Animal Nutrition Digestion

What is the goal of digestion?

Hydrolyze (add water) to food compounds into smaller units that can be absorbed

Hydrolysis is catalyzed by enzymes

Where does most digestion and absorption happen?

The small intestine because that is where most enzymes are working

Microbial fermentation

Fiber cannot be hydrolyzed by enzymes so the only way the animal can get nutrients from fiber is by this

They need a compartment where food can sit around long enough to get fermented

How are microbial wastes useful from animals?

If an animal ferments food in the foregut then the microbes serve as a protein supplement if they pass down into the small intestine

How can large herbivores thrive on a diet of poor fiber?

They have a larger fermentation compartment to retain food longer so microbes have time to ferment the food and they can extract more nutrition from slowly digesting the fiber

How are ruminants masters of efficiency?

They focus on getting everything out of the food they eat because particles cannot leave the rumen until they are small and dense

They can use the protein produced by fermenting microbes as a source of amino acids

Headgut

This part of the digestive tract includes the oral cavity and lips/bill/teeth

Prehension

Grasping or taking food

Prehensile lips= like a rhinos flexible lip

Mastication

Chewing

Glutition

Swallowing

Saliva main purpose

Main function is to lubricate food so food can slide down esophagus

In ruminants= phosphate and bicarbonate buffers to keep rumen pH at 6/7

Foregut

The digestive tract that includes the esophagus and the stomach

Also includes crop, proventriculus, and gizzard

Peristalsis

Wavelike contractions of gut wall that propels food through tract

Used by the esophagus

Crop

Expansion of the esophagus in birds

It’s main function is storage and it can regurgitate food for young and it enables fermentation

Proventriculus

Teh glandular stomach in birds (pH is 4) and secretes hydrochloride acid and pepsin (enzyme that digests protein)

Gizzard

Muscular stomach in birds that finely grinds ingests to be digested

Proteolytic enzymes

Specialized proteins that break other proteins down

Ex: trypsin

Stomach (hind gut fermenters and non fermenters)

It is glandular with three regions

• cardia (secretes mucus)

• Fundus (secretes acid, enzymes, musics)

• Pylorus (secretes mucus)

Acidic, pH is 1-3

Functions of hind gut fermenters and non fermenter stomach

Food is liquified for delivery as chyme to the small intestine

Acid can help kill bacteria

Proteolytic enzyme action is initiated

Food can be consumed quickly and then slowly released to promote more complete digestion

Layers in the foregut of ruminants

1. Gas layer- fermentation gasses (CO2, methane) which has to be removed by eructation

2. Fiber met- composed layer of the more recently ingested forage and fiber floating on the liquid layer

3. Liquid layer- water along with small dense particles

Eructation

Oral ejection of gas from the digestive tract

Foregut in ruminants

Houses large population of anaerobic microbes for fermentation and can convert carbohydrates (including fiber) and protein of plants to VFA and microbial protein

Contractions alter the size of the sacs and move food particles around for bacterial colonization, rumination, eructation, and passage to omasum

Rumen epithelium

This has papillae in the foregut in ruminant to increase surface area for absorption of volatile fatty acids

Rumen epithelium

This has a honeycomb shape in the foregut in ruminants for filtering and retaining large particles

Fermentation in the rumen

While particles are actively fermenting, gas is produced and makes the particles buoyant, digestion and rumination decrease the particle size

Rumination in the rumen

Fiber food is regurgitated and chewed for a minute, then reswallowed several times

Purpose: breaks down course fiber and then stimulates saliva to buffer rumen pH and allows microbes to efficiently ferment food

Passage in foregut of ruminants

During contractions, small dense particles move forward and pass to the omasum

Some particles can be digested further in the small or large intestine

Most particles pass before they are completely digested

Omasum

Only in the foregut of ruminants

Epithelium in leaf like projections for water absorption and filtering coarse particles

Abomasum

This is the true stomach in the foregut of a rumen

Acidic with thick mucus to protect lining from the acid And they have a continuous digesta flow

Reticular groove

Only in the foregut of ruminants

This creates a tube to connect the esophageal opening directly to the omasum so milk for calves can skip the rumen so the milk will not be fermented and lose nutritional value

Midgut

Mainly the small intestine where most digestion occurs

Stomach acid is neutralized to bring pH above 6 and bile salts from gall bladder emulsify lipids

The enzymes secreted from pancreas and intestinal walls digest proteins, starch, disaccharides, and lipids

Villi (small intestine)

These are microscopic projections that increase the surface area for absorption

Hindgut

This part of the digestive tract includes the cecum, large intestine, and rectum

This is the final storage of digesta

Absorption of inorganic ions and water

The protein synthesized as microbes grow and divide passes on to the feces instead of teh small intestine= animal can only use microbial protein if it eats the poop (coprophagy)

Where in the hindgut does fermentation happen?

