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COLLECTION: GOAT HANDBOOK
ORIGIN: United States
DATE INCLUDED: June 1992
Extension Goat Handbook
This material was contributed from
collections at the National Agricultural Library.
However, users should direct all inquires about the
contents to authors or originating agencies.
DOCN 000000048
NO F-5
DIGESTION
G. F. W. Haenlein R. Caccese; U. of Delaware, Newark
P. H. Sammelwitz; U. of Delaware, Newark
Anatomy and Physiology
1 Digestion in the goat is the process
by which ingested feed materials are broken down and
readied for absorption through the gastrointestinal
(GI) tract in order to furnish energy and building
blocks needed by its metabolism. All feed sources
originally are the result of plants utilizing the
energy of the sun and the minerals of the soil, without
which the herbivorous goats and other farm animals
could not exist.
2 In the food chain, the primary
consumers are the herbivorous animals. They have specially
developed fore-stomachs and GI tracts that enable
them to utilize cellulose and other forms of plant
polysaccharides. They are also maintaining large symbiotic
populations of bacteria and protozoa in these fore-stomachs,
which can break up the complex structures of plant
celluloses into digestible sugars and a variety of
useful organic acids. Some energy from this breakdown
is used by the bacteria to synthesize their own body
mass, proteins and vitamins, which are then later
utilized by the herbivore when the bacteria pass on
down the GItract and are themselves digested by their
host, e.g. the goat.
3 Herbivores are classified into:
pre-gastric fermentors, ruminants or pseudo-ruminants;
and post-gastric fermentors, whose cecum has evolved
to function similarly to a degree as the forestomach
of ruminants. Cows, sheep, goats and deer are ruminants,
horses and rabbits are members of the second group.
4 Pre-gastric fermentation enables
the most efficient utilization of roughages, which
are fermented and chemically degraded before passing
into the small intestine, where most nutrient absorption
takes place. In post-gastric fermentation, the bacterial
break-down occurs in the cecum, after roughages have
passed through the small intestine. Such animals derive
benefit from cecal fermentation by resorting to the
practice of coprophagy (feces eating).
5 Feedstuffs basically consist of
six kinds of nutritive materials: carbohydrates, fats,
proteins, vitamins, minerals and water. Through digestion,
they become available to the goat by way of absorption
in simple chemical forms such as sugars, fatty acids,
glycerol, amino acids and salts.
6 Mouth The initial step in digestion
is prehension, the conveyance of food into the mouth.
In the goat, the lips, teeth and tongue are the primary
prehensile organs. The lips of goats and sheep are
most important in seizing feed, while the cow makes
more use of the tongue for grasping and tearing of
grasses and forbs. Thus, the goat is capable of greater
selectivity in its grazing. Goats, like other ruminants,
lack an upper set of incisor teeth. Instead they possess
a hardened dental pad, against which they bite.
7 Mastication, or chewing, is necessary
for the reduction of feed to smaller particle sizes.
Goats, exhibit an accentuated lateral movement of
the jaws while chewing, which greatly increases the
grinding action of the teeth. These lateral movements
result in the molars developing a sharp, pointed surface
on the inner edge of the lower teeth and on the outer
edge of the upper teeth. Due to the fact that the
upper jaw is wider than the lower jaw, only one side
of the mouth can be used at a time to grind the feed.
8 While grazing or browsing, the
feed is only chewed enough to be mixed with saliva
to form a bolus of suitable size to swallow. Saliva,
which is a mixture of serous and mucous fluids, is
released by the stimulation of chewing from five pairs
of glands. The parotids, which are located behind
the angle of the jaw; the submaxillaries, on the inner
sides of the parotids; the sublinguals, on each side
of the tongue base; and the inferior molar and buccal
glands in the cheeks. Saliva functions primarily as
an aid in mastication and swallowing, along with providing
some enzymatic (amylase) breakdown of starches. Saliva
contributes very importantly to the buffering of rumen
fermentation, provides nutrients to rumen microorganisms
and certain anti-frothing properties.
9 Pharynx and Esophagus When the
bolus has been formed, it is passed through the pharynx
into the esophagus, which is a musculo-membranous
passageway connecting the pharynx to the rumen. The
bolus is projected forcefully through the esophagus
and into the rumen. The rapid movement is made possible
by the presence of striated muscles in the esophagus
of ruminants, not found in most other animals.
10 Compound Stomach Upon entering
the rumen, the bolus is subject to microbial fermentation.
In young ruminants, liquid feed will by-pass the rumen,
entering into the omasum instead. This is accomplished
by means of the esophageal groove, a fold of mucous
membranes that can be formed in response to the mechanics
of sucking or pursing. The rumen (paunch) is the largest
compartment of the compound stomach in the adult ruminant
consisting of four compartments (the rumen, reticulum,
omasum and abomasum) or three fore-stomachs and the
true stomach. The rumen has a capacity of 4-5 gallons
in the adult goat. The pH in the rumen is kept within
a range of 5-7 normally. The rumen serves as fermentation
vat, organ of maceration and grinding, bacterial digestor,
and organ of absorption. The bacterial populations
may count 10 billion cells per gram. Ciliated protozoa
usually are also present in the rumen, besides a number
of flagellated microorganisms. Rumen muscular contractions
churn the boluses with the other contents of the rumen.
