Horses’ have one of the most complex, sensitive, and convoluted digestive systems of any livestock. Almost all mammals have stomachs, except for a small handful of evolutionary anomalies, but how many stomachs does a horse have? As trickle grazers, who are also flight animals, the horse’s stomachs and digestive systems have evolved to have some very unique characteristics.
Horses have a non-ruminant monogastric digestive system. Thus, horses only have one stomach. Horses can quickly graze large areas but are not as efficient at extracting energy from the food as ruminants that chew the cud. Cows and other ruminants have complex multichambered stomachs.
The stomach is an acid-secreting sac that attaches to the distal end of the esophagus and precedes the small intestines, other parts of the gastrointestinal tract, and accessory organs. Almost all vertebrates (animals with a backbone) have a stomach, except for the non-conformist platypus.
Some animals have monogastric digestive systems, which means they only have one stomach. However, other animals have multichambered complex stomachs that facilitate ruminant digestion. Which digestive systems do horses have, ruminant or non-ruminant? Do horses have a stomach and if they do, do they have one, two, three, or more stomachs?
What Is A Stomach
All life on earth requires energy to perform the functions of living, i.e., growing, moving, reproducing, thinking, etc. Plants are primary producers that gain their energy by capturing and utilizing solar energy. The remaining organisms must consume plant matter directly, as is the case with herbivores, or indirectly like carnivores which eat herbivores and other animals.
The ingested food must be consumed through the mouth and stored in the stomach before passing to the rest of the digestive system for the mechanical and chemical extraction of energy and micronutrients from the food.
Scientists and anatomists are sticklers for detail and like to have all the nitty-gritty details clarified so that everyone understands the terminology. As such, they have agreed that a true stomach must possess three features:
- A stomach is an enlargement of the proximal lumen. It may have a spindle shape (early-stage evolution) or a sac-like shape (later-stage evolution similar to the modern stomach).
- A stomach must have a distal sphincter-type structure to control the movement of food from the stomach to the rest of the gastrointestinal tract.
- Lastly, a true stomach must secrete acid to assist in the breakdown of protein.
Evolution Of The Stomach
The gastrointestinal tract and the stomach evolved many millennia ago. Biologists typically study fossils when trying to understand how animals evolved. The majority of the fossils found show only the skeletal anatomy and demonstrate poor preservation of soft tissue. As a soft tissue structure, scientists cannot rely on fossils to provide clues into how, when, and why the stomach evolved.
Instead, scientists use comparative anatomy and physiology to formulate hypotheses on the development and evolution of the stomach. Comparative studies use regression analysis (i.e., working backward) of existing animal species to make educated guesses about when stomachs first appeared in historical animals.
Chordates (animals with a backbone) originally appeared approximately 450 million years ago. The earliest chordates are the prehistoric fish, Acrania, and Agneta. The modern-day descendants of these ancient fish are the lancelets and lampreys, who are both stomachless and do not secrete hydrochloric acid in any part of their gastrointestinal tracts.
The first acid-secreting organs resembling a true stomach were found in the elasmobranchs (sharks and rays) approximately 350 million years ago. The acid-secreting stomach first developed in response to increased protein content in the chordates diet.
The hydrochloric acid secreted by the stomach breaks down complex protein chains into digestible amino acids allowing the animal to access the energy-rich nutrients. As a higher-level chordate, the horse definitely has a stomach.
Animals With No Stomachs
Since the evolution of the stomach, almost all higher-order chordates, including amphibians, reptiles, birds, and mammals, all have acid-secreting stomachs with very few exceptions.
Fish belonging to the teleost class once had stomachs but have since lost their stomachs, as have two types of egg-laying mammals, the platypus, and the spiny anteater.
Scientists continue to be puzzled by the platypus and the spiny anteater because, according to them, both these species should have stomachs. However, as egg-laying mammals, these are rule-breaking rebels, so perhaps it’s not so surprising that they decided stomachs were unnecessary!
Animals With One Stomach
Animals with monogastric digestive tracts have one stomach. Almost all carnivores, omnivores, and some herbivores have one stomach, including humans and horses. Monogastric herbivores like horses have a well-developed cecum with a complex balance of hindgut bacteria to facilitate hindgut fermentation.
Animals With More Than One Stomach
Animals with more than one stomach are always herbivores; they may be true ruminants like cows, goats, and sheep or pseudo-ruminants like camels and alpacas. A true ruminant has four chambers in its stomach the rumen, reticulum, omasum, and abomasum.
Pseudo-ruminants have similar digestion to ruminants, but they do not possess a rumen. Instead, they possess a three-chambered stomach with a reticulum, omasum, and abomasum.
Ruminants Vs Non-Ruminants
Herbivores are animals who extract nutrients from plant material. Most plant cells are encapsulated by a tough layer of cellulose. In order to access the nutrients in the plant cells, herbivores need to break down the cellulose layer surrounding the cell.
Both ruminant and non-ruminant herbivores have evolved to break down fibrous cellulose through fermentation. However, the method of fermentation, location, and efficacy of the fermentation process differs between ruminants and non-ruminant animals.
Animals like cattle and deer lack the enzymes necessary to break down cellulose and instead have evolved to have a four-chambered stomach to facilitate foregut fermentation. The four chambers are the rumen, the reticulum, the omasum, and the abomasum.
When a cow or other ruminant eats vegetation, the food is passed into the rumen. It is mechanically broken up and mixed in with the bacteria necessary to begin the fermentation process. The bacterial flora in the rumen facilitates the breakdown of cellulose, allowing the ruminant to access the plant-based nutrients via bacterial digestion. Fermentation results in the production of large quantities of gas, which is why animals like cows are prone to developing bloat.
The food is then regurgitated courtesy of the reticulum. The cow “chews the cud” before swallowing the partially processed food. Once re-swallowed, the food passes into the omasum, where it is further broken down through mechanical processes.
Following the omasum, the stomach contents are passed to the abomasum, sometimes referred to as the “true stomach.” In the abomasum, acidic secretions containing the enzyme lysozyme destroy the bacteria, releasing the nutrients the bacteria obtained from the plant. Thus, the cow obtains the plant nutrients indirectly by means of bacterial fermentation.
Horses and other Perissodactyla animals have evolved to facilitate hindