ruminant digestion[1]

GWR 227206 Integrative Veterinary

Physiology

Integrative Veterinary Physiology

Ruminant Digestion

Dr G W Reynolds

Room 2.17 Riddett Building

Institute of Food Nutrition & Human Health

Extn. 7507

Email [email protected]

1

http://www.massey.ac.nz/

mailto:[email protected]


Physiology

Herbivores

One of the most successful groups of

terrestrial animals

Their main energy source is plant structural

carbohydrates (cellulose, lignin).

Mammals do not produce cellulase

Herbivores have established a symbiotic

relationship with suitable microbes (produce

cellulase)

Specialised regions within the GI tract that

serve as large fermentation chambers.

In the odd toed ungulates e.g. the horse, this

fermentation chamber is a greatly enlarged

hindgut.

In ruminants there is an enlarged region

between the oesophagus and the gastric

stomach – the forestomach

[Latin ruminare means to chew again]

2



Physiology

Advantages of Ruminants

The forestomach allows

ruminants to exploit as their

food source the most abundant

carbohydrate form on the

planet.

It allows ruminants to thrive in

niches where the quality of the

herbage is too low to support

nonruminants

For example above the Arctic

Circle (musk oxen), in high

mountains (llamas, yaks) and

in the hot deserts (camels,

goats).

3



Physiology

Advantages of Ruminants

The microbes synthesis relatively

high quality proteins from

low quality plant proteins

non-protein nitrogen

recycled nitrogen-containing end

products of metabolism (e.g. urea)

The high quality microbial proteins

become available to the ruminant

when the microbes are digested

The microbes supply the host

animal with vitamin B complex,

provided there is an adequate

supply of the trace element cobalt

for B12.

Tree goats - Morocco

4



Physiology

Disadvantages of a Ruminant

Ruminants must spend

~ 4-7 hours per day gathering food

~ 8 hours per day chewing the cud

Failure to effectively expel the

large volumes of gas produced

during fermentation can be life

threatening

Ruminants have no direct control

over the digestive activities of the

microbes

Can indirectly influence the rate of

fermentation

5



Physiology


End products of fermentation are

mainly the volatile fatty acids

(VFAs)

acetic, propionic and butyric acids,

Pathways for intermediary

metabolism must be geared to

their use.

Propionic acid is the only VFA

capable of being converted to

glucose

The ruminant has a high glucose

requirement during the later

stages of foetal growth and

lactation.

Propionic acid

Acetic acid

Butyric acid

6


http://en.wikipedia.org/wiki/File:Propionic_acid_flat_structure.png

http://en.wikipedia.org/wiki/File:Acetic_acid_flat_structure.png

http://en.wikipedia.org/wiki/File:Butyric_acid_flat_structure.png


Physiology


In the wild state these

disadvantages may be of little

consequence.

Under intensive farming systems

easily fermented concentrates

often replace the normal roughage

diet

Physiological strategies regulating

fermentation may be unable to

cope with the greatly increased

rate of VFA production

Can cause a variety of digestive

and metabolic disorders.

7



Physiology

Ruminant Stomach

Ruminants have a single „compound‟ stomach divide into 4 distinct

divisions.

Fermentation occurs in the first three, reticulum, rumen and

omasum, which are non-secretory

Collectively referred to as the forestomach.

The 4th division, abomasum, is the „true‟ stomach and is analogous

to the simpler form of stomach found in dogs, cats and humans

It secretes hydrochloric acid, which kills the microbes and sterilises

the digesta.

Pepsinogen produced by the abomasum begins the breakdown of

protein, mainly microbial cell wall protein.

8



Physiology

Ruminant Stomach – Right Side

ventral sac abomasum

omasum

reticulum

9



Physiology

Ruminant Stomach – left Side

reticulumabomasum

Ventral rumen

Caudal

Blind sac

atrium dorsal sac

10



Physiology

Forestomach

The interior surface

of the rumen forms

numerous papillae

that vary in shape

and size from short

and pointed to long

and foliate.

Reticular epithelium is

thrown into folds that

form polygonal cells that

give it a honey-combed

appearance. Numerous

small papillae stud the

interior floors of these

cells

The inside of the omasum

has broad longitudinal

folds, reminiscent of the

pages in a book. The

omasal folds represent ~

1/3rd of the total surface

area of the forestomach

11


Innervation of Ruminant Stomach

The innervation to the stomach

Vagus nerves

splanchnic nerves,

Both supply motor (efferent), and

sensory (afferent) nerves.

The left and right vagi merge in the

lower thoracic region and then divide

to form dorsal and ventral branches

Vagal (parasympathetic) efferent

activity is essential for the orderly and

sequential contractions of the

forestomach

Stimulation of splanchnic motor

(sympathetic) nerves inhibit motility


Physiology 12




Physiology

to celiacmesenteric ganglion

to liver & pylorus

Dorsal vagus

Ventral vagus

Reticulum

Rumen

Abomasum

Omasum

From: Dyce, K. M., et al (1996) Textbook of Veterinary Anatomy. 2nd Edition. W.B Saunders Company

13



There is a predominance of sensory over motor fibres in the vagal (10:1)

and splanchnic (3:1) nerves, hence both are predominantly sensory nerves

The vagal sensory fibres innervating the stomach are associated with:

Tension receptors

slowly adapting mechanoreceptors

located in the muscle layers

activated by

passive distension of the stomach

active contraction of the smooth muscle of the stomach wall

Epithelial/mucosal receptors.

rapidly adapting mechanoreceptors and chemoreceptors

are most numerous in the reticulum, cranial sac of the rumen, abomasum (and

duodenum)

respond to tactile stimuli (light brushing), acids, alkali and hyper- and hypotonic solutions

The splanchnic sensory fibres transmit information from serosal receptors, which

are especially concentrated at the attachment of the mesenteries.


Physiology 14


Blood Supply to Ruminant Stomach

The blood supply to the stomach is via branches of the

celiac artery.

Blood flow to the stomach increases markedly after

feeding when fermentation end products are being

absorbed.

The nutrient rich blood drains into the portal vein and

passes through the liver before being returned to the

heart via the caudal vena cava.


Physiology 15


Blood Supply to Ruminant Stomach


Physiology

From: Dyce, K. M., et al (1996)

Textbook of Veterinary Anatomy.

2nd Edition. W.B Saunders Company

16


Microbial Fermentation

The microbes in the forestomach are mostly

anaerobic bacteria, but also yeast-like fungi and

protozoa.

They break down the ingesta by hydrolysis and

anaerobic oxidation.

The protozoa feed on ruminal bacteria, starch and

polyunsaturated fatty acids in the ingesta.

They are very sensitive to changes in intraruminal

conditions and their presence in ruminal fluid is a

good indicator of its „normality‟.

Very little is known about the importance of the

ruminal fungi in fermentation.


Physiology

Rumen protozoa

Rumen fungi

Rumen bacteria

17


Microbial Fermentation

The plant structural carbohydrates in

roughage are β-1,4 linked glucose units

(cellulose) and β-1,4 linked xylose units

(hemicellulose)

Slowly broken down by the ruminal cellulolytic

bacteria to VFAs.

Carbohydrate in grain-based feed contains α-

linked glucose units

rapidly degraded by amylolytic bacteria to VFAs and

lactic acid.

Both types of bacteria are classified as

primary bacteria

Secondary bacteria convert the lactic acid to

propionate and hydrogen to methane.

This last reaction re-oxidises reduced co-

enzymes making them available again as

hydrogen acceptors.

A small amount of O2 is taken in with the food

and water; some O2 diffuses from arterial

blood across rumen wall

Quickly used up by facultative anaerobic bacteria


Physiology

Cellulose

Starch

18


Ruminant Digestion

The digesta in the forestomach do not form an

homogenous mass

Instead the most recently ingested food is added

to a raft of fibrous material that floats on the

underlying soupy fluid

Above this raft is a layer of ruminal gas

During clinical examination, palpation of the

lumbar fossa should detect the gas layer above

the textured fibrous raft.

Failure to remove (by eructation) the gas causes

distension of the stomach (bloat)

Excessive hardness of the raft is a sign of

ruminal impaction, and softness or absence of

the raft indicates the animal has not recently

consumed roughage


Physiology

From: Dyce, K. M., et al (1996)

Textbook of Veterinary Anatomy.

2nd Edition. W.B Saunders Company

19


Digestion in the Rumen


Physiology

Animal Organ Acetic Propionic Butyric

Sheep Rumen 64 20 16

Ox Rumen 71 15 14

Horse Caecum 73 20 7

Dog Colon 51 36 13

Average composition of the mixed volatile fatty acids from the rumen and large

intestine, given as percentages of total acid. (Adapted from AT. Phillipson, Nutr. Abstr.

Rev., 17:12-18, 1947-48.)

20


Digestion in the Rumen

Most of the VFAs produced by the microbes are absorbed into the venous

blood across the wall of the forestomach and used by the ruminant as

substrates for metabolism.

About half of the VFAs are absorbed in an undissociated state by passive

diffusion

the remainder is absorbed by facultative diffusion in exchange for

bicarbonate

During absorption, most of the butyrate is metabolised to β-hydroxybutyrate

used as a substrate by most tissues, and especially the mammary gland during

lactation.

About 1/3 of the propionate is metabolised to lactic acid during absorption.

Most of the acetate is absorbed unchanged.


Physiology 21


Protein Digestion/Regeneration


Physiology

About half the dietary protein is degraded in the forestomach

Overfeeding of protein can lead to excessive levels of ammonia production, ammonia

toxicity and energy expenditure for conversion (detoxification) of ammonia to urea in the

liver

Dietary Nitrogen

Salivary urea

Amino acid

synthesis

Tissue

proteins

22


Rumen Gases

The production of gases by

the microbes reaches a

peak of up to 40 L/h in

cattle 2-4 hours after a

meal, when fermentation

rate is at its maximum.