The large intestine and cecum are partially compartmentalized or sacculated by folds in the wall so it can slow down passage rate for more fermentation; most of the sugar, fat, and protein have been digested and absorbed so mostly fiber

BUT it does not have a mechanism to select large particles so passage rate will be faster through the hind gut than the rumen

Selectors

These animals choose best (most digestible) parts of plants to eat (sheep)

Non Selectors

These animals eat the less digestible parts of the plants like stems (cattle)

Browsers

These animals prefer material above shoulder height (goat and giraffes)

Grazers

These animals prefer her age at ground height (cows/buffalos)

Rate of passage

This is indirect with rate of digestion; a slower rate of passage gives more time for fermentation

Factors that felt is anatomy of the tract like size, the amount of food consumer per day (more food means faster passage rate), intake of proteins and lipids (slow passage rate), and temperature outside (cold weather increases passage rate)

Three chemical digestive processes

1, Acid secretion- hydrochloric acid and lower pH of stomach, also hydrolyze food compounds, softens things up for smoother digestion

Digestive enzymes- pancreases adn gut

Bile salts- detergents for fats

Three mechanisms for absorption

1. Simple diffusion

2. Facilitated diffusion

3. Active transport

Disaccharide digestion

These are hydrolyzed to monosaccharides in the border of the small intestines

Maltese, sucrase, and lactase are produced in teh cells lining the small intestine

How is starch digested

Amylose and amylopectin are hydrolyzed into disaccharides (maltose);

Amylase secreted by pancreas

How is fiber digested?

Cellulose and hemicellulose are broken down by chitinase, but not a lot of animas have this

Carbohydrate digestion- is it fast or slow?

Sugars= digested very quickly when sucrase is present

Starch= less processed, the more slowly digested

What type of absorption method do carbohydrates use?

Glucose= actively transported against the gradient

Fructose= facilitated diffusion with help from protein

Protein digestion steps

1. We need to break down the protein chain into smaller chains

2. Amino acids can be cleaved off on at a time from the ends

Digested in stomach and small intestines and then absorbed in small intestines

Endopeptidases

Hydrolyze peptide bonds at a specific amino acid in the chain

Exopeptidases

These clip off amino acids at the ends of the peptide chains

Pancreatitis

Proteolytic enzymes are released as inactive enzymes because if they are activated where they are made, they will break down the organs where they came from, and this will happen in dogs

Is protein digestion fast or slow?

They are digested slower, you will feel more full after eating this

Protein absorption

They are absorbed as amino acids by active transport mechanisms

Microbial fermentation of carbohydrates

This occurs in the stomach compartments and large intestines and allow animals to digest fiber

Microbes produce fermentation acids (acetic, Propionic, butyric) or volatile fatty acids

Microbial digestion of proteins

Gut microbes break down proteins into peptides, amino acids, and ammonia (especially important in ruminants)

Microbes use those as nitrogen sources for the synthesis of new microbial proteins

Lipid digestion

1. Bile salts are secreted by the liver

2. Emulsify the triglycerides so they come into contact with lipase to break lipids down in small intestines

3. Products are absorbed from Michelle’s near the intestinal walls and released

Symbiotic relationship between animals and microbes

Both just want to maintain itself, grow, and reproduce but they help each other in different ways

How do animals care for microbes?

1. Provide fermentation vat where it is a warm, wet anaerobic environment that is mixed

2. Adds more substrate periodically

3. Adds buffers to keep pH neutral in ruminants

4. Removes microbial waste products like VFAs and hard to digest materials

How do microbes care for animals?

They provide animals with nutrients they cannot get from their diet like fermentation acids (ruminants, the VFAs stimulate growth of rumen papillae)

Microbial protein can be synthesized from poor quality proteins (rumens only)

Microbes detoxify some plant compounds that are toxic (rumens only)

Fermentation

Bacteria breaks down complex carbs to simple sugars and then ferment sugars to acids and gasses without oxygen

Why is the rumen anaerobic?

There is no light, so no photosynthesis so no oxygen

Also an oxygen free environment allows specialized microbes to ferment and break down tough plant fiber that the animal’s body cannot digest

Cellulolytic Bacteria

Fiber fermenters

Energy sources: cellulose and hemicellulose

N source: ammonia

Products: acetate, H2, CO2

Amylolytic bacteria

Starch fermenters

Energy sources: starch and simple sugars

N source: ammonia, peptides, amino acids

Products: propionate, butyrate, acetate

Proteolytic bacteria

Protein fermenters

Energy and N source: amino acids

Products: ammonia and isoacids

Carbohydrate fermentation in ruminants

Rapidly= VFAs production and drop in rumen pH

Slowly= longer retention in rumen and old feed is taking more space and animal cannot eat as much

Sugars ferment faster than starches which is faster than fiber

Amylopectin is faster than amylose

Wet feeds ferment more quickly

Protein fermentation in ruminants

Rapidly= ammonia production

Plant protein degrades faster than animal proteins

Heated proteins degrade slower