Ruminant Digestion Notes for ADN
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Transcript of 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