Hindgut Fermenters

Question 1

Enzymatic digestion

Answer 1

Bonds between molecules susceptible to mammalian enzymes (secreted by the animal)

Carbohydrates (CHO)
Dry Matter (DM)
Proteins
Fats/lipids
Soluble CHO (α linkages)

Question 2

Microbial fermentation

Answer 2

Bonds between molecules are not susceptible to mammalian enzymes but are to enzymes secreted by microbes

Insoluble CHO (β linkages)

Question 3

GIT and feeding strategy of hindgut fermenters

Answer 3

Natural diet of herbivores is low in nutrients compared to the diet of carnivores
Diet of herbivores is high in CHO, low in protein and low in energy
➢Consume large quantities to satisfy nutrient and energy requirements
➢Spend vast amount of time eating (species variations – rabbits dawn and dusk, behavioural
considerations)
• Need a GIT that enables enzymatic digestion of soluble compounds and microbial fermentation of insoluble compounds
• Enzymatic digestion is a relatively quick process (0.5-2 hours in equids) takes place in the stomach and/or small intestine
• Microbial fermentation is a much slower process (24-48 hours in equids) so requires large
fermentation chambers
➢Ruminant = foregut (rumen)
➢Hindgut fermenters = caecum and colon

Question 4

Small mammals

Known as caecal fermenters

Answer 4

Guinea pig • Hamster
• Rabbit
• Koala

Question 5

Large mammals

Known as colon fermenters

Answer 5

• Orangutan • Equids

* Rhinoceros • Elephant

Question 6

Omnivore species also utilise hindgut fermentation although benefit from a diet higher in

Answer 6

protein and soluble CHO

Question 7

Key features of the GIT of hindgut fermenters;

Answer 7

oDentition suitable for biting and grinding plant material oSmall stomach
oEnzyme production
oSmall intestine for absorption of soluble molecules oLarge fermentation chambers
oMicrobial populations

Question 8

Dentition of hindgut fermenters

Answer 8

• Due to the high food intake the teeth of herbivore
animals continually grow (rabbits/guinea pigs) or
erupt (equids)
o Rat - only incisors continually grow
• Incisors for cutting leaves and twigs
• Flat molars positioned close together to create one occlusal surface for grinding fibrous ingesta to small enough particles to swallow
• Implications for management – must provide fibrous foods/materials for chewing to wear down teeth

Question 9

Salivary glands of hindgut fermenters

Answer 9

Role of saliva is to lubricate food when swallowed and buffer the acidic content of the stomach (most species have parotid, mandibular and sublingual glands, rabbits also have zygomatic glands)
• In rabbits saliva is produced continually from the mandibular gland, and in response to food intake and chewing by the remaining glands
• In horses saliva is only produced in response to chewing
• Saliva composition varies between species;
o Bicarbonate = buffers acidic stomach contents = present in most hindgut fermenters
o α-amylase = present in rabbit and rat saliva but not horse = starch digestion although action is low o Water and mucus = lubrication

Question 10

Small stomach of hindgut fermenters

Answer 10

The stomach of hindgut fermenters takes up a small proportion of the total GIT volume 10% horse < 53% sheep (rumen)
15% rabbit 69% cat
• Have a trickle feeding strategy, not large meals
• Ingesta passes relatively rapidly through the foregut compared to foregut fermenters
although dependent on diet o 0.5-2 hours for horse
o 3-6 hours for rabbit
o 2 hours for guinea pig o 0.25-2 hours for rat
• Hindgut fermenters are thought of as simple stomached compared to foregut fermenters which are compartmentalised

Question 11

Stomach

Answer 11

• Entrance to the stomach is through the cardiac sphincter = prevents regurgitation in horse, rabbit, guinea pig and rats
o What they eat must pass through the GIT = management implications
• HCL secretion in equine stomach is continuous = ingesta helps to prevent gastric ulcers • Exit from stomach is through pyloric sphincter

Question 12

Cardiac / oesophageal of stomach

Answer 12

Non-glandular region, pH 6-7, no secretions = no protective mucus lining

Question 13

Fundic and Pyloric

Answer 13

Main secretory part, parietal cells secrete HCL, chief cells secrete pepsin, mucus secreted to protect again acid corrosion, pH 1-2 in horse and rabbit