The rumen microbes break down plant fibers into sugars
and organic acids. Soluble compounds are absorbed
through the rumen walls which are covered with many
papillae, while the larger food particles may require
to be remasticated before they can pass out of the
rumen. This involves also the reticulum, the second
of the four stomach compartments.
11 The act of rumination is accomplished
by a combination of factors. There is an increase
in fluid pressure of the reticulorumen compartments
accomplished by extra-reticular contraction; there
is also, a contraction of the diaphragm, creating
negative pressures in the trachea; and the sudden
contraction of the diaphram tightens the esophagus
which is normally in a collapsed, flacid state. This,
results in the passage of reticulorumen material into
the esophagus. Anti-peristaltic movements of the esophagus
transfer the bolus to the mouth, where ''cud-chewing''
occurs. The bolus is rechewed at the goat's leisure,
re-insalivated and swallowed again. The feed materials,
because of smaller particle size, have a comparatively
higher specific gravity and will eventually pass through
the reticulum into the omasum. The total residence
time of feed spent in the rumen is usually about 8
hours.
12 The predominant end products of
fermentation in the rumen are the volatile fatty acids
(VFA), which consists of 60-70acetic acid, 15-200001:0000ropionic
acid, 10-15butyric acid and smaller amounts of longer
chain VFA's. Some alcohol is also formed in small
amounts. These fermentation products from the rumen
are absorbed through the wall and pass into the bloodstream.
About 900f the energy required by the ruminant is
provided by these short chain fatty acids.
13 Rumen fermentation results in
large quantities of gas, principally methane and carbon
dioxide. Amount and rate of gas evolution is directly
related to type of feed. For this reason, ruminants
are prominent belchers, and if the belching process
is inhibited, e.g. by foaming, the animal will bloat.
The mechanics of eructation (belching) are complex,
and are associated with secondary rumen and specific
reticulum contractions. Rumen gas is forced out by
contraction of the dorsal blind sac of the rumen.
It is allowed to enter the esophagus by the relaxation
of its lower sphincter diaphragm. The esophagus fills
with gas, the lower sphincter closes, and the upper
pharyngo-esophageal sphincter relaxes. The gas is
released into the nasopharynx, while the nasopharyngeal
sphincter is closed. The glottis remains open while
the lips are closed, so that much of the gas enters
into the lungs and respiratory system.
14 There is a free flow of material
between the rumen and reticulum and vice versa. The
reticulum is the second and smallest of the four compartments
of the goat compound stomach, making up only about
50f total volume. It is lined with a mucous membrane
that is folded into hexagon like patterns (honeycomb).
The reticulum receives the heavier substances from
the rumen and serves as a muscular transit reservoir.
Contraction of the reticulum aids in the rumination
process, but is not vital. Rumination will occur even
if the reticulum is removed.
15 Feed passes then into the 3rd
compartment, the omasum, which consists of many folds
(maniply) and comprises about 70f the total volume
of the compound stomach of goats. Here, excess moisture
is removed from the ingested feed, and finer grinding
occurs here also, even after rumen-recticular fermentation.
The omasum is not involved in the rumination process.
16 Feed then passes into the abomasum,
or true stomach. Glandular secretions of gastric enzymes
and hydrochloric acid are produced, and its functions
are similar to the stomach of monagastric species.
It makes up about 80f the total volume of the compound
stomach and is essential to life. The abomasum consists
of 3 parts, the fundic, body and pyloric regions.
The pyloric region joins the small intestine, separated
by the pyloric sphincter muscle. Glands of the fundic
area, near the omasoabomasal orifice, secrete a mucus
which protects the walls of the abomasum from protein-degrading
enzymes and hydrochloric acid, secreted by the fundic
and pyloric glands. The fundic glands are the most
important for digestion. A specialized secretion from
the fundus combines with vitamin B12 to stimulate
the production of red blood cells.
17 Gastric juice, consists of water,
inorganic salts, organic substances and hydrochloric
acid, and between four to six liters per 24 hrs are
secreted in goats. The enzymes rennin, pepsin and
gastric lipase are part of the organic constituents
of gastric juice. Rennin is the enzyme for milk coagulation,
acting on milk casein by producing a gel-like mass,
in preparation for the protein digestion by pepsin.
18 Pepsin, from its precursor pepsinogen,
converts proteins into peptides, intermediary products
of digestion. Further breakdown of proteins does not
occur in the abomasum. Gastric lipase degrades fats
eventually to fatty acids and glycerol, operating
in conjunction with bile (an emulsifying agent) after
the chyme (the liquid feed) leaves the stomach and
reaches the small intestine. Feed chyme is moved through
the compound stomach and abomasum in peristaltic waves
of contraction. Some movement is also attributed to
reticular actions that lift the body of the abomasum
by virtue of its thin muscular attachment. The acidity
of the abomasal contents usually remains close to
pH of 3.
