Agriculture Reference
In-Depth Information
Starch is hydrolyzed by pancreatic amylase to maltose
while maltase and isomaltase are hydrolyzed to glucose
that is actively absorbed. This process is facilitated by the
intestinal enzyme maltase present at the brush border of
the epithelial cell.
Digestion of lipids (rumen protected) begins by emulsi-
fi cation of fats as they combine with bile acids/salts and
form micelles (hydrophilic outer side). Pancreatic lipase
then hydrolyzes emulsifi ed triglycerides to form
kids. Exposure to the dam, to fecal matter, and to the
environment readily exposes newborn kids to aerobic and
anaerobic microbes, the latter eventually colonizing the
rumen.
Preruminant Phase
During this period (up to 3 weeks of age), the newborn is
nursing. Most milk bypasses the rumen and fl ows directly
through the reticular groove of the reticulo-rumen to the
omasum and abomasum. Sucking stimulates salivary and
abomasal secretion more than drinking from a bucket.
Saliva of the newborn contains esterase, and the aboma-
sum secretes rennin (chymosin) and hydrochloric acid at
this stage of life. Exposed to rennin, milk clots and sepa-
rates into a hard curd containing butterfat and protein curd
(casinogen) that remains in the abomasum for digestion.
The remaining whey fraction (albumins, globulins, lactose)
leaves the abomasum in bursts. Butterfat is hydrolyzed to
glycerol and fatty acids by lipase originating either from
milk (mammary glands) or from estrase in saliva. Curd
proteins are exposed to further hydrolysis by rennin and
acid. In the intestine, the curd, whey proteins, and lactose
are completely digested. The intestine of newborns has low
maltase activity and therefore cannot fully utilize starch.
Intermediary metabolism is driven by glucose, and blood
glucose concentrations are sensitive to insulin.
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monoglycerides and free fatty acids that can diffuse into
epithelial cells. Ruminants may absorb triglycerides
without further digestion (Wrenn et al., 1978). In the intes-
tinal epithelial cells, fatty acids and monoglycerides are
re - esterifi ed to form triglycerides. Combined with choles-
terol, cholesterol esters, phospholipids, and a small amount
of proteins (lipoproteins), they form chylomicrons for
uptake into portal blood and transport. Under most condi-
tions supply of triglycerides is limited because most lipids
have been hydrolyzed to glycerol and fatty acids in the
rumen, and unsaturated fatty acids also have been saturated
therein.
Soluble vitamins and minerals are primarily absorbed
from the SI though some are absorbed from the large
intestine. Within the colon of ruminants, fermentation may
begin again (postgastric fermentation), but the extent of
fermentation is limited because chyme already has been
fermented in the rumen and enzymatically digested in the
SI. However, water and some electrolytes are absorbed in
the small and particularly in the large intestine of animals
selected under desert conditions with limited access to
water.
β
Transitional Phase
In this phase (3-8 weeks of age), young ruminants will
ingest progressively larger amounts of roughages and dry
food. These stimulate development of salivary glands and
the reticulo-rumen. A population of ruminal microbes
becomes established due to ingestion of food, water, cud,
fecal matter, and other environmental contaminants.
Fermentation of feeds produces VFA that stimulate the
growth of reticulo-rumen papillae and omasal leaves. To
handle the gases produced and larger particles, the muscle
wall has the rumen development and rumen motility initi-
ating the processes of eructation and rumination. As this
development progresses, intermediary metabolism shifts
from a glucose driven- to a VFA-driven system that is less
insulin - sensitive.
DIGESTION IN YOUNG RUMINANTS
Newborn Phase
When born, ruminants have a small, nonfunctional fore-
stomach with no microorganisms and no acid or pepsino-
gen secretion. Colostrum directly enters the abomasum
and fl ows to the duodenum where immunoglobulins (IgM
antibodies,
-globulin) are absorbed intact through phago-
cytosis by the intestinal mucosa. Colostrum contains an
antitrypsin factor that prevents digestion of antibodies
within the duodenum. Through this process, the newborn
kid develops immunity to most diseases to which the adult
doe had been exposed. This absorption proceeds for only
some 24-48 hours after birth. Colostrum also provides
nutrients including lactose and some microbes (lactoba-
cilli). Limited energy reserves due to limited glycogen
storage by the liver and ineffi cient use of lactose as a
source of energy are the main causes of death in newborn
γ
Pre - weaning and Post - weaning Phase
During this period (8 weeks to adulthood), reliance on milk
reduces because milk production by the doe decreases.
Reticular groove closure becomes erratic and usually is
absent in adults. The forestomach proportions and motility
change to attain those of an adult, salivary esterase dimin-
ishes and salivary urea is present, and abomasal rennin
is replaced by pepsinogen, all of which refl ect inborn