Question 14

Pyloric region of the stomach

Answer 14

Gastrin secretion = stimulates HCL production

Question 15

Enzymatic secretions

Answer 15

• Enzymatic digestion takes place in the stomach and small intestine
• Breaks down the ingesta to their individual molecules to allow absorption
e.g. protein is not one solid mass of protein, it is made up of single molecules (amino acids) joined together to make protein units that form protein compounds
• The α bonds that link the individual molecules are the target of this breakdown action
• The enzyme forms a complex with the target compound causing the bonds to break and the individual molecule to be released

• The reaction must occur multiple times to break all the bonds and release all of the individual molecules
• Once released the molecules are able to be absorbed through the gut wall where they enter the circulation and travel to the liver for processing and use by the body
• The complexes formed between enzymes and compounds is not random o Particular enzymes form complexes with specific molecules
o Results in specific nutrients being broken down and available for absorption at particular locations in the GIT

Question 16

Small intestine of hindgut fermenters

Answer 16

• Small intestine is the main location of enzymatic digestion and absorption of soluble molecules (proteins, lipids, vitamins, minerals, sugars and starch)
• Structure is comparable between species although differences in dimensions and GIT proportions exist
• Three main sections;
1) Duodenum – first section digesta reaches after the stomach (most acidic section)
2) Jejunum – middle and longest section where nutrients are absorbed
3) Ileum – final section where nutrients are absorbed (least acidic section)

Question 17

the mucosa

Answer 17

The inner most layer of the small and large intestines is called the mucosa
• The mucosa is not flat:
o Finger-like projections called villi (singular = villus)
extend inwards towards the centre of the intestine
Villi are only present in the SI
o Depressions called crypts surround each villus
Villi
• Structure increases the intestine surface available for nutrient absorption
• Villi are covered with tiny projections called microvilli (often referred to as the brush border)
• Each villus contains a circulatory network for transporting absorbed nutrients into the blood
• Cells within the crypts produce enzymes for digestion - goblet cells within the mucosa layer produce protective mucus

Question 18

Passage of matter through the small intestine in hindgut fermenters

Answer 18

• Ingesta passes through the small intestine of hindgut fermenters usually within 1-3 hours
• Considerably longer than a dog which pass ingesta through its entire GIT in approximately
8 hours
• The ileum joins the large intestines (caecum and colon)
• In equids this is a straight forward arrangement with ingesta passing from the ileum into the caecum through the ileocaecal valve and then leaving the caecum through the caecoventral colonic valve
• In rabbits the arrangement is more complex due to a three-way connection between the ileum, caecum and colon = ileocecocolic junction

The arrangement in rabbits facilitates selective retention of fluid and small particles and expulsion of fibrous material
• Adaptation by rabbits to avoid carrying large volumes of fibrous material in the GIT

Question 19

Large fermentation chambers

Answer 19

Soluble nutrients within the ingesta should have undergone the majority of their digestion before reaching the hindgut
• Insoluble/fibrous portion is what remains and must undergo microbial fermentation to break the bonds linking the molecules
o Hemicellulose
o Cellulose
o Lignin (completely undigestible)
• Takes time so requires large chambers to store ingesta as goes through the process – caecum and colon

• The caecum and colon are the main sites of microbial fermentation in hindgut fermenters o Provide space for mixing of ingesta with microbial populations and a large surface area for absorption of
nutrients, fermentation products and water
• In most species the sites of fermentation are sacculated, increasing the surface area of the structure
o Some also have flexures involved in selective retention of ingesta
• The mucosa lining of the caecum and colon contains crypts but no villi
• Goblet cells produce mucus to protect against acids produced during the fermentation process
• Optimum pH for microbial populations is 6-7, therefore bicarbonate and phosphate are secreted into the large intestine to neutralise the acidic products of fermentation

Question 20

Large fermentation chambers in Equids

Answer 20

• Microbial fermentation takes place in the both the caecum and the colon in equids
• Literature often refers to equids as colon fermenters (see slide 8) mainly because of their
enlarged colon
➢Caecum = 25-35 litres, 16% GIT capacity ➢Large colon = 81 litres, 38% GIT capacity

• Course particles high in fibre are selectively retained in the caecum and large colon
o Logical as main sites of microbial fermentation
• Absorption of fermentation products takes place in the caecum and colon
• Ingesta then passes through into the small colon
• Finalsiteofwaterabsorption
• Formationoffaecalballs

