Life requires a permanent consumption of energy that every living being must cope with to ensure the functioning of its organs and the integrity of its body structures. Thus, animals ingest various types of food that must be transformed into nutrients, that is, reduced to “elementary bricks” that can be used directly by the body. Digestion is the set of mechanical, secret, and chemical acts that enable this transformation. Every self-respecting rider and owner should know how the digestion process works in horses.
The proximal part of the horse’s digestive tract
In the horse, the ingestion of liquids is done by pumping with the intervention of the cheeks and tongue, which causes a depression in the oral cavity. The upper lip, which is very sensitive and mobile, is used to pick up and sort food. Plants are cut by the sharp incisors, then crushed by the grinding molars. As in all herbivores, chewing is long and energetic, to allow the reduction of coarse food, rich in poorly digestible substances. To ingest 1 kg of hay, a horse takes about 3,000 chews in 40 minutes! The teeth are well adapted to this task as they grow throughout the animal’s life. If the teeth are not worn down enough or only partially, when the horse does not eat enough rough feed, sharp enamel points called “teeth on teeth” form at the edges of the molars and can injure the tongue and the inside of the cheeks. The pain then prevents the horse from eating and chewing normally. Herbal medicine can be helpful in easing the digestion process.
During chewing, the salivary glands produce a large amount of saliva (5 to 8 1 per hour) that allows the food to be hydrated (formation of the “food bowl”) and facilitates swallowing. Due to the highly developed soft palate, which divides the pharynx into a respiratory and digestive tract, it is necessary to provide feed adapted to the animal’s chewing ability, since swallowing is only possible with finely ground food bowls.
The esophagus is a long and narrow tube in the horse, which makes it prone to accidental obstructions. After swallowing, the bowl of food is carried to the stomach. Due to its small capacity and relative inability to dilate, it is “emptied” several times during a meal. Its functioning is regular and remains the same whether the animal is fasting or fed at will. It is organized in periods of 2 hours, of which 4/5 correspond to a powerful contractile activity that allows the mixing of the food bolus, its impregnation by gastric secretions and its transformation into a kind of thick soup (the chyme) that corresponds to the beginning of the chemical transformation of the compounds. A small part of the gastric contents is expelled through the pylorus during each contraction of the distal part of the stomach. It then reaches the middle part of the digestive tract.
The middle part of the horse’s digestive tract
This is formed by the small intestine, where secretions from the glands are discharged and promote the further chemical transformation of the food particles. These secretions are alkaline and allow the acidity of the gastric chyme to be neutralized. The conditions are then ideal for the activity of the enzymes of the pancreatic juice (proteases, lipases, amylases), which is secreted mainly at mealtime. Due to the absence of gallbladder in the horse, bile is not stored, but continuously discharged. It allows fats to disperse into very fine droplets that can then be “attacked” by pancreatic lipase. Proteins, fats, and soluble sugars are well digested by the horse’s small intestine. The insoluble sugars “trapped” in the walls of the plant fragments undergo almost no processing.
Once they have entered the intestinal lumen, the nutrients resulting from the “enzymatic disassembly” of the food particles are absorbed by the mucosal cells and then released into the blood and lymph for distribution throughout the body. Like the stomach, the small intestine has a cyclical function organized in 2-hour periods (whether the animal is fed or not), of which 4/5 correspond to successive contractile activities of two types: contractions propagated over several tens of centimeters that ensure the progression of the intestinal chyme downstream, and localized contractions that isolate part of the intestinal contents and ensure their mixing. In the duodenum, these localized contractions occur when the stomach is at rest and are responsible for the reflux of part of the duodenal contents through the pylorus into the gastric cavity. This periodic reflux explains the wide variations in gastric pH (from 1.5 to 7.5) that can be detected in equids. Food transit is very rapid (2-6 hours) in the relatively short, small intestine of the horse. Feeding large meals, especially when rich in concentrates, increases this tendency.
The distal part of the digestive tract of the horse
The undigested material in the small intestine then reaches the distal part of the digestive tract or “large intestine”. This is composed of a series of huge “pockets”: the cecum, the colon, and the rectum. The total volume of the large intestine is estimated at 140 1 in horses, 70 1 in donkeys, and 30 1 in ponies. The cecum and colon are considerably enlarged in horses. This large volume, associated with a high temperature (about 40°C) and the presence of numerous microorganisms, allows them to act as true fermentation tanks. Most of the insoluble sugars contained in the walls of the plant particles undergo a microbial attack in these compartments and are thus transformed into nutrients that can be assimilated directly by the body. These phenomena are facilitated by the clear slowing down of the intestinal contents in the cecum and colon (16 to 56 hours, depending on the diet).
The cecum has a capacity of 30 to 40 1. As soon as it is distended by the emptying of the small intestine, its walls develop a localized contractile activity (2 to 5 phases of 3 to 6 minutes per hour) that allows for the agitation of the cecal contents (this is the “agitation system” of the fermenter) and a propagating contractile activity (about 30 times per hour) that ensures the progression of the cecal contents toward the colon. The solid particles remain in the cecum for an average of 4 to 8 hours. When they leave the cecum, they enter the colon, which has two parts: the large colon and the small colon.
The large colon is an elongated reservoir with a capacity of 80-130 l. Food particles move through it due to the existence of propagated contractile activity that directs them toward the small colon, but also undergo, in certain areas, considerable deceleration due to contractile activity propagated in the opposite direction to the previous one (retrograde activity). This activity allows solid particles to remain in the large colon for very long periods (12 to 48 hours, depending on the diet), which gives the microorganisms enough time to digest them efficiently. The small colon extends the large colon toward the rectum. It is home to important localized contractile activity that slows the progression of matter and facilitates important water reabsorption. This dehydration of the contents facilitates the fragmentation of the stool and the molding of the manure, which is stored in the rectum and then eliminated during defecation. It corresponds to a propagated contraction of the rectal walls with relaxation of the anal sphincter. The emission of feces is frequent in herbivores (10 to 24 times every 24 hours).
If understanding the digestive system allows one to better consider the question of feeding, it is always important to consider the activity of the horse. The needs of a leisure horse will be different from the feeding needs of a competition horse. To provide a balanced diet to your horse and best meet its nutritional needs, discover the products of the Royal Horse brand!
