Anatomy and Physiology of the Equine Digestive Tract

This part of my blog focuses on the anatomy and physiology of the equine gastrointestinal tract. Most horse owners do not have a background in equine nutrition. By the end of this blog post you should be able to describe the anatomy of the gastrointestinal tract of the horse.

NATURAL ENVIRONMENT OF THE HORSE
Horses are highly sociable animals and having social time gives them a higher chance of survival. Horses communicate by body language since they can't actually "talk". Horses are prey animals and have a fight or flight instinct (hide or run).
 * Horses travel 4-16 miles per day
* Horses graze little and often
  - Grazing 16-20 hours per day
  - Low quality feeds, high in fiber

DIET
 Horses are non-ruminant herbivores known as a "hind-gut fermenter". This means that horses have only one stomach, as humans do. However, unlike humans they have to digest plant fiber that comes from grass and hay. Unlike ruminants which digest fiber in plant matter by use of a multi-chambered stomach, horses use microbial fermentation in a part of the digestive system known as the cecum or caecum to break down the cellulose.

In practical terms, horses prefer to eat numerous small feedings steadily throughout the day, as they do in the wild when grazing on pasture. This isn't always possible in modern times due to stabling practices and human schedules that favor feeding horses twice per day. It is important to remember the underlying biology of the animal when determining what to feed; how often and amounts.

The digestive  system of the horse is very delicate. Horses are unable to regurgitate food, except from the Esophagus. So if they overeat or eat something poisonous or fermented, vomiting is not an option. They also have a long, complex large intestine and a balance of beneficial microbes in their cecum that can be upset by rapid changes in feed. Because of these factors, they are very susceptible to colic which is a leading cause of death in horses. To prevent this, horses require clean, high-quality feed, provided at regular intervals and may become ill if subjected to abrupt changes in their diets. Horses are also sensitive to molds and toxins. So it is very important not to feed contaminated fermentable materials such as lawn clippings. Fermented silage or "haylage" is fed to horses in some places; however contamination of failure of the fermentation process that allows any mold or spoilage may be toxic.
* Intestinal system designed to process:
  - Higher amounts of low quality forage
  - Ingested on almost continual basis
  - Principal diet should be forage

DEPARTURE FROM NATURAL STATE
* Continual grazing and moving
* Not voluntarily confined

CHANGED USE
 * Mechanisation of agriculture:
  - Working animal to leisure or sports horse
 * Types of feedstuff changed
  - Include energy dense feeds- cereal grains
  - Meet energy requirements of sports horse

CHANGED FEEDING MANAGEMENT
 * Meal feeding:
  - Large meals of cereal grain 2x's daily
* Grain supplementation greater than forage
  - Often less than recommended
  - Minimum 1kg DM forage/100kg LW
* Whilst intestinal system requires largely unmodified

ENCOUNTERED SOME PROBLEMS
 * Health Implications
  - Gastric ulceration
  - Hind-gut acidosis
  - Azoturia
  - Laminitis
  - Colic
* Outcomes
  - Poor performance
  - Irreparable damage
  - Euthanasia

WHAT CAN WE DO?
 * Consider digestive anatomy and physiology
  - Maintain gut health
* Understand behavioral needs
  - Maintain good welfare
* Develop dietary management regimes that avoid or even overcome diet-related problems

GIT AS A WHOLE
* Horses are classified anatomically as non-ruminant herbivores
  - Non-ruminants= enzymatic digestion (limited fiber degradation in LI)
  - Ruminant= fiber digestion in the rumen
  - Horse= sit between these
* GIT begins at lips and ends at anus
* GIT consists of 3 functional regions
  - Stomach
  - Small Intestine
  - Large Intestine
(Stomach and Small Intestine being the foregut & Large Intestine being the hindgut)

GIT CONSISTS OF:
 - Mouth
- Pharynx
- Esophagus
- Stomach
- Small Intestine
- Caecum
- Large Colon
- Small Colon
- Rectum
ASSOCIATED ORGANS CONSIST OF:
- Teeth
- Tongue
- Salivary Glands
- Liver
- Pancreas

 THE MOUTH
* Digestion begins in the mouth
* Upper lip guides food into the mouth
* Feed torn by upper and lower incisors
* Molars and premolars grind the forage into smaller particles
* Chewing is essential for digestion (regular dental checks are important)
* Horses chew 70-90 times per minute and saliva is produced in response of chewing