Question 21

Large fermentation chambers in Rabbits

Answer 21

• Microbial fermentation takes place predominantly in the caecum of rabbits with selective retention of fine particles and fluids
o Known as caecal fermenters o Caecum = 40% GIT capacity
• These fine particles undergo microbial fermentation
o Products of fermentation are both absorbed in the caecum and expelled as cecotrophs (soft faeces)
• Fibrous material (insoluble CHO, >0.5mm in length) are selectively progressed through the colon for expulsion
o Important for promoting GIT motility as insoluble CHO promote GIT transit

• Rabbits have a number of unique GIT structures
o Thickened circular muscle between the proximal and distal colon called the fusus coli that controls
colonic motility
o Vermiform appendix at the end of the caecum rich in lymphoid tissue (immunity role) which also secretes bicarbonate and water
• Rabbits excrete faeces in two forms due to performance of coprophagy

Question 22

Cecotrophs(softfaeces):

Answer 22

Excretedduringrestperiods(~4hoursafterfeeding)
Rich in amino acids, VFAs, enzymes, vitamins B and K, microorganisms (↑ protein)
Coated in mucus to protect against acid in the stomach (stay for 6-8 hours) Pass into the small intestine where contents are digested and absorbed
= pseudo ruminant

Question 23

Hard faeces

Answer 23

excreted during feeding phase (dawn and dusk), indigestible fibre

Question 24

Microbial populations

Answer 24

• Microbial populations are present in the caecum and colon of hindgut fermenters o Bacteria
o Fungi
o Protozoa
• The animal provides an environment in which the microbes can survive and grow in exchange for them breaking down the insoluble parts of the plants eaten
= symbiotic relationship
• Microbes have to adapt to changes in diet which can take a number of weeks o Requires dietary changes to be made gradually
o Equids = can cause colic due to disruption of the fermentation process

  Microbes attach themselves to cell wall compounds Microbes then excrete their own enzymes that are able to breakdown the links between the individual molecules of cellulose and hemicellulose The individual molecules are then processed by bacteria to produce Volatile Fatty Acids (VFAs) and gases VFAs are absorbed through the gut wall into the blood for use by the body as energy sources    • Products of fermentation of benefit to the animal o Volatile fatty acids (Butyrate, propionate and acetate) o Heat o B vitamins and vitamin K o Amino acids – only in those performing coprophagy o Gases are also produced – can cause issues if become trapped

Question 25

Equids

Answer 25

(transit time 48 hrs +)
• Little / no salivary amylase
• Cannot regurgitate due to gastric sphincters
• Small acidic stomach (10% GIT capacity) –
constant HCL secretion
• Efficient enzymatic digestion in SI (soluble
elements)
• Insoluble CHO (fibre) undergo microbial
fermentation in the caecum and large colon
• VFAs absorbed in caecum and colon
• Microbial proteins not utilised and are
excreted
• Microbial population relies on processing of
insoluble CHO
• Microbes sensitive to dietary and pH changes =
digestive upset
• Requires fibre based diet

Question 26

Rabbits

Answer 26

(transit time ~19 hrs)
• Salivary amylase present
• Cannot regurgitate due to gastric sphincters • Small acidic stomach (15% GIT capacity)
• Efficient enzymatic digestion in SI (soluble
elements)
• Unique ileocecocolic junction
• Unique selective retention of fine particles and fluid
by colon
• Fine particles undergo fermentation in caecum
• VFAs absorbed in caecum
• Insoluble CHO expelled
• Produces cecotrophs which yield amino acids, VFAs,
vitamins B and K
• Microbes sensitive to dietary and pH changes =
digestive upset
• Requires balance of soluble and insoluble fibres
38

Question 27

Guinea Pigs

Answer 27

(transit time ~20 hrs)
• Salivary amylase present
• Cannot regurgitate due to gastric sphincter and shape of stomach
• Small stomach (4.1-4.5) – all parts of the stomach are secretory (glandular)
• Lack enzyme L-gulonolactone oxidase, which synthesizes ascorbic
acid (vitamin C) from glucose
• Efficient enzymatic digestion in SI (soluble elements)
• Insoluble CHO (fibre) undergo fermentation in the well developed caecum
• 65% GIT capacity
• VFAs absorbed in caecum
• Microbial proteins not utilised and are excreted
• Poorly developed colon
• Perform coprophagy (uncertainty about cecotroph production) - yields
some amino acids and B vitamins upon ingestion
• Microbes sensitive to dietary and pH changes = digestive upset
• Microbial population relies on processing of insoluble CHO

