ruminant digestion notes for adn

8/7/2019 Ruminant Digestion Notes for ADN

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Ruminant Digestion Notes for ADN

Key Reference Dukes Physiology of Domestic Animals ed Swenson and Reece,

11th edn 1993

Or Textbook of Veterinary Physiology Cunningham &Everell 1997

Costs and benefits of ruminant digestion (Dukes, p 390)

a. Benefits

Ruminants are ecologically successful due to pregastric fermentation

1. Allows utilization of fibrous diets not suitable for nonruminants

2. Permits break down of cellulose, releasing cell contents and making CHO in

cellulose available for digestion

3. Allows higher biological value microbial protein to be made from low value plant

protein, non protein nitrogen and recycled nitrogen products e.g. urea

4. Provides all Vitamin B complex (if sufficient Co for Vit B12)

5. Utilize feeds containing toxic compounds

b. Costs

1. Large amount of time spent chewing food (4-7 hours) and cud (8 hours)/day

2. Need adequate, almost continuous food supply

3. Complicated mechanisms needed to keep rumen fermenting efficiently

- saliva addition, mixing movements, eructation of gas, absorbtion of end products,

regurgitation and rumination,

4. Intermediary metabolism must use end products- VFAs,


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1. Development of the young ruminant

From birth to maturity substantial changes occur

Adaptation from milk to fibrous food diet

1. Increase in forestomach size

From 40% total stomach volume at birth to 90% as adult

Within 8 weeks stomach reaches adult proportion

Development is dependent on exposure to solid food

2. Epithelial development

Occurs as rumen develops

Exposure to VFAs stimulates papillary development

Accelerated by highly digestible foods, e.g. grain

3. Microbial colonization

Stomach is sterile at birth

Bacteria quickly colonize- gradually develops into correct environment for strict

anaerobes

Protozoa flora may not occur until later

4. Commence fermentation

5. Acquire patterns of motility

Eructation and regurgitation can occur at 3 weeks

6. Decrease in reticular groove reflex

Reticular groove diverts milk from rumen to abomasum

7. Commence eating fibrous food- can start eating solid food at 2 weeks

8. Switch from glucose-dependence to utilize volatile fatty acids


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Four main phases:

a. Newborn (0-24 hours)

Small, nonfunctional forestomach

Abomasum does not secrete pepsinogen or acid

Antitrypsin factor in colostrum prevents intestinal digestion

Colostrum- rich in IgM etc, also Vit A, D, E, also Ca++, Mg++

Absorbed intact through intestinal mucosa by phagocytosis

Lactose is digested- necessary for BAT metabolism in hypothermia

b. Preruminant (1-21 days)

Milk is main food, but some solid food intake begins

Sucking- saliva is producted, contains esterase- begins milk lipid hydrolysis

Reticular groove functions to divert milk from forestomach

The oesophageal (reticular) groove reflex

Function: To bypass the rumenoreticulum and deliver milk directly to the

abomasum in the developing ruminant

Action: The lips of the vertical spiral groove roll inward, closing off the rumen

and reticulum and creating a tube which directs material from the oesophagus,

quickly through the omasum towards the abomasum

This is accompanied by inhibition of rumenoreticular and omasal contractions

Afferent limb: Chemoeceptors in the posterior oral cavity and pharynx are

stimulated by sucking or drinking. Imput via glossopharyngeal CN IX

(also stimulated by solutions with Na + and Cu ++ ions)

Control centre: Medulla oblongata

Effectors: Vagus nerves to supply reticular wall myenteric plexus (?)

Activation of reflex: copper salts in sheep, sodium salts in calves


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Teeth surfaces wear irregularly- rough surface, and grow continuously

During eating- fast, irregular chewing, during rumination- slower, regular

Importance: grind up stems, leaves to increase area for microbe attack

Deglutition- striated muscle oesophagus- sequential contractions coordinated by

medulla

Salivation- stimulated by mastication- buccal mechanoreceptors

Saliva- very high daily output- e.g. 10 L sheep, 100L cattle per day

Glands produce basal secretion, increased by stimuli

Parotid- produce half of saliva output

Also- palantine, buccal, pharyngeal, inferior molar glands

Very responsive to stimuli in mouth, oesophagus, forestomach

Parasymp stimulation (ACH) increases secretion

Control of secretion: salivary centres in hindbrain

Others- submaxillary, sublingual, labial- small amounts saliva

Composition of parotid saliva- isotonic, higher K+, HCO3-, HPO4

=

High pH (8.1)- important to neutralize VFAs

Phosphate- recylced to microbes

Nitrogen- 77% from urea- recycled to microbes


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2. Rumen and reticulum motility in adult ruminants

