Equine Nutrition/Digestive anatomy and physiology

Overview Of The Gastro-Intestinal Tract
The horse's Gastro Intestinal Tract or GIT, is comprised of the mouth, pharynx, oesophagus, stomach, small intestine (3 parts: duodenum, jejunum and ileum), large intestine (made up of three parts: the caecum, large colon, and small colon),rectum and anus. The teeth, tongue, salivary glands, liver and pancreas are all associated organs. In a mature horse the entire digestive tract is ~30 metres (~100 feet) long and varies greatly in diameter and surprisingly the stomach itself is no larger than a rugby ball. The stomach and small intestine are referred to as the fore gut and the large intestine as the hind gut. Horses are unusual because they are classified as non-ruminant herbivores. They have digestive enzymes in their small intestine, but use microbial fermentation in the hind gut.

The horse's GIT was designed to process fibrous feeds which are required for both a healthy gut, and to satisfy behavioral needs. It was also designed to have food/digestive matter throughout the whole of its GIT all of the time. As a nomadic animal, they are designed to eat small amounts, over many hours in the day and cover a large territory to do it. It is important with the domesticated horse, to mimic this feeding pattern as best as possible and to pay close attention to changes of diet and appropriate types of feeds in the right balance, to avoid GIT disturbances that can lead to reduced diet digestibility and potentially poor health.

The Mouth
Digestion of food begins in the horse's mouth. The food gets guided to it by the horse's lips, particularly the upper one which is more mobile. In the case of grass or hay, the food is torn or picked up by the horse's front teeth, the incisors. Horses have 12 incisors in total, 6 on the top jaw and 6 on the bottom. Once the food is picked up, it is then moved further back into the mouth with the help of the horse's tongue where the food will meet the molars. Horses have 12 pre-molars in and 12 molars situated behind the pre-molars. The horse uses a grinding (circular) motion to reduce the particle size of the food (a horse will chew up to 70 to 90 times a minute when eating). It is worth noting that horses have open rooted teeth unlike humans which means that they continue to grow throughout a horses life, essential when grinding their food which wears the teeth down. Chewing is an essential part of digestion, as the food particle size is reduced, the surface of the food increases and saliva is mixed into it, which has an effect later on in the digestion process. Regular dental checks are essential to maintain good health in a horse, as hooks and sharp edges can form on the teeth after periods of time especially in horses who are fed concentrates, and who are fed forage in hay nets above the ground, as this is an unnatural position for a horse to use its teeth effectively.

Saliva is produced by the mechanical action of chewing. Horses do not salivate in anticipation of eating and so mastication is essential for the salivary glands to produce saliva. There are digestive enzymes present in saliva which aid in breaking starch into simple sugars. Saliva also buffers the highly acidic stomach environment.

The Stomach
The size of stomach is relatively small in comparison to the overall gastrointestinal tract. The size of the stomach is likened to that of a rugby ball and has an 8 litre volume capacity. It is also relatively inelastic, which is why the practice of feeding "little and often" to not overfill the stomach, is critical. A full grown horse shouldn’t eat more than 2 kg (4.5 lb) of feed at one meal as the stomach can’t handle a greater capacity. Some food matter can move through the stomach in as little as 20 minutes.

The anatomy of the stomach itself is comprised of two sections: the Non-Glandular (also referred to as the "top" section, or squamous section) where the food enters, and the Glandular (also referred to as the "bottom" section, or gastric section) where the highly corrosive acid - hydrochloric acid, is produced. The non-glandular section of the stomach has no mucous layer to protect from the acidity of the stomach but is protected both by the buffering effect of the saliva produced by chewing and the physical barrier of the chewed fiber. The glandular section is protected by a mucous layer against the acidic pH level of 1.5-2 of the stomach contents. Very little digestion takes place in the stomach, and while there is some protein breakdown, no absorption of nutrients occurs here.

The food matter then moves to the Small Intestine.

The Small Intestine
The small intestine is named for its diameter rather than its length. It is 21 - 25m long, makes up 75% of the total gastrointestinal tract (GIT) and contains up to 30% of the volume of the horse's GIT. The inner wall of the small intestine is covered in 0.5-1mm long finger-like projections called 'villi' that themselves may have microscopic projections called micro-villi. These allow an increased surface area which maximizes the absorption of nutrients from digesting content. The outer layers of the intestine have longitudinal and circumferential bands of smooth muscle that rhythmically contract and relax in response to involuntary nervous and hormonal signals. These involuntary contractions are called 'peristalsis' - which facilitate the movement of food boluses down the length of the GIT. These waves move around 30 cm/minute and a bolus of food will take between 45 minutes to 2 hours to traverse the entire length of the small intestine. The small intestine absorbs protein, fats and oils, and some soluble carbohydrates.

The small intestine is comprised of three parts:

1. Duodenum
The duodenum is entered via the pyloric sphincter of the stomach. The acidic bolus of food from the stomach is neutralized by bicarbonate secreted from the Brunners Glands, bile salts from the liver via the bile duct, and digestive enzymes such as trypsin, lipase and amylase from the pancreas. The pH level of the stomach contents is 1.5-2 (very acidic). Bile buffers the bolus to ph 7-7.5 allowing digestive enzymes to activate and ions to be transported across the intestine wall to the blood; bile salts surround and emulsify fat molecules to allow further enzymatic digestion by lipase enzymes from the pancrease; and pancreatic enzymes allow the breakdown of starch, protein and fat into their constituent "building blocks".

2. Jejunum
The jejunum is the middle section of the small intestine and is mainly involved with the absorption of the most basic products of carbohydrate and protein digestion - monosaccharides and amino acids.

3. Ileum
The ileum is the last section of the small intestine and is characterized by the presence of Peyers Patches towards the distal end. Peyers Patches are regions of lymphoid tissue that produce protective cells that prevent potentially harmful microbes from the hindgut from colonising the small intestine. The ileum terminates at the Ileo-Caecal Valve.

Caecum
The caecum is located at the junction of the small and large intestine. It the first part of the large intestine after the ileum. It is lined with mucus-secreting glands and has no digestive enzymes. This organ is 1.2 metres long and has a capacity of ~30 litres. It makes up ~15% of a horse's overall gastrointestinal tract. Digesta enters the large intestine through the ileo-caecal valve (this valve prevents back flow of the digesta) and leaves through caeco-colic valve. The caecum is often referred to as a 'large fermentation vat', and is likened to the rumen in a cow. Here, a slow process of fibre break down occurs with the help of millions of bacteria and protozoa. Heat is produced as a byproduct of the fermentation process, so it is important for the horses gut to have a steady supply of forage.

Large Colon
The large colon is between 3 and 3.5 meters long, with a 90 litre capacity. Here, fibre continues to ferment, and nutrients continue to be absorbed through the gut lining.

Small Colon
The small colon, is the same length as the large colon, 3-3.5 metres, but is smaller in diameter. The main function of the small colon is to absorb water which helps to keep the horse hydrated. Fecal balls are formed in the small colon.