Chapter 19: Digestion
• Chapter overview:
– Chapter 19 presents the physiology of digestion
and nutrition, including:
• anatomical views and digestive parts identification
• the process of digestion and absorption
• factors affecting digestibility of feeds
Digestion and Absorption:
• The process of digestion includes:
– The prehension of food or feed
– The mechanical chewing and grinding
– Mixing with digestive acids and enzymes to
chemically break down the foodstuffs
• The process of absorption includes:
– Transport of the digested foods across the
intestinal mucosa to the blood or lymph system
The General Mechanical Process:
• Mastication - chewing
• Deglutition - swallowing
• Regurgitation - movement of digesta in
reverse order from the stomach to mouth
• Defecation - voiding the fecal, or waste,
materials from the body
Animal Diet Types:
• Species may be divided into dietary
preference groups:
– Herbivores consume plant materials, examples
are horses and cattle
– Carnivores consume flesh of other animals,
examples are dogs and cats
– Omnivores consume both plants and flesh,
examples include the primates
Differing Digestive Tracts:
• Farm animal species have a variety of
digestive systems
– Ruminants have four distinct stomach areas
• examples include bovine, ovine and caprine
– Nonruminants (also termed monogastrics)
display considerable variety
• hogs, dogs, and cats have a single, simple stomach
• poultry have a two part stomach
• horses have a large, functional cecum
Nonruminant Digestive System:
• Mouth - prehension and chewing of food;
some carbohydrate enzyme activity
• Esophagus - route of food from mouth to
stomach
• Stomach - addition of hydrochloric acid
and protein digesting enzymes, mixing and
holding
Nonruminant Digestive System:
• Small intestine - primary site of digestion
and absorption
• Large intestine - major site of water
absorption and preparation of digesta for
excretion
Nonruminant Variations:
• Poultry
– Mouth: no teeth for chewing
– Esophagus: a “crop” is contained within the
esophagus for food holding and moistening
– Stomach: divided into proventriculus
(glandular area) and ventriculus (crushing
area)
– Large intestine: short and exiting into the
cloaca; two large ceca with limited function
Nonruminant Variations:
• Horse
– Large intestine differences:
• cecum is very large (may contain 50% of digesta)
• cecum provides some nutrients to the horse via
microbial fermentation
Ruminant Digestive System:
• Many parts of the tract are similar to
nonruminant monogastric description
• Significant differences include:
– Mouth contains no upper incisors
– Stomach is divided into four major parts
•
•
•
•
rumen
reticulum
omasum
abomasum
Ruminant Digestive System:
• Rumen
– Largest digestive area of the compound
stomach
– Majority of fermentation of feedstuffs occurs
here
– Majority of absorption of byproducts of
fermentation - volatile fatty acids or “VFA’s”
Ruminant Digestive System:
• Reticulum
– Receives feed from esophagus
– Initiates mixing, regurgitation, and eructation
– Environment for fermentation of feedstuffs
Ruminant Digestive System:
• Omasum
– Third area of the stomach receives the digesta
outflow of the rumen/reticulum
– Some water absorption and further subdivision
of feed particles may occur
Ruminant Digestive System:
• Abomasum
– The fourth and final stomach compartment, but
very similar in function to the pig stomach
– Acid and enzyme stomach
– Final holding and mixing area before the small
intestine
Ruminant Digestive System:
• Selected terms:
– Regurgitation - controlled reverse movement
of coarse feedstuffs from reticulum/rumen via
esophagus to mouth for rechewing
– Eructation - expulsion of accumulated
fermentation gases from rumen via esophagus
– Rumination - refers to the processing of
feedstuffs in the reticulum/rumen, to include
fermentation, regurgitation, eructation
Chemistry of Digestion:
• Digestion involves enzymes and acids
produced by the host animal or microbes
working in symbiosis with the host
– Enzymes break specific chemical bonds in
feeds
– Domestic animals produce enzymes to digest
nonfibrous carbohydrates, fats, and proteins
– Only microbes have enzyme systems to digest
fibrous carbohydrates, such as cellulose
Digestion of Carbohydrates:
• General carbohydrate digesting enzyme
(amylase) in the mouth begins digestion;
little amylase is found in horses and none in
ruminants
• Carbohydrate digesting enzymes (amylase)
from the pancreas, and intestinal mucosa
(sucrase, maltase, lactase) complete
carbohydrate digestion
Digestion of Proteins:
• Protein digesting enzyme (pepsin) and
hydrochloric acid in the stomach begin
significant digestion
• Protein digesting enzymes (e.g. trypsin)
from the pancreas and intestinal mucosa
complete digestion in the small intestine
• Young nursing animals – rennin coagulates
milk allowing more complete digestion
Digestion of Fat:
• Fat digesting enzyme (lipase) in the
stomach begins digestion
• Fat digesting enzymes from the pancreas
(lipase) and intestinal mucosa complete
digestion in the small intestine
• To assist in fat digestion, bile from the liver
emulsifies fat into smaller droplets in the
small intestine
Factors Affecting Digestibility:
• Rate of passage - in general, increased rate of
passage of digesta through the tract reduces
digestibility, factors increasing rate of passage
include:
– Increased level of feeding/intake (ruminants)
– Finer processing (such as grinding) of feed
Note: grinding grain usually increases digestibility
but grinding hay decreases digestibility
Factors Affecting “Conversion”:
• Feed “conversion” refers to the amount of
productivity per unit of feed consumed
• Factors impacting feed conversion include:
– Age/weight - younger animals are more efficient
– Level of feeding - limiting feed generally increases
efficiency
– Inheritance - feed conversion is moderately heritable;
correlation between ADG and efficiency is high