The equine digestive system is not dissimilar to other mammalian digestive systems in that it provides the animal with sustenance, nutrients, vitamins, minerals and disposes of waste food products which are not digestible.
The horse has 12 incisor teeth which, when guided by the lips clip off grass and other vegetation for grinding by the pre molars and molars. The horse has hypsodont teeth meaning they are high crowned and continue to erupt strong enamel crests capable of grinding grasses. The salivary glands, which can produce 10 gallons of saliva per day contain bicarbonate and amylase which starts the digestive cycle. The mastication of the food into small clumps pass through the oropharanx and to the oesophagus.
The oesophagus is approximately 50 to 60 inches and stretches down the neck to the stomach. The stomach is quite small compared to the horse’s size and has evolved to suit a steady trickle of food. The capacity being just 8 to 12 litres with food passing as quickly as 15 minutes through to the small intestines. It also has a very strong sphincter at the entrance from the oesophagus allowing accelerated movement from the horse at any time, but this also means the horse is unable to vomit or release gases produced by the stomach during times of digestive upsets or excessive fermentation. The stomach’s location within the horses also prevents regurgitation due to it being centralised and not in contact with the ribs or abdomen.
The first part of the stomach the saccuc caecus, where hydrochloric acid breaks down food solids and pepsin breaks down proteins. As food moves through the stomach to the pyloric region fermentation is reduced and pepsin protein break down is increased before entering the small intestine.
Unlike the human stomach for instance, the stomach of the horse continually produces gastric acid. This can cause problems for the horse which has restricted unnatural feeding habits due to stabling and/or heavy competition schedules or particular yard management policies.
Domestication has resulted in horses not always having access to a correct continual food source and therefore digestive disorders not seen in wild horses such as gastric ulcers, colic and microbiome disturbances are created.
Gastric ulcers tend to form on the non-glandular region of the stomach lining rather than the glandular region. When the horse has an empty stomach ulcers form on the exposed non glandular lining which has little protection against gastric acid. Horses with access to constant high fibre diets are very unlikely to develop ulcers. However horses with restricted access to high fibre and/or high grain diets and/or stressful environments are likely to develop them.
Foodstuff passes through the pyloric (final) region of the stomach to the small intestines which is a tube approximately 25 metres long and with a capacity of roughly 45 litres. It consists of the duodenum, jejunum and ileum and secretes its own enzymes to facilitate major digestive processes. The small intestine is the primary location for soluble carbohydrate absorption as glucose and sugars from grains. The horse does not have a gall bladder so the small intestines are supplied bile via ducts straight from the liver and additional digestive enzymes from the pancreas. Pancreatic enzymes breaks down protein into amino acids while the bile breaks down fats. The small intestine is also the primary location for fat digestion and absorption. Simple chain carbohydrates as glucose sugars are absorbed through the intestinal wall to the blood stream and sent to the liver.
Most of the carbohydrate and amino acid digestion and absorption from grains is done in the small intestines. But equally important vitamins A, D, E and K and minerals calcium and some phosphorus are all absorbed in the small intestines.
It can take 30 to 90 minutes for food to be fully digested within the small intestine and the successfully absorbed through the walls and into the blood stream. If the digestive process does not have enough time or quantities of grain are too high semi digested foods as soluble carbohydrates and starches can pass into the hind gut. The fermentation process of the hind gut to these carbohydrates and starches produces lactic acid which effects the PH of the microbiome within the gut. If the PH balance is affected with certain microbial bacteria being killed off toxins are released into the blood stream. This can then lead to bouts of colic, digestive up sets and/or laminitis in some horses.
The remaining food still to be digested is structural carbohydrates in the form of fibrous cellulose material, from grasses, hay, hayage and straw chop or chaff, It is almost unaffected by the digestive enzymes of the stomach and small intestine.
This material is passed from the ileum to the hind gut which is composed of the cecum, large colon small colon, rectum and anus.
First to revive the digesta is the cecum, a large fermenting organ unique to the horse which has a capacity of approximately 35 litres. It contains bacterial and microbial protozoa populations which produce enzymes able to breakdown fibrous structural carbohydrates into fatty acids and digestible energy, which along with B vitamins and a major source of phosphorus are absorbed through to the blood stream as a source of cellular energy and for metabolic functions.
It is within the cecum and colon that the microbial populations are specific to the type of feed stuff digested on a regular basis. This is why the diet should always be adjusted slowly as to aid in the adaption of the cecum and colon microbiome, avoiding digestive disruptions. The capacity of the cecum and large colon is large and designed to slow down the passage of food to aid in the complete digestion of all nutrients, vitamins, minerals etc. It is also a major component in the absorption of water. The small colon is the gateway to the rectum and the excretion of faecal matter. The small colon absorbs any last moisture and water not absorbed by the cecum and large colon and forms balls of faecal matter.