The principal gases

produced are CO2 (60%),

CH4 (30-40%) and variable

amounts of N2, H2S, H2 and

02.

They are eliminated almost

entirely by the process of

eructation.


Physiology

Composition of rumen gases in a dairy cow. From L.E. Washburn

and S. Brody, Mo. Ag. Exp. Sta. Res. Bull.: 263, 1937.

24


Control of Fermentation

Ruminants exert limited control over the rate at which

fermentation proceeds by altering or satisfying several

requirements. These include: The regular supply of macerated substrate (chewed food) for microbial digestion.

Removal of the end products of fermentation (VFAs, microbial products, gases).

The movement of undigested particles to the abomasum, and thence to the small

intestine.

Mixing of the forestomach contents to prevent local accumulations of inhibitory

end products, and aid absorption of VFAs.

Providing a highly buffered fluid environment for the micro-organisms.

Providing stable conditions of temperature, osmotic pressure, and pH.


Physiology 25


Salivary Secretion

Ruminants produce large volumes

of saliva Sheep: 6-16 L/day

cf blood volume ~ 3.5 litres

Cattle: 60-160 L/day cf blood volume ~ 40 litres

The secretions from the major

glands are isotonic with blood plasma

have no significant amylase activity

change their composition following salt

depletion (K+ exchanged for Na+ due to

the action of aldosterone),

contain urea and alkali

maintain a basal secretory rate even

after denervation.

The main salivary glands of the sheep (from Kay, 1960).

1: parotid, 2: submaxillary (submandibular), 3: inferior molar,

4: sublingual, 5: buccal, 6: labial. Glands are paired on both

sides of the head and mouth.


Physiology 26


Salivary Secretion

The reticulum, rumen and omasum produce no secretion of their own and depend for the

provision of a liquid medium on: water contained in food

water in drink

saliva

The parotid glands are serous glands provide the major fraction of salivary secretion into the rumen.

In sheep a single gland may secrete 1-4 L/day,

In cattle, estimates for a single gland range from 20-80 L/day

Parotid saliva is approximately isotonic with tissue fluid helps stabilise the osmotic environment for the microbes.

Has a high electrolyte content:

The high levels of HCO3 and HPO4-- are important for their buffering action and maintaining a pH

range of 5.5 - 7.0 despite VFA production.

Urea in parotid saliva provides an energy source for the microbes


Physiology

Na+ K+ HCO3- HPO4

-- Cl-

Saliva

(mmol/l) 170 13 112 48 11

Plasma

(mmol/l): 140-180 3.9-5.4 ~27 2-7 95-105

Electrolyte concentrations in ruminant parotid saliva

27


Control of Salivary Secretion Salivary glands are

innervated by

parasympathetic and

sympathetic divisions of

ANS


Physiology

Courtesy Dr J Patterson, Swinburne University, Melbourne

28


Control of Salivary Secretion

As in other animals afferent stimuli arise in the mouth from taste and

mechanical stimulation of the gums.

In ruminants, important sensory inputs also arise from lower in the gut.

Moderate stretch in several areas of the gut is an effective excitatory

stimulus to parotid secretion. Oesophagus - especially the thoracic part

The opening of the oesophagus (the cardia)

The reticulum- especially on its medial wall near the reticular groove and reticulo-omasal

orifice

Omasal canal

Vagal afferent fibres carry excitatory stimuli from these regions to salivary

centres.

Ensures a steady flow of saliva enters the rumen even when the animal is

not eating or ruminating

The stimuli mentioned so far all increase or excite parotid secretion


Physiology 29



The stimuli mentioned so far

all stimulate parotid secretion

Recordings in conscious

animals suggest there are also

inhibitory mechanisms

operating

When sheep and cattle eat

Initially there is a very high rate

of secretion from the parotid

glands

After about 30 minutes of

eating this declines and falls to

below pre feeding levels

This is despite the fact that

feeding continues and

excitatory stimuli from taste,

buccal and oesophageal

stimulation are present


Physiology

Parotid and submandibular salivary secretion rates during eating.

Submandibular flow is shown by the dotted line and parotid flow by the

solid line. Feeding is marked by F and rumination by R. (From D.H.

Carr, pers comm)

30



Possible inhibitory stimuli:

High (excessive) levels of stretch of the

oesophageal and gastric regions mentioned

earlier will inhibit salivary secretion

Mediated by vagal and splanchnic afferent

nerves

Important as digesta and secretions

accumulate in the stomach

Increases in blood osmotic pressure reduces

parotid secretion.

Plasma osmolality may increase by over 5%

during feeding Increased metabolite production in gut

Movement of water into gut

Drop in blood volume

Reduction in salivary flow helps maintaining

stable conditions in the ECF during

digestion.


Physiology

Changes in the osmolality of jugular plasma, the rate of parotid salivary

secretion and reticular contractions before and during feeding. (From D.H.

Carr, pers comm).

