Chapter 19: Digestion

1
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

2
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

3
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

4
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

5
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

6
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

7
Nonruminant Digestive System

• Small intestine - primary site of digestion and
  absorption
• Large intestine - major site of water absorption
  and preparation of digesta for excretion

8
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

9
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

10
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

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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 VFAs

12
Ruminant Digestive System

• Reticulum
• Receives feed from esophagus
• Initiates mixing, regurgitation, and eructation
• Environment for fermentation of feedstuffs

13
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

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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

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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

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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

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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

18
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

19
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

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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

21
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