Question 28

Rats

Answer 28

(transit time 11-18hrs)
• Salivary amylase present
• Cannot regurgitate due to gastric sphincter
• Small stomach (pH 3.5-5) – all parts of the stomach are secretory
(glandular)
• Expands upon eating
• Efficient enzymatic digestion in SI (soluble elements)
• Insoluble CHO (fibre) undergo fermentation in the well developed caecum
• Expands upon eating
• VFAs absorbed in caecum
• Microbial proteins not utilised and are excreted
• Poorly developed colon
• Produces cecotrophs which yield amino acids, VFAs, B vitamins upon
ingestion

Question 29

Dietary requirements in general

Answer 29

• The GIT of all hindgut fermenters are developed to process insoluble CHO / fibrous material
o Ensures microbial populations have adequate food supply o Promotes GIT motility
oProvides energy and vitamins
• Species variations for the amount and type of fibre the diet should contain
• When they receive an inappropriate diet that is either too high or too low in insoluble CHO issues develop
oToo low fibre = reduced wearing of teeth, reduced motility, reduced microbial activity/growth, reduced energy
oToohighfibre= impactedGIT,reducedcaecalactivity(rabbit)

Question 30

Carbohydrates

Answer 30

Starch and sugars provide readily digestible energy = glucose Insoluble CHO provide fermentable energy = VFAs

Question 31

Proteins

Answer 31

In all living cells, used in growth, repair and normal bodily functions

Question 32

Fats/lipids

Answer 32

Used in cell membranes, hormone production, absorption of fat-soluble vitamins and storage of energy

Question 33

Vitamins and Minerals

Answer 33

Essential for many functions within the body and formation of hormones, vitamins, amino acids and tissues

Question 34

Energy

Answer 34

Not a nutrient, gained from utilisation of nutrients – CHO, fats and proteins

Question 35

Dietary requirements of Equids

Answer 35

• Should be eating 2-2.5% BW per day as dry matter (minimum of 1.5% BW should be fed as forage) o Some ponies can eat up to 5% BW per day = causes obesity
• Energy (DE) and protein requirements for maintenance, light and moderate exercise satisfied with fibre based diet supplemented with concentrates / additives
• More demanding exercise requires more energy dense diet so replace some of the fibre feeds with concentrate feeds
• Concentrate feed fortified with vitamins and minerals
• Vitamin and mineral supplements useful if only feeding forage or out at pasture as do not add extra energy or protein

Question 36

Dietary requirements for Rabbits

Answer 36

• Limited research on rabbit nutrient requirements
• Should be eating 2.4-2.7% BW per day as dry matter
o Sweet feeds are consumed in preference = issues with obesity
• Rabbits select soft, nutrient dense vegetation (younger plants) = diet usually higher in CP and soluble CHO than other herbivores
• Nutrient composition of the diet influences cecotrophs production
o High protein diets = decrease cecotroph production, low protein diets = increase cecotroph production o Excessively high and low fibre diets = decrease cecotroph production due to increased caecal retention o Cecotrophs contribute to daily nutrient intake only marginally – varies with diet fed
46

47. Dietary requirements
    Rabbits
    • Main energy sources are sugars absorbed in SI and VFAs from caecal fermentation
    o Rabbits do not digest starch as well as other monogastrics so need to avoid overloading the system
    • Access to grass is ideal as enables normal behaviour, abrasive to teeth, high in fibre with good protein levels and provides good source of vitamins
    o When not grazing ad libitum access to grass hay (meadow hay) is ideal, higher in fibre and lower in protein
    o Some texts recommend alfalfa hays/pellets but higher in energy and protein and can lead to obesity
    • Recommended to feed pelleted commercial feeds over mixes to avoid selective feeding o Fibre content >18%
    o Optimum pellet size of 0.63cm long, 0.47cm diameter

Question 37

Dietary requirements for Guinea Pigs

Answer 37

• Should be eating ~4% BW per day as dry matter
• Guinea pigs are very susceptible to dietary changes so changing food type or brand must
be a slow process to avoid reduced intakes
• More efficient at digesting fibre than rabbit, likely due to rabbits selective expulsion of fibre
• Debate over production of true cecotrophs
o Use what is termed a ‘mucus trap’ in a longitudinal furrow in the colon
o Traps bacteria, proteins and fine particles from caecum then transports them back to the caecum for fermentation
o Fibrous particles are expelled as hard faeces
o This selective system is less efficient than the rabbits but thought to still produce cecotrophs

