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