a. Primary, mixing or "A" cycle

Basal, continuous contractions

Function: to continously agitate ingesta, saliva, microbes, delay ingesta for more

thorough digestion and breakdown

1. Biphasic contraction of reticulum

Sheep- 5-7 secs long, cattle- 5-12 secs long

In cattle see a relaxation notch in contraction

First reticular phase: Mixing contraction

Second reticular phase: Evacuation contraction

This contraction raises level of fluid in reticulum so it flows into rumen

2. Monophasic contraction of dorsal ruminal sac

slower, caudally moving rumen contraction

3. Ventral ruminal sac contraction

either- uniphasic- dorsal sac only

- biphasic- dorsal and ventral sacs

Ratio of reticulum to rumen contractions: 1:1 or 1:2

Contraction frequency: every 35-45 secs during eating, 75 secs during rest

cycle lasts 20-30 seconds

Primary contractions- spread caudally and are driven by the reticulum

b. Secondary contraction or eructation or "B" cycle

Spread cranially in rumen

1. Caudoventral ruminal blind sac contracts


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and reticular groove lips, cardia and ROO- monitor tension in wall

b. Epithelial/mucosal receptors

Location: epithelial receptors in b.m. of epithelium of forestomach

esp rumen pillars- stretch receptors

Dual function- 1. mechanoreceptors-excited by moving light touch

- 2. chemoreceptors in rumen, reticulum walls

Afferents: vagus nerves

Integrated- gastric centres (paired) in medulla oblongata

Coordinates cyclical activity with regurgitation and eructation

Efferents: vagus nerves

Stimuli which affect motility see page 404 Dukes

Extrinsic activity inhibited by abomasal distension

Stimulate activity by coarse food particles and gas

e.g. coarse food- dense rumen mat- increases resistance to movement of pillars,

tension receptors are triggered- leads to increased motility

Vagotomy -1 vagus N- compensates and functions normally

-2 vagus N- abolishes extrinsic reticular contractions,

general anaesthesia also abolishes reflexes

Omaso-abomasal activity (page 405 Dukes)

The ROO is a bottleneck limiting digesta movement from ruminoreticulum

Thereby limits food intake by animal

Motility of omasum- slow, progressive contractions


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sheep- 15-30 seconds after biphasic reticular contraction

cattle- independent of reticular contraction

Motility of abomasum- Fundus- shallow contraction

Antrum- powerful contraction

Abomasum is intermittently emptied

Abomasum secretes digestive juices pH < 1.4, folds cover 85% of fundic mucosa

Distension of duodenum slows abomasal emptying

Intestinal motility

Ruminants show continuous motor activity, unlike monogastric animals

Extrinsic N (splanchnic and vagus) regulate the migrating myoelectrical activity

Rumination

Rumination- act of remasticating rumen ingesta

Rumination cycles- single and repeated 60 secs each

-continue up to 8hr/day, esp. at night, during rest

Time spent ruminating depends on food texture and food volume in rumen

Stimuli to ruminate:

a) tactile stimulation of rumen and reticulum epithelium

b) stimulate craniocaudal pillar receptors- information on volume and texture

block receptor function with alpha-adrenergic drugs e.g. Ad, Norad, xylazine

Control centre: rumination centre- probably ventral hypothalamic area

Very potent drive to ruminate, esp on coarse feed

Pseudorumination: follow cycle but do not bring up bolus

Steps: a) Regurgitation


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b) Remastication and reinsalivation

c) Redeglutition

a) Regurgitation

Activity: Fast movement of digesta from ruminoreticulum, through cardia, up

oesophagus to mouth

Commences with long lasting extra contraction of reticulum (1-4 secs)

Followed by usual biphasic contraction (overall= triphasic contraction)

Floods open cardia with material from reticulum

Then brief inspiratory effort with tongue and soft palate blocking mouth and nose-glottis closes as cud traverses pharynx- accompanied by head movement

