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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 feedstuffs
The process of absorption includes:– Transport of the digested foods across the
intestinal mucosa to the blood or lymph system
General Terms
Prehension: to take into the mouth Mastication: chewing Deglutition: swallowing Regurgitation: the backward flow of food
through the esophagus
Three Major Categories
Carnivore: consume flesh of other animals, examples are dogs and cats
Omnivore: consume both plants and flesh, examples are primates
Herbivore: consume plant material, examples are horses and cattle
Omnivore
Combination of carnivores/herbivores More complicated GIT than carnivores Colonic digesters
– Pigs, humans
Cecal digesters– rat
Herbivore
Cow, horse, rabbit– Each has a different type of GIT
Cow- ruminant Horse- simple stomach, large cecum, large
sacculated LI Rabbit- larger stomach, very large
sacculated cecum, unsacculated LI
Salivary Glands
3 pair– Parotid– Mandibular– Sublingual
Water - moistens food aids in swallowing Mucin - lubrication for swallowing Bicarbonate salts Enzymes
Different Digestive Tracts
Farm animals have a variety of digestive systems– Ruminants: have 4 different compartments to
the stomach• Examples include cattle, sheep, goats
– Nonruminants (also known as monogastrics)• Hogs, dogs, and cats have a single, simple stomach• Poultry have a two part stomach• Horses have a large, functional cecum
Pregastric vs Postgastric
Pregastric: Fermentation that occurs in the rumen of ruminant animals. It occurs before food passes into the portion of the digestive tract in which digestion actually occurs.
Postgastric: The fermentation of feed occurs in the cecum, behind the area where digestion has occurred.
Pregastric vs Postgastric
Ruminants– More efficient – Less intake
Non- ruminant herbivores– Only postgastric– Less efficient– Greater intake
Monogastric Systems
Mouth: prehension and chewing of food; some carbohydrate enzyme activity
Esophagus Stomach
– Storage– Muscular movements (break down food)– Secretes Digestive Juices (hydrochloric acid)
• pH about 2
Small Intestine
Duodenum– Active Digestion Site– Produce enzymes
• Pancreas– Helps to neutralize ingesta entering the SI
• Liver– Produces bile; breaks down fats
• Intestinal Walls
Small Intestine
Jejunum– Active in nutrient absorption
Ileum– Active in nutrient absorption
Villi pH 6 to 7
Large Intestine
3 Sections– cecum– colon– rectum
Active in water resorption Secretion of some minerals *Bacterial Fermentation*
Horses are Different
Saliva– contains no enzymes– may secrete up to 10 gallons/day– stimulated by scratching
Esophagus– only one way peristaltic movement
• Impossible for regurgitation
Horses are Different
Stomach– much smaller in comparison to other species– not very extensive muscular contraction– So how should we feed differently?
Small Intestine– same as pig but no gall bladder
• Can’t handle a high fat diet
Horses are Different
Large Intestine– over 60% of GIT– 4 parts
• cecum
• large colon
• small colon
• rectum
Cecum and Large Colon
Similar to Rumen– bacterial cellulose breakdown– bacterial protein breakdown– VFA production– Water Soluble Vitamin production
Avian Species
Beak– no teeth– can be used to reduce particle size
Esophagus– ingesta holding and moistening– Salivary Amylase– Fermentation in some species
Avian Species
Gizzard (ventriculus)– thick muscular wall– particle size reduction (similar to mastication)– nonglandular– normally contains grit– no enzymatic secretion
Avian Small Intestine
Functions in digestion and absorption of feed and nutrients just as in other monogastrics
pH is slightly acidic Most enzymes found in mammals except?
Avian Large Intestine
Contains 2 blind pouches instead of ? Mostly water absorption Some bacterial activity but less than in most
mammals very short in comparison
Ruminant Digestive System
Mouth– what is unique about the teeth?– Can only chew on one side of mouth at a time
Saliva production is continuous Production about 12 gallons/d
Reticulum
Honeycomb most cranial not truly separated from rumen no enzymatic secretion walls are tough, tend to catch heavy objects
Rumen
Large compartment extends from diaphragm to pelvis
papillae Fermentation chamber Majority of absorption of byproducts and
conversion to volatile fatty acids
Rumen Digestion
Reticulorumen provides a favorable environment for bacterial fermentation
Continuous turnover of digesta and removal of fermented digesta
Anaerobic fermentation
Omasum
Manyplies (Stockmans Bible) short blunt papillae very muscular no enzymatic secretion reduction of particle size water resorption
Abomasum
True Stomach First Glandular portion of the tract Very similar in structure and function to
nonruminant stomach
Other Unique Points
Esophageal Groove– cardia to omasum
– milk bypass
Rumination: The process where rumen contents are regurgitated, remasticated, and reswallowed for further digestion
Eructation: expulsion of accumulated fermentation gases from rumen via esophagus
Rumination
Regurgitation of ingesta with mastication– reticular contraction that concentrates ingesta at
the cardia– increased inhalation of air at same time– contraction of diaphragm– ballooning of esophageal walls– ingesta sucked into esophagus – returned to mouth by reverse peristalsis
Rumination
Excess liquids are swallowed Mastication commences More time spent masticating here than
initial intake Amount of time ruminating is a function of
diet composition
Rumen Contents and Motility
Rumen contents are not uniform Occur in stratified layers Change from ventral to dorsal Rumen mat
– high concentrate diets eliminate mat– more viscous fluid in high grain diets– lowest dry matter in high forage diets
Rumen Contents and Motility
Motility of rumen mixes contents Divided into
– primary contractions– secondary contractions
Contractions require up to 50 seconds to complete
Rumen Contents and Motility
Contractions begin with reticulum Progress dorsally Finish with ventral blind sac and ventral
pillars
Rumen and Acute Acidosis
Optimal rumen pH is 6.7 Variation in pH is normally +/- 0.5 Introduction of high grains result in
– breakdown of rumen mat– proliferation of facultative anaerobes– these produce high levels of lactic acid
Rumen and Acute Acidosis
Lactate is a much stronger acid that other VFAs
In severe cases lactate can make-up well over 50% of total rumen acids
Succinate and Formate can also appear in high quantities
Rumen and Acute Acidosis
Increased acids can reduce pH as far as 4.0 Severe rumenitis occurs at these pH levels Absorption of lactic acid results in systemic
acidosis All chronic acidosis results in rumen
parakeratosis
Factors Affecting Digestibility
Rate of passage: 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