31



Sympathetic actions on the gland Decrease the blood supply through vasoconstriction

Cause contraction of myoepithelial cells within the gland and expulsion

of saliva

Followed by compensatory pause in flow as myoepithelial cells relax

Add protein to the saliva


Physiology 32


Salivary Secretion

In summary, the principal functions of saliva in ruminants are to:

Return urea to the rumen for microbial protein synthesis.

Add fluid for proper microbial actions in the large reticulorumen fermentation vat.

Supply bicarbonate and phosphate buffers to keep the pH of the reticulorumen

within the normal limits (5.5 to 7.0)


Physiology 33


Motility of Reticulum & Rumen

Several different methods can been used to study motility of the

reticulum and rumen in intact animals

Palpation

Auscultation

Intragastric pressure changes recorded using balloons or open tipped

catheters positioned in the stomach via

Nasogastric tube

Rumen fistula

Radiography

Barium sulphate (insoluble)

Radio-opaque markers surgically attached to stomach wall

Electromyography

Partial Exteriorisations


Physiology 34


Partial Exteriorisations

Partial exteriorisations are prepared by bringing small parts of the reticulum

and rumen into a position immediately beneath the skin. Contractions of the

stomach are then seen as skin movements.


Physiology

Method of partially exteriorising the rumen. The circles represent

stitches. (From C.S.W. Reid, Proc. N.Z. Soc. Anim. Prod., 23: 169-

188.)

35


Reticulorumen – A sequence

Two basic types of contraction sequence

The first commences with a double contraction of the

reticulum

Contraction then spreads from the cranial to the

caudal regions of the rumen

Involves dorsal rumen first and then ventral rumen

This is a “backward” moving contraction.


Physiology 36


Reticulorumen – A sequence

This type of contraction has been variously termed: Primary contraction.

Mixing cycle

A sequence

It recurs in sheep at about 30-90 sec - at the shorter

interval if the animal has been recently fed, and toward

the longer if fasted.

The functions of the contraction include: Mixing of digesta and distribution of microbes

Mechanical breakdown of digesta

Bringing the products of fermentation to absorptive surfaces

Aiding the movement of digesta onward in the gut


Physiology 37


Reticulorumen – B sequence

The second type of contraction does not involve the

reticulum

Starts as a contraction of caudal ventral blind sac

Spreads to posterior then anterior regions of dorsal

rumen

Ends with a contraction of ventral rumen sac

This is a “forward” moving contraction.


Physiology 38


Reticulorumen – B sequence

This contraction has been variously termed:

Secondary contraction.

Eructation (belching) cycle.

B sequence.

B sequences follow a variable time after A sequences - not always

in a 1:1 ratio.

Animal will eructate (belch) with this contraction, depending on the

rate of gas production.


Physiology 39


Control of RR Motility

Organised movements of the reticulum and rumen cease after

the vagus nerves have been cut

after atropine has been administered

The evidence from anaesthetised animals indicates that both sensory and

motor fibres controlling RR contractions are contained in the vagus nerves

In anaesthetised animals reticulum contractions can be caused by

Stimulation of the intact vagus nerve.

Stimulation of the peripheral end of the cut cervical vagus nerve.

Stimulation of the central end of a cut cervical vagus nerve, provided the other

vagus nerve is intact.

The vagi contain both motor (efferent) fibres to the stomach and sensory

(afferent) fibres from it, both of which are normally involved in the regulation

of gastric contractions


Physiology 40



Mechanical stimulation (touch or stretch) of the regions of the gut

listed below is an effective stimulus to contractions of the reticulum

and rumen.

The mouth (gums)

Thoracic oesophagus

Reticulum (especially medial wall)

Reticulorumenal fold

Reticulo-omasal orifice

Slight stretch of the abomasum

All these are areas that would normally be stimulated during eating

and digestion


Physiology 41


Vagus Indigestion

„Vagus indigestion‟ (abnormal motility patterns of RR) is due to damage to

receptor areas rather than to vagi themselves - especially damage around

medial wall of reticulum


Physiology

A nail is embedded in the

epithelium of the reticulumA nail has penetrated the

reticulum, causing traumatic

reticuloperitonitis (hardware

disease) and the death of this cow

Images from Noah’s Arkive, University of Georgia.

Metal door spring removed from

a cow‟s reticulum

42



Chemical stimuli delivered to the gut may also be involved in reflex

stimulation of RR contractions.

The addition of HCl at pH 1to the abomasum stimulates reticular

contractions, provided the vagal branches to the abomasum

remained intact.

This observation has been criticised because abomasal pH rarely

falls below pH 2.5.

However acid is secreted by parietal cells at about pH 0.9

Provided the receptors are close to the site of acid secretion,

stimulation would be possible.


Physiology 43



Co-ordination of the afferent

information from chemo- and

mechano-receptors occurs in reflex

centres.

These are located in the dorsal vagal

nucleus of the medulla oblongata

They regulate the efferent vagal

discharge.

Inhibitory as well as excitatory stimuli

influence the activity of the medullary

centres.

Excess stretch is inhibitory.