THE STOMACH
* Size- relatively small and inelastic
  - Capacity of 8 litres (size of a rugby ball)
  - Important not to over fill (no more than 2 kg per meal)
  - Reason why we feed little and often
* Divided into 2 sections (non-glandular and glandular)
  - Non-glandular section is where food enters (no mucous layer)
  - Glandular is where hydrochloric acid is produced (protected by mucous layer)
* Acidic- pH 1.5 to 2 in the glandular region
* Little digestion of feed- some protein breakdown (no absorption)
* Rate of passage- rapid (20 minutes)

THE SMALL INTESTINE
* Size- a 21 to 25m long narrow tube
  - 30% of horse's GIT volume (75% of its length)
  - Joins the stomach to the large intestine
* Three main parts
  - Duodenum (first part after the stomach)- mixing and neutralizing
  - Jejenum
  - Illeum (last part before the LI)- absorption of nutrients
* pH of digesta entering the SI ranges from 2.5 to 3.5
* Bile (secreted from the liver) buffer the pH to approximately  7 to 7.5 (no gall bladder)
* Brunner's Glands secrete bicarbonate into the lumen- further neutralize digesta

THE SMALL INTESTINE CONT'D
* Neutral pH required for:
  - Transport of nutrients across gut wall
  - Optimal activity of enzymes (anylases, lipases, etc)
* Surface area increased by:
  - Villi (0.5 to 1mm long)
  - Crypts (covered by columnar epithelium)
  - Crypts secrete enzymes and mucous to cover intestinal surface
* Absorption greatest in proximal jejenum
* Rate of passage between 45 minutes and 2 hours
  - Depends on diet, type of food, meal size and amount of forage in the diet
* Digesta moves at 30 cm per minute via peristalsis (muscular contractions)

THE LARGE INTESTINE (ALSO KNOWN AS THE HINDGUT)
 * Size- approximately 7 metres long (capacity 150 litres)
  - 60% of horse's GIT volume
* Three main parts
  - Caecum (first part after the ileum)
  - Large colon
  - Small colon
  - Rectum
* Large intestine lined with mucous-secreting glands- no digestive enzymes
* Digestion & absorption in LI depends on microbial fermentation feed residues from SI
* Caecum- 1.2 metres long (capacity of 30 litres)
  - 15% of horse's GIT volume
  - Large blind-ended sack-like structure
  - Digesta enters LI through ileo-caecal valve; leaves through the caeco-colic valve (both at top)
  - Large fermentation vat (similar to the rumen of the cow)
* Large Colon- 3 to 3.5 metres long (capacity of 90 litres)
  - Right and left ventrical colon
  - Left and right dorsal colon
  - Four parts connected by bends known as flexures (often sites of impaction)
* Small Colon
  - Same length as large colon
  - Smaller diameter
  - Main function= absorb water
* Horses do not produce enzymes to digest fiber
* Microbes ferment digest- produce end products
  - Volatile fatty acids (VFA)= acetate, butyrate and propionate
  - Gases= carbon dioxide (CO2) and methane (CH4)
* Microbes- over 400 species (probably more)
  - Bacteria
  - Fungi
  - Protozoa
* pH of LI= 6 to 7 (very important to maintain correct pH)
  - Bicarbonate and phosphate salts secreted
  - Rapid absorption of VFA
* Microbes need time to adapt to any changes in diet
* Abrupt changes:
  - Reduced diet digestibility
  - Colic
  - Acidosis
* Hindgut designed to process fiber
* Problems occur when low fiber diets are fed
* Particularly when high levels of starch are included in diet
* High amount of starch and sugars in LI
  - Undesirable
* Rapidly fermented:
  - High levels of VFA
  - Lactic acid produced
  - Low pH
* Adverse affect on LI environment
  - Death of fiber degrading bacteria
  - Colic
  - Acidosis/laminitis

THE GIT- CONCLUSION
 * Designed to process fibrous feeds
* Fibrous feeds require for:
  - Healthy gut
  - Satisfy behavioral needs
* Sudden changes to diet or inappropriate diets can lead to:
  - GIT disturbance
  - Reduced diet digestibility
  - Ill health

Abbreviations used in this section:
LW= Live Weight
DM= Dry Matter
LI= Large Intestine
SI= Small Intestine
GIT= Gastrointestinal Tract
VFA= Volatile Fatty Acid
CO2= Carbon Dioxide
CH4= Methane
LAB= Lactic Acid Bacteria 

VIDEO "THE HORSE'S DIGESTIVE SYSTEM"