Material refluxes into oesophagus-Rapid antiperistalsis by striated oesophageal

muscle 0.2 m/sec, 5 X faster than peristalsis

Tongue squeezes fluid out of bolus and reswallowed

b) Remastication

Slower, more regular chewing than primary mastication

40-50 secs/60 secs of rumin. cycle is remastication

Dependent on texture and quantitity of digesta

Chewing occurs on one side of mouth in each rumination

Chewing stimulates salivation and 1o and 2 o rum-retic movements

c)Reinsalivation

Serous rumination saliva= secondary saliva

Source: parotid gland on side of chewing (not mandibular gland)

parotid continously secretes saliva at basal level


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secretion increased by parasymp ACH stimulus- initiated by stimuluation of

mechanoreceptors near teeth, also in oesophagus and rum-retic

Compare to primary saliva- buccal gland- mucous saliva

sublingual, mandibular gland- seromucous

Increasing dry matter % (esp. particle size) will increase both types of saliva

Saliva supplies 70% water in rumen reticulum, plus phosphate, bicarbonate

Role of saliva:

-source of fluid- provides liquid to suspend particles, wet dry ingesta

-copious alkaline buffer for VFAs

-recycling urea as source of nonprotein nitrogen for microbe protein synthesis

-recycling phosphate for microbial nucleic acid and phospholipid synthesis

-provide an antifoaming agent for rumen

d)Redeglutition

Occurs after 20-70 chews, depending on consistency of cud

Brief pause - 5 secs then next cud is regurgitated

Eructation

High volume of gases which collect in pocket above mat in dorsal sac

Gas- 0.5- 2 L/min (cow) produced by microbes

from salivary bicarbonate and acids

Expel rumen-retic gases: 65% CO2, 25% Methane, 7% Nitrogen,


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Contractions: secondary contractions arise in ventral blind sac (see above)

Nasopharyngeal sphincter is closed

A lot of gas is inspired- recycled by lungs

Strong flavours from digestion- reach lungs, blood, milk

Impairment of eructation: frothy bloat- lack of pressure on cardia

Oesophageal obstruction with food- simple bloat


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Rumen Environment (page 327 Cunningham)

To function effectively as a fermentation chamber the rumen requires:

1. Substrate for fermentation- usually 10-15% dry matter content

2. Stable temperature, near 37C

3. Osmolality near 300mOsm

4. Negative oxidation/reduction potential (-250 to - 450 mV) (by O2 removal)

5. Remove undigestible wastes, mix ingesta

6. Microbe removal appropriate to proliferation rates

7. Suitable pH- (copious alkaline saliva)

8. VFAs produced by fermentation- must be buffered, removed

Therefore precise maintenance of homeostasis is required

*Stratification of contents of rumen and reticulum

Fermenting material is selectively retained, residue is passed on to abomasum

1. Top layer- gas- mainly CH4 and CO2- feel in sublumbar fossa

2. Fibrous raft- occupies most of dorsal sac- feels doughy on palpation

fibrous food material, light as it contains air

has high density of microbes, new food is added to raft

3. Liquid fraction of raft- mixing of saliva, fermentation products

4. Soupy material in reticulum, cranial, ventral rumen sacs- contains fine particles propionic>acetic

2. If chain length is longer (Bu>Pr>Ac)

Acetic acid is major VFA absorbed

Mechanism of absorbtion:

1. Half by passive diffusion (in undissociated state)

2. Facilitated diffusion- half exchanged for bicarbonate


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- epithelial cells contain carbonic anhydrase, produces carbonic acid, dissociates to bicarbonate and

H+ ions, which associate with VFA

- 1 VFA absorbed: 1 bicarbonate generated- so pH is buffered in rumen

- This accounts for about 50% of rumen VFA buffering, remainder is by salivary bicarbonate

VFA metabolism:

1.Acetate- Major VFA produced

Rumen has high capacity for acetate absorbtion

Used by most tissues for metabolism, to form acetyl Co-A in citric acid cycle, used to make f.a. in

mammary gland for immediate energy

Stored as glycogen, fat, protein, and used in phospholipids, sterols

2. Propionate - Other major VFA produced

Most absorbed by rumen, 30% converted to lactate in ep. cells

Propionate is removed by liver, converted to oxaloacetate for Kreb's cycle or can be converted to

glucose, stored as glycogen

Pyruvate- metabolized to acetate, butyrate, H2, CO2, propionate

Propionate- formed from pathway involving succinate

H2 concentration is low in rumen as it is used by methanogens- hydrogen sink

If inhibited then propionate, succinate act as H2 sink

Therefore when methanogenesis is inhibited propionate production increases

3. Butyrate- some modified to beta-hydroxybutyrate (ketone body) in rumen epithelial cells-

the rest is metabolised in liver to BHB

(ruminants- ketone bodies- from rumen, monogastrics- ketone bodies from partial oxidation of long-

chain f.a.)