Generally this applies to any of the

areas where moderate stretch is

excitatory, but especially to the

abomasum.


Physiology 44


Detecting Contractions of Reticulum

and Rumen in “the field”

Reticular contractions

identified by a tinkling sound

heard through stethoscope

Caused by rumen fluid flowing

back into reticulum as it relaxes

Contraction of rumen detected

by palpation in sub lumbar

fossa region

A hardening and bulging caused by

contraction of dorsal rumen

This allows A sequences and

B sequences to be

distinguished seperately


Physiology 45


New-Born Ruminant Stomach

The forestomach in the new-born ruminant is anatomically and functionally under-developed

It contains no microbes (i.e. is sterile) and does not contribute significantly to the digestion milk


Physiology

Rumen

Reticulum

Omasum

Abomasun

Age in weeks

Weig

ht as a

perc

enta

ge o

f birth

weig

ht

(birth

weig

ht

= 1

00)

Abomasum

Omasum

Oesophagus

Dorsal sac

Rumen

Ventral

sac

Pylorus

Posterior

Blind sacs

Small intestine,

caecum & colon

46


Rumen Development


Physiology

Rumen

Oesophagus

Omasum

Reticulum

Birth to 2 weeks

Abomasum

Oesophagus

ReticulumOmasum

Abomasum

Rumen

Oesophagus

Abomasum

Rumen

Omasum

Reticulum

Oesophagus

Rumen

Abomasum

OmasumReticulum

8 weeks

3-4 months Mature

Rob Costello, Dairy Technical Specialist, Merrick's Inc.

47



In the new-born ruminant, swallowed milk is directed from the oesophagus directly into

the omasum and abomasum by the reticular groove.

When contracted forms a conduit between the cardia and the reticulo-omasal orifice


Physiology

From: D.A. Titchen and J.C. Newhook,

1975. In Digestion and Metabolism in the

Ruminant. pp 15-29. Editors IW. McDonald

and A.C.I. Warner. Australia, University of

New England Publishing Unit.

48



Within the abomasum, the newly ingested colostrum and milk is subjected

to the clotting action of the enzyme rennin.

Rennin has a pH optimum of near 6.5 and produces a hard clot or curd

consisting of the butterfat and the curd proteins (caseinogens) precipitated

as calcium caseinate)

The whey fraction, which contains important immunoglobulins, passes into

the small intestine

The antibodies are absorbed, passively immunising the young animal

In cattle the immunoglobulins constitutes about 70% of the whey protein of

colostrum

The immunoglobulins survive digestion in the gut because: Neonates are achlorhydric, i.e acid, and pepsin, secretion is absent or low

Colostrum contains an inhibitor to the proteolytic enzyme trypsin.


Physiology 49



The immunoglobulins are absorbed by pinocytosis

They coalesce to form a single large globule in the basal portion in the cell

There are both specific, receptor mediated absorption mechanisms, and non-specific absorption

mechanisms

egg albumin, gelatin and serum protein added to the colostrum is also absorbed

Adrenocortical hormones appear to cause premature „closure‟ of the gut

Such hormones are produced under conditions of stress

Lambs born under stressful conditions (climatic or nutritional) may have impaired globulin

absorption.


Physiology

Species Prenatal Postnatal

Ox, Goat, Sheep 0 +++ (36 hr)

Pig 0 +++ (36 hr)

Horse 0 +++ (36 hr)

Dog + ++ (l0 days)

Rat + ++ (20 days)

Rabbit +++ 0

Man +++ 0

Transmission of Passive immunity and time after birth for „gut closure‟.

50


Reticular Groove

Milk taken when young ruminants suck passes directly to the abomasum

The milk flows directly to the abomasum where the enzyme rennin causes coagulation

This delays its passage into the intestine

Other swallowed fluids, e.g. saliva, are directed into the reticulum

The reticular groove forms a channel from the oesophagus, through the reticulum to the reticulo-

omasal orifice.

It has two well defined „lips‟ which flank a floor.

The lips make a functional extension of the oesophagus directly to the reticulo-omasal orifice.


Physiology 51


Reticular Groove – Reflex Control

Contraction of the reticular

groove occurs as a reflex

response.

When the vagi are cut,

suckled milk enters the

reticulorumen.

In decerebrate

preparations the

reticular groove can be

made to contract by introduction of water into

the posterior region of the

mouth cavity

touching the posterior

mouth cavity with a probe

Cranial laryngeal nerve

stimulation (sensory)


Physiology 52


Reticular Groove – Reflex Control

The responses of the reticular groove illustrate many of the classical

properties of visceral reflexes.

Latency: 2 -4 seconds may elapse between delivery of stimulus and contraction of the groove. .

Summation: One afferent stimulus alone is often insufficient to excite a visceral reflex.

Introduction of water alone into the mouth, or stimulation of afferent fibres in the cranial laryngeal nerve, is

ineffective.

Inhibition: Groove closure inhibited when the abomasum is full/stretched (vago-vagal reflex).

Causes spillage of milk into the reticulum and rumen.