19 Small Intestine Feed passes from
the abomasum into the duodenum, the first segment
of the small intestine, in a series of spurts of 30-40
ml each, as the pyloric sphincter opens. Such spurts
may last for 10-15 minutes, after which little feed
passes. Some feed may be returned to the abomasum
from the duodenum by antiperistaltic contractions;
in goats, about 40 of the chyme reaching the duodenum,
may be returned to the abomasum for a short time.
The rate and volume of discharge from the abomasum
is dependent upon the amount of feed that is present
in the duodenum.
20 While digestion in the abomasum
occurs in an acid environment, duodenal digestion
depends on an alkaline medium. By the time the feed
chyme has reached the lower end of the duodenum, the
pH must have risen from 3 to about 8 normally. Pancreas,
liver and glands of the duodenum contribute secretions
that make the medium alkaline.
21 The pancreas is a diffuse dual-purpose
gland located along the duodenum and secretes enzymes
for starch, fat and protein digestion via the pancreatic
juice into the intestinal tract, when stimulated by
the hormones pancreozymin and secretin which are produced
in the small intestine. The pancreas also produces
the hormones insulin and glucagon.
22 Another glandular secretion, the
bile, is produced by the liver and stored in its gall
bladder (horses and rats have no gall bladder, but
goats do). Cholecystokinin, a hormone that comes from
the small intestine, controls release of bile from
the gall bladder. The purpose of bile is to emulsify
fats in the chyme, thus enabling lipase enzymes to
work effectively. Bile also aids in alkalinization
of the chyme in the duodenal tract.
23 There are four general types of
glands located in the small intestine. Brunner's glands
are situated in the submucosa of the duodenum, producing
an amylase enzyme which is needed for the reduction
of sugars. The crypts of Lieberkuhn are deep folds
in the intestinal membranes, and although they are
not true glands, they contain enzyme producing cells.
Peyer's patches are congregations of lymphoid tissue
in the walls of the small intestine. They control
populations of local bacteria, function in antibody
production and aid in fluid filtration. Specialized
secreting cells of Paneth, the crypts of Lieberkuhn,
and goblet cells of the intestinal tract secrete an
intestinal mucus.
24 Movements of the intestines churn
the contents, and mix in the digestive secretions.
Intestinal motility also brings the chyme into contact
with intestinal membranes for enhanced absorption
of nutrients. Peristalic contractions propel the chyme
through the intestine, and assist the flow of blood
and lymph in the walls of the intestine.
25 There are three main aspects of
intestinal movement. One is of a propulsive nature,
including peristaltic and antiperistaltic motions.
Pendular movements and rhythmic actions are related
to intestinal absorption. Third, there are control
movements which initiate or stop peristalsis by the
creation of waves.
26 After the duodenum, feed chyme
passes through the section of the small intestine,
called ileum and jejunum to the large intestine.
27 Large Intestine The large intestine
mainly functions to remove water from the intestinal
contents, and to concentrate the feed residues for
excretion. In order to maximize water removal efficiency,
the large intestine causes delay in the rate of passage
of intestinal contents. While feed may transverse
through the small intestine in about 3 hours, it takes
approximately 18 hours to move through the large intestines.
This is especially impressive considering that the
large intestine of goats is only about 61/2 feet long,
while the small intestine is almost 36 feet long.
The extent of water resorption from the large intestines
in goats is relatively high, as evidenced by their
dry faces, and is an important water conservation
mechanism in goats.
28 Material passes from the cecum,
a blind pouch located at the juncture of the small
and large intestine, to the first part of the large
intestine by peristaltic movements, which undergo
periods of high activity and almost inactivity. During
high activity, pressure waves of 6-36 seconds can
occur, with a movement occurring about once each minute.
The extent of digestion and absorption of nutrients
in the large intestine has not been studied closely.
29 The last foot of the large intestine,
the rectum, is a modified storage organ for the fecal
material. Upon accumulation of sufficient amounts,
nervous stimulation triggers defecation. The anus
is located at the termination of the digestive tract.
It is made up of two sphincter and a retractor muscle,
all of which are normally closed, except during defecation.
30 Studies of goats have indicated
that it takes about 11-15 hours for feed material
to pass through the digestive system. Maximum excretion
is achieved in about 30 hours but does not reach completion
until 6-7 days later. In goats kids, the rate of passage
increases to a constant level at and after weaning.
31 The compound stomach accounts
for 670f the total digestive capacity of the goat,
while the small intestine holds 21 The cecum, of little
significance in ruminants, has a capacity of only
20f the total. The large intestine and rectum account
for the final 10
32 The ruminant digestive system
of the goat, by virtue of its large rumen ''holding
tank,'' is working non-stop throughout the adult life
of the animal. Other animals have digestive organs
and secretions that alternate between periods of stress
and inactivity, while the goat must continually manufacture
digestive juices and enzymes day and night, 24 hours
a day. A breakdown in this complex process for even
a brief period can result in acute and potentially
deadly situations for the ruminant.
DIGESTION
COLLECTION;GOAT HANDBOOK
ORIGIN;United States
DATE_INCLUDED;June 1992
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