Others- lactic acid- produced by amylolytic bacteria degrading starch

At low rumen pH propionate bacteria are inactive, so amylolytic bacteria produce both D (-) and L (+)

forms of lactic acid

Lactic acid- strong acid (pK= 4.6) - rumen pH falls

Lactic acid - absorbed at 10% of VFA rate


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L isomer is metabolized to pyruvate in liver more rapidly thanD isomer

Accumulation causes metabolic acidosis

Blood sugar levels in ruminants

ruminant- 40- 50 mg%- very slow glucose response curve

other species- 80-100 mg%

Glucose--rapidly degraded in rumen, unlike monogastric==VFAs

Changing patterns of glucose use in developing ruminant

Cellulose

beta-1 linked compounds (cellulose, hemicellulose, fructosan, pectin) are degraded by primary

cellulolytic bacteria

Carry out fermentation to VFAs but dont produce methane (performed by secondary methanogenic

bacteria)

Cellulolytic bacteria- slow metabolism, division, require NH3

Cellulolytic and methanogenic bacteria - pH optimum 6.2-6.8

Produce CO2, CH4, VFAs, 70 acetate: 15 propionate:10 butyrate

Increase on high roughage diet

Starch

alpha-1 linked starches (amylose, amylopectin) and simple sugars (sucrose etc)

degraded by primary amylolytic bacteria

Faster fermentation, shorter division times, lower pH optimum- 5.5- 6.6

Require NH3 and amino acids for protein synthesis, dont form methane

Secondary methanogenic bacteria and propionate bacteria are required to make methane and

propionate (pH optimum 6.2-6.8)

Predominate on high concentrate diet, can increase rapidly- lead to rapid overproduction of VFAs and

lactic acid

Produce 55 acetate: 25 propionate: 15 butryate


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Digestion of nutrients

Nitrogen metabolism

Two nitrogen sources in rumen

1. Saliva- recycling of N2 in ruminants

2. Diet- nitrogen, nitrates in protein, which is a low level in diet, often not very

digestible, low biological value (low in essential amino acids)

Protein fermentation- hydrolysed by bacteria, protozoa and fungi

Half of dietary protein is degraded in rumen- variation in breakdown

Protozoa- mainly hydrolyse bacterial protein

Also breakdown amino acids to VFAs, NH3 and branched chain- VFAsUrea- broken

down to NH3 by bacteria or rumen epithelium

Urea moves across rumen wall from blood via urease activity in wall

Ammonia fixation- by glutamate dehydrogenase and glutamine synthetase- bacterial

enzymes

Derived from deamination of a.a. , conversion of non-protein nitrogen compounds,

e.g nitrites, nitrates, amides, urea etc.

Fermentable CHO is required to supply carbon and energy to make a.a.

Advantages of microbial protein synthesis-

-all essential a.a. are synthesised

-microbial protein is more digestible to host than plant protein

however breakdown and resynthesis consumes energy

To increase protein during times of high demand (e.g. late pregnancy) various

treatments to inhibit rumen proteolysis e.g. formalized protein, heat treatment

Monensin- ionophore which improves cation transport across membranes- inhibit

microbial methane production

Lipid metabolism page 395 Dukes


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Lipids are small constituent of the ruminant diet

Plants are low in lipids- occur as structural components of leaves and in seeds


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Modifications to rumen function (Dukes, p 410)

Protected nutrients

Plants with low digestibility proteins (e.g maize) are poorly fermented

so they remain intact until abomasum, intestine, increasing utilization

Formalin treatment ("protection") can decrease fermentation of proteins

Lipids can also be protected, e.g. to deliver polyunsaturated f.a. to gut

Antibiotics

8% of energy content of food is lost as methane

Antibiotics used to suppress methanogenic bacteria

Monensin is used for this, increases propionate production

Probiotics

Addition of selected microbes to boost one area of fermentation

ruminant digestion notes for adn

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