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 Deglutition

- chewing - 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: –

Regurgitatio

n - 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