The rapid ingestion of a large volume of milk (e.g. once a day feeding) may

cause ineffective clotting of the curd and rapid emptying of the abomasum.

This in turn results in overloading of the small intestine with protein,

bacterial overgrowth in the intestine and diarrhoea (scours).


Physiology 53


Reticular Groove – Adult Animals

In about 70% of adult sheep the reticular

groove can be made to contract by giving

copper salts orally

This has been used to direct drenches

against nematodes directly to the

abomasum

Nicotine and arsenic were the active

components of the drenches

To achieve a lethal concentration for the

worms (not the sheep) they had to be

delivered to the site of parasite action

(abomasum)

In adult cattle sodium salts have the effect of

causing groove contraction

From time to time there is a resurgence of

interest in using the reticular groove

mechanism to aid animal production


Physiology 54


Reticulo-Omasal Orifice (ROO)

Passage of material from the reticulum

to the omasum occurs via a relaxed

ROO, which in sheep is about 1 cm in

diameter.

In the cow, for the greater part of the A

sequence, the ROO is loosely open

(some 60-70% of the time).

During the first phase of the diphasic

reticular contraction the orifice closes,

it opens at the peak of the second

phase of contraction of the reticulum

Relaxation (opening) of the ROO is

controlled by the vagus

The neurotransmitter is most likely VIP


Physiology

Reticulum

R.O.O.

Pressure recordings

(balloon)

Motility of the Reticulo-Omasal Orifice

R.O.O.

55


Reticulo-omasal Orifice (ROO)

When the ROO opens, digesta

flow into the omasum and the

omasum relaxes

Relaxation of the omasum

combined with contraction of the

reticulum contributes to the flow

of digesta into the omasum

Contractions of the omasum can

be inhibited by distension of the

abomasum

This may provide a means for the

abomasum to control the volume

of ingesta entering it


Physiology

Pressure records

Reticulum

R.O.O.

Omasum

56


Omasum

The function of the omasum is unclear.

May help regulate flow of digesta from the

reticulorumen to the abomasum

Fluid and electrolyte and VFAs are absorbed by the

omasum.

The surface area of the leaves is large - about 1/3 of

the total epithelial lining of the reticulorumen

The leaves undergo movement, especially near their

bases.

Contractions stimulated by infusions of VFAs into the

omasal lumen.

May not be essential for life

Some ruminants, e.g. primitive deer, (lesser mouse

deer) have no omasum. This is a browsing type of

ruminant


Physiology

Omasal folds

57


Eructation

Removal of gas produced by micro-organisms during fermentative digestion

Gas produced in the first 3 divisions of the ruminant stomach

Rate of production increases after eating and decreases slowly until the

next feeding.

Most of the gas is produced in the reticulorumen

Estimated that 1.2 - 2 litres of gas are formed per minute in the rumen of a

500 kg cow.

Most of the gas is the result of bacterial action and CO2 liberation from

salivary bicarbonate


Physiology

CO2 CH4 N2 O2 H2 H2S NH3

65% 27% 7% 0.60% 0.20% Trace Trace

58


Dorsal Rumen

Oesophagus

Reticulum

Ventral Rumen

Caudal Ventral

Blind Sac

Eructation

Eructation usually begins with a B

sequence contraction.

This sweeps gas toward the

oesophagus at the cardia.

Whether eructation occurs at this

stage depends on clearing the cardia

of ingesta

The sphincter at the diaphragm

relaxes, and the oesophagus fills with

gas

When the oesophagus has filled with gas the diaphragmatic sphincter closes and the

pharyngo-oesophageal sphincter relaxes

Gas moves up the oesophagus - in cattle aided by a rapid anti-peristaltic wave of

contraction in the oesophagus - perhaps passively in sheep


Physiology 59



Physiology

From: Van Soest, Peter J (1994) Nutritional Ecology of the Ruminant, Edition: 2nd. Publisher: Comstock Publishing

60


Ruminant Methane Production


Physiology

Since 1999 atmospheric methane

concentrations have levelled off

World population of ruminants has

increased at an accelerated rate

Prior to 1999 there was a strong

relationship between change in

atmospheric methane

concentrations and the world

ruminant populations

Since 1999 this strong relation has

disappeared.

This change in relationship

suggests that the role of ruminants

in greenhouse gases may be less

significant than originally thought

Global large ruminant equivalence and atmospheric methane

concentrations

Large ruminant equivalence (*1,000,000)

Methane concentration (ppb)