Guinea Pigs
• Diet needs to provide vitamin C (ascorbic acid) as Guinea pigs cannot synthesise their own ascorbic acid
• Commercial mixes fortified with vitamin C although levels deteriorate if feed not stored correctly
o Dark, cool area
o Can supplement in water but need to ensure guinea pig drinks
• Similar to rabbits, forage is recommended to enable normal feeding behaviour, wear teeth and support hindgut health
• Pellets are also recommended to avoid selective eating of certain ingredients in mixes

Question 38

Feeds for hindgut fermenters - forages

Answer 38

• Forage-based feeds include grass, hay, haylage and chopped forages
• Grasses have the advantage of being a cheap, natural food source and appetising o Nutrient value very variable – species, maturity of plants, selective grazing behaviour
o Equids are susceptible to sugar content, excessive intake = laminitis
o Be aware of weeds/poisonous plants in grazing areas
• Hay can be either grass or legume (alfalfa)
o Grass hays are lower in energy, protein and calcium than alfalfa hays, but higher in fibre
o Processing and storage has considerable effect on nutrient level and hygienic quality ➢ Slow drying/baled when damp = microbial growth
➢ Slow drying and rain = leaches nutrients
➢ Warm and damp storage = microbial growth
• Dust or moulds likely to cause respiratory issues to all species

• Meadow grass hay is thought of as lower in nutrient value than seed hay
o Recommended for rabbits and guinea pigs
• Rabbits at HAU Companion Animal House fed Timothy hay as no access to grass
• Guinea pigs have a small amount of Timothy hay as well as meadow hay • Forage should form the basis of the equine diet to ensure GIT
health
o Grass hays are most commonly fed (seed or meadow)
o Alfalfa hay fed to performance horses as higher energy ➢More commonly fed in Europe/USA

• Haylage is most commonly fed to equids
• Grass cut at earlier growth stage than hay
o Wilted in field for 48hrs then wrapped to produced airtight environment
o Produces an anaerobic environment in which lactic acid bacteria ferment sugars in the forage
o Fermentation reduces sugar content, although considerably less than happens in silage - relies on maintenance of the anaerobic environment
• DM content is lower (500-700g/kg) compared to hay so should feed more haylage than hay
o Need to test the batch to determine DM and nutrient content as so variable • Less dusty than hay = good for equids with respiratory issues
Not recommended for rabbits or guinea pigs as too nutritious

• Straw can be fed to equids in small quantities to provide additional fibre
• LowdigestibilityvaluemeansitcanbecomeimpactedintheGIT so limited intake recommended
• Often used in chopped forages mixed with hay
• Not recommended for rabbits or guinea pigs due to impaction
risks
• Forages can be fed in long or short (chopped form) to equids with short chaffs being suitable for those with dental issues
• Long forages are recommended for rabbits and guinea pigs to encourage chewing and prevent excessive intakes

Question 39

Feeds for hindgut fermenters – concentrates

Answer 39

• Commercially available concentrate feeds are recommended over feeding individual cereal grains
o Balanced and fortified to meet nutritional requirements o Known composition
o More practical – purchase and storage
• Mixes and pellets are recommended for equids as no records of selective feeding with mixes
• Pellets are recommended for rabbits and guinea pigs to avoid selective feeding of mixes o Ingestion of only certain grains leading to dietary imbalances
• Should always follow manufacturers guidelines to ensure balanced diet is received

• Equine feeds developed for horses workload o Maintenance / light work
o Medium work
o Heavy work / performance
o Breeding / lactation
• Can also get feeds formulated for specific reasons o Low starch/sugar
o High fibre nuts/cubes

• Rabbit and guinea pig feeds are less varied • Some formulated for older rabbits
• Breeding rabbit formulations (higher CP levels)
Recommended nutrient composition:
• Rabbit: CF = >18% CP = 12-16%
• Guinea pig: CF = 10-18% CP = 20-30%

Question 40

Feeds for hindgut fermenters – succulents

Answer 40

• Succulents are recommended to enrich the diet but not as a form of providing nutrients
o Varied nutrient value = varied intake Most common succulents fed:
• Equids = carrots, apples, turnips, swedes, parsnips
➢ Cut length ways to avoid becoming lodged in oesophagus
• Rabbits and guinea pigs = parsley, broccoli, spinach, sprouts, bell peppers, kale, cauliflower, dandelion leaves, carrot (in small amounts)
➢Can also be fed fruits in small amounts (one a week) – apple, pears, melon, banana, kiwi