions move ingested particles
caudally to join the fi brous raft in the rumen. Ruminal
contents, particularly the raft, are meshed, kneaded, and
slowly rotated with powerful rumen contractions. Soupy
material common to the reticulum, cranial sac, and ventral
sac is randomly pushed out of the reticulum to the omasum
and postruminal tract by reticulo-ruminal contractions
when the reticulo-omasal orifi ce is open.
The process of rumination includes four steps: regurgi-
tation, re - insalivation, re - mastication, and re - swallowing.
During regurgitation, a bolus of feed gathered near the
cardia area of the rumen is pulled back to the mouth, an
extrareticular contraction that precedes the usual biphasic
contraction and opening of the cardia with antiperistaltic
movement of the esophagus bringing the bolus to the
mouth. There, the bolus is re-chewed, mixed with addi-
tional saliva, and re-swallowed.
Much of the fermentation in the rumen occurs within
the fi brous raft. Rumen contractions persistently attempt to
break the raft and release the gases for removal via eructa-
tion (belching). During the secondary cycle, aided by con-
traction of the dorsal sac, gases are moved cranially into
the reticulum while the raft and fl uids are pushed ventrally.
If and when the gas layer clears the cardia of its fl uid,
eructation is evoked.
Rumen Motility, Rumination, and Eructation
Powerful contractions by the reticulo-ruminal wall include
a primary cycle (a mixing cycle or “A” sequence) as well
as a secondary cycle (eructation cycle or “B” sequence).
Both are initiated by excitation of vagal nerve fi ber recep-
tors distributed throughout the reticulum and rumen
regions. The primary cycle consists of a double contraction
of the reticulum followed by caudally moving contractions
of the dorsal ruminal sac and ventral ruminal sac. This
serves to pump ruminal fl uids atop the fl oating raft in the
rumen allowing fl uid to percolate through the raft remov-
ing small particles for removal from the rumen. The sec-
ondary cycle usually occurs at the end of alternate primary
cycles and consists of contractions of the caudoventral
ruminal blind sac, a cranially moving contraction of the
caudodorsal ruminal blind sac followed by the middorsal
ruminal sac and the ventral sac. At the end of this cycle,
the point where the esophagus enters the rumen is cleared
of liquid so that the headspace gas can escape. Compared
to motility before a meal, rate of motility often is doubled
during and after feeding. The motor activity responsible
for these contractions originates in the bilaterally paired
gastric centers in the medulla oblongata of the hindbrain.
Reticulo-ruminal motility is important for mixing of
digesta, rumination, particle size reduction through attri-
tion, eructation, and VFA absorption.
Two types of sensory receptor mechanisms are respon-
sible for the vagal nerve input: tension receptors and epi-
thelial/mucosal receptors. Tension receptors are located in
the muscle layers of all parts of the GI tract. They monitor
the tension present in the muscular wall imposed by
passive distension and thus may be responsible for limiting
intake of low quality feeds based on bulk or mass. The
epithelial receptors are located close to the luminal epithe-
lium of the forestomach whereas mucosal receptors are
Fermentation
The primary sites of microbial fermentation within the
digestive tract of goats are the rumen and reticulum. The
host animal and the diverse population of microbes in a
symbiotic relationship ferment feeds yielding products that
are useful nutritionally. Microbes that inhabit the rumen
and reticulum, being provided with the unique anaerobic
environment by the host ruminant discussed above,
perform several functions: (1) fermentation of structural
(cellulose and hemicellulose) and nonstructural (sugars
and starch) carbohydrates into readily metabolized energy
sources such as volatile fatty acids, (2) conversion of non-
protein nitrogen from plants and metabolically recycled
urea to a high biological value microbial protein, (3) syn-
