The DIGESTIVE system is formed by the gastrointestinal tract (mouth, esophagus, stomach, and intestines) and the liver. The functions of the digestive system are eating and digesting the food, absorbing the nutrients and sending them to the bloodstream, and eliminating the non-digested material. The URINARY system is closely associated with the digestive system. The urinary system is formed by the kidneys, ureters, bladder, and the urethra. The functions of this system are controlling the water balance and levels of electrolytes in the horse’s body, and excreting waste substances out of the body.
The horse’s gastrointestinal tract may be compared in part with a ruminant’s (e.g., cows) gastrointestinal tract and in part with a humans’. When compared with a ruminant, the horse has a slightly smaller and less efficient gastrointestinal tract. The big rumen of a ruminant animal is the first place where food is carried by the esophagus in order to ferment the fiber with millions of specialized bacteria and protozoa. Thus, fermentation is the first stage of digestion for a ruminant animal.
Humans and horses have a relatively similar structure of their gastrointestinal tracts. Their main differences are the cecum’s size and activity. Because of a different diet not based on forages, humans have a small cecum, with very reduced fermentative action of fiber from salads, grains (cereals), and fruit. Conversely, the horse digests large amounts of fiber from forages (such as grass and legume) in the cecum, by the action of a large microbial population.
The horse’s digestion, as in humans, starts with the CEPHALIC PHASE, when the brain signals the gastrointestinal tract to receive and digest food, even before eating. This action may start for two reasons:
- Detection of palatable food by the specialized sensory organs (smell, vision, and/or hearing).
- Hunger, in answer to the “biological watch,” triggered by either the horse’s daily feeding schedule or an empty stomach.
The cephalic phase of digestion is very important as the gastrointestinal tract gets ready to do its work. In order to profit from this phase, horses should be fed at the same time every day with palatable and nutritious food.
From the time the horse’s gastrointestinal tract actually is in contact with the foodstuff, there are four phases:
- ORAL PHASE: This phase occurs in the mouth. The lips choose the food to be eaten, such as forage or grain. If the horse is grazing in a pasture, the incisors cut the grass. Food becomes wet in the mouth by large amounts of saliva produced by the salivary glands. The tongue carries the food towards the back of the mouth to be chewed by premolars and molars and then swallowed.
- ESOPHAGEAL PHASE: This is a fast phase that occurs in the esophagus. After food is swallowed, it is conducted by the esophagus to the stomach (whether the horse’s head is kept low, while grazing, or kept high), without any physical or chemical change. Curiously, in normal circumstances, horses cannot vomit like humans and other animal species.
- GASTRIC PHASE: This phase occurs in the stomach. The stomach is described as a small bag where food is taken from the esophagus. Gastric juices are secreted in the stomach in order to start the hydrolysis of the foodstuff, reducing food into smaller nutrients, which can be absorbed by the intestine in the next phase. Some carbohydrates are fermented in the stomach due to the presence of bacteria. A third function of the stomach consists of controlling the passage rate of the foodstuff to the small intestine.
Because a horse in a pasture grazes about 17 hours a day, the stomach is usually not empty at any time. Considering this natural habit and the stomach’s reduced capacity (around 1/10 of the whole gastrointestinal tract capacity), any horse kept in a stall or paddock should have free access to grass/hay (unless the horse is an “easy keeper,” with a very slow metabolism), and, sometimes, grain in small, frequent meals.
- INTESTINAL PHASE: The small and large intestines have the greatest capacity in the gastrointestinal tract, and most changes to the foodstuff occur there. The absorption of nutrients and the excretion of the non-digested portion of the foodstuff also occur in the intestines
- Small Intestine: The small intestine is a long, thin tube, about 50 feet long. One of its functions is the hydrolysis of proteins and carbohydrates into the most simple absorbable forms due to the action of enzymes secreted into the small intestine. The emulsification of fats (lipids) also occurs in the small intestine, due to the action of bile salts. Another function of the small intestine is to absorb nutrients (amino acids, glucose, fatty acids, vitamins, and minerals).
- Large Intestine: The large intestine is a much wider tube than the small intestine, but much shorter (about 20 to 22 feet long). The large intestine has three main parts: cecum, colon, and rectum, that work in different ways.
The cecum is an elongated bag (about 4 feet in length), the top end of which joins both the small intestine (at its end) and the colon (at its entrance). The main function of the cecum is fermenting the fiber from forages and any carbohydrates that are passed by the small intestine. Fermentation occurs through specialized microbial populations (bacteria and protozoa), similar to those in the rumen of ruminant animals. Volatile fatty acids (acetic, propionic, butyric, valeric) are the results of this fermentative action. They are absorbed in the cecum to be used later as energy. The cecum is also the primary site in the gastrointestinal tract where water is absorbed.
The colon, in its different sections (ventral, dorsal, transverse, and small), is the largest portion of the large intestine. The passage rate in the colon is slow, as it is the last opportunity for digesting the foodstuff, and the water content has been reduced in the cecum. The ventral colon also has a small microbial population specialized in fermenting fiber.
The rectum is the last portion of the gastrointestinal tract. The function of the rectum is to absorb water and compact the material to be eliminated as manure.
Note: Read here about The nutrients required by horses and their digestion