Figure 1. Global atmospheric methane concentrations from NOAA (2007) and

cattle equivalents from FAO (2007). Large ruminant equivalences are calculated

using 8 sheep or goats as being equivalent to a large animal

1850

1800

1750

1700

1650

1600

1550

1500

FAO: Food and Agriculture Organisation

IAEA: International Atomic Energy Agency

NOAA: National Oceanic and Atmospheric Administration

1979 1981 1983 1985 1987 1989 1991 1993 1995 1997 1999 2001 2005

Year

61


Ruminant Methane Production

Large ruminants produce some 15-

20% of the global production of

methane

Ruminants on low quality feeds

possibly produce over 75% of the

methane from the world's population of

ruminants

Supplementation can reduce methane

production as

% digestible energy consumed

per kg live-weight gain


Physiology

R. A. Leng Department of Biochemistry, Microbiology and Nutrition,

University of New England, Armidale, N.S.W. 2351

No supplements

Urea/mineral

supplements

No supplements

Urea/mineral

+ bypass protein

supplements

The effects of supplements on methane emissions for

ruminants on low feeds. (A) The effects of improving the

efficiency of rumen fermentative activity on methane production/kg

of digestible energy consumed. (B) the production of methane/kg

gain in supplemented cattle (feed conversion efficiency (FCR)

(9:1) or un supplemented cattle (FCR = 40:1) fed straw-based diets

(after Saadullah 1984)

% d

ige

stib

le e

ne

rgy

Fe

rme

nte

d to

me

tha

ne

(A) (B)

Me

tha

ne

pro

du

ctio

n

(kg

/to

n L

wt. G

ain

)

20

15

10

5

0

1200

800

400

0

62

FCR is the mass of the food eaten divided by the body mass gain over a

specified time


Control of Eructation


Physiology

Gaseous distension of the rumen stimulates eructation and secondary contractions of the rumen

Occurs only if the animal is in an upright or nearly upright position

Does not occur under general anaesthesia

Caused by stimulation of receptors in the caudal region of the dorsal sac of the rumen,

Is a vago-vagal reflex.

Receptors in the cranial part of the rumen and reticulum are important in determining whether or

not eructation occurs with the secondary contractions

Eructation does not occur when the area around the cardia is covered with ingesta, water, foam

This is important because any liquid that was regurgitated into the mouth with the eructated gas

can enter the airways and lungs (glottis open during eructation).

Fluid

No cardiac

opening

Cardiac

Opening Distension Distension Secondary cycles

63


Eructation

Other stimuli, apart from gaseous

distension of the rumen, are

important in causing eructation

Feedlot cattle in USA are often

subject to a chronic and appetite-

depressing bloat when fed a

concentrate food.

This can be corrected by ensuring

the incorporation of some

scabrous material (roughage) in

the diet.

Apparently tactile stimulation of

the RR is important


Physiology 64


Rumination

Rumination is the process by which

ingesta are regurgitated, rechewed, re-

ensalivated and reswallowed.

This second period of mastication

appears to have two main functions: To reduce food particles size.

To facilitate microbial attack on the plant

material

Comparisons have been made of the

rate of digestion of lucerne stalks

placed in nylon bags in the rumens of

sheep.

The lucerne was cut either

transversely or longitudinally, placed in

a nylon bag and then examined at

intervals with scanning EM.


Physiology

Cut transversely

Unaltered after

40 hrs

Cut longitudinally

Unrecognisable

After 6-7 hrs

65


Rumination

Rumination has been shown to be very effective in reducing particle

size of food

Observations were made on sheep with oesophageal fistulae

Boluses were collected on their way to or on their way from the

mouth (after re-chewing)


Physiology

Particle size To mouth From mouth

Coarse 30% 10%

Medium 37% 51%

Fine 33% 39%

66


Rumination

Involves both the digestive and respiratory systems.

Rumination is characterised by:

An extra reticular contraction before the normal diphasic contraction

Respiration momentarily stops and an inspiratory effort made against a glottis at

the peak of the extra phase of reticular contraction

The bolus moves rapidly up the oesophagus to the mouth.

Excess fluid from the bolus is swallowed

Bolus is remasticated

The process is completed in 45-60 seconds by re-swallowing the

bolus.

The whole sequence is repeated after the previous bolus is re-

swallowing.

Rumination commonly continues for an hour or more.


Physiology 67


Electromyogram (EMG)

Recordings During

Rumination in a Sheep

A. EMG of reticulum showing

normal biphasic contraction

B. A normal biphasic contraction

preceded by an extra

contraction

C. EMGs of the oesophagus near

the glottis (1), upper thorax (2)

and close to the cardia (3), and

the reticulum (Re)

D. Jaw and respiratory

movements

The regurgitation phase (AP) of

rumination is closely followed

by swallowing of the excess

liquid on two occasions (P1

and P2) and later of the bolus

(P3).


Physiology

A.

B.

C.

D.

68


Rumination


Physiology 69


Rumination

Periods of rumination

and feeding in sheep and

cattle on legume pasture

Although diurnal patterns

differ, both species tend

to graze in the morning

and evening and rest

during the hotter part of

the day


Physiology 70


Rumination

The physical nature of the diet influences the age at which rumination commences

It may begin in lambs and calves as soon as two weeks after birth

A diet containing roughage causes rumination to start earlier than a diet of solely

milk.

The drive for roughage is strong - milk fed calves will eat the wooden rails of pens and lambs

pull wool from other lambs and chew and swallow it

Tactile stimulation (touch) or light stretch particularly of the area around the cardia,

reticular groove, reticulo-ruminal fold and reticulo-omasal orifice is very effective.

Thus the desire to ruminate seems to be related in large part to the volume of

contents in the reticulorumen, and to the tactile stimulation by coarse material in the

rumen

Rumination is associated with characteristic changes in the demeanour of animals

and often occurs at night.

A reduction in external stimuli (visual and auditory) may be a pre-requisite.


Physiology 71


Rumination

Current theories on the basis of rumination suggest that

the initial reticular contraction at the time of regurgitation

has two functions: To flood the area of the cardia with ingesta which is then available for

regurgitation.

To raise the pressure in the stomach - there may be an increase of 5-6

mm Hg during this contraction.

The inspiratory effort against a closed glottis produces a large reduction

in pressure within the thorax, and hence within the thoracic oesophagus.

The steep gastric-oesophageal pressure gradient - it may be 40-60 mm

Hg in cattle – drives the movement of digesta into the oesophagus when

it relaxes.

The bolus is then swept to the mouth by a reverse peristaltic wave.


Physiology 72


Abomasum

The abomasum is structurally and histologically comparable to the simple stomach of monogastric

animals.

There are three regions which are defined by the type of gland:

A very small cardiac region encircling the omasoabomasal orifice which has cardiac glands

The body or fundus which contains gastric or fundic glands

The pyloric region which contains pyloric or antral glands.

The body has large spiral mucosal folds and this results in about 90% of the total surface area

being formed by the body region. The pyloric region consists of the antrum, which has no folds but

only surface rugae, the pyloric canal and sphincter.


Physiology

Pyloric sphincterOmasoabomasal orifice

73


Abomasum – Gastric Glands

In the fundic region, the glands

contain parietal (or oxyntic) cells which

produce HCl

chief (or peptic) cells which produce

pepsinogen

mucous neck cells

The chief cells are present in the

lower third of the gland

The parietal cells occupy much

of the middle third

Endocrine cells are found mainly

in the lower part of the gland

The gastric pits are lined with

surface epithelial cells.


Physiology 74


Abomasum

The cardiac glands contain the same cell types as the

fundic glands.

There are is larger number of mucous cells which

produce a very viscous mucus.

The pyloric (antral) glands are coiled and produce both

mucus and pepsinogen,

There are virtually no parietal cells.

The gastrin-secreting G cells are present in antral

glands.


Physiology 75


Abomasum

The composition of gastric juice in the ruminant is similar to that in

other mammals

pH averages about pH 2.8

contains pepsin and intrinsic factor

The principal functions of the abomasum are:

Digestion of protein through action of pepsin

Rumen microorganisms entering the abomasum are killed by the acid and

digested to provide nutrients for the host

Secretes intrinsic factor which is essential for absorption of Vitamin B12 in the

ileum

In the immature ruminant the abomasum secretes the enzyme rennin which

causes rapid clotting of milk.

Pepsin is also produced in the preruminant animal and is involved in proteolytic

digestion of the milk


Physiology 76


Abomasal Secretions

Both abomasal secretion and the inflow

of digesta from the omasum are

continuous.

Sheep secrete about 5 litres of gastric

juice per day.

The volume, acidity and pepsin content

increase with feeding and rumination,

decreases with starvation

Experimental sheep may also develop a

conditioned secretory response to the

sight and smell of food.

Abomasum secretions

stimulated by vagal nerve stimulation,

cholinomimetics

Inhibited by atropine i.e. parasympathetic

blockers


Physiology 77


Abomasal Secretions

Control of abomasal secretions

can be studied using surgically

prepared pouches


Physiology

Secretion from pouch increases

when animal feeds

78


Abomasal Secretions

The increase in abomasal secretion with feeding is associated with an

increased inflow of digesta into the abomasum.

Part of the response due to distension of the stomach which is one of the

known stimuli to abomasal secretion in ruminants


Physiology 79


Abomasal Secretions

Distension also stimulates the release of the antral hormone gastrin.

Gastrin secretion is important in ruminants (as well as monogastrics) in mediating food-stimulated gastric secretion.

The composition of the digesta coming into the abomasaum appears to be a key factor.

Gastrin is released by ammonia and peptone in the abomasal fluid and when the pH of the abomasal contents rises.

The high pH of incoming digesta appears to be a powerful stimulant to gastrin release in the sheep


Physiology 80


Abomasal Parasites

Abomasal nematodes

Ostertagia circumcincta (sheep) 0stertagia ostertagi (cattle)

Trichostrongylus axei

Heamonchus contortus

All impair the secretory activity of the abomasum.

The abomasal pH rises since acid secretion is inhibited, serum

gastrin increases and there may also be raised serum pepsinogen

levels.

Diarrhoea, reduced digestibility and disturbed protein metabolism

and utilization

The high serum gastrin may reduce both rumino reticular and

abomasal motility and also the animal‟s appetite.

H. contortus is a blood-sucker and can cause serious anaemia and

death.


Physiology 81


ruminant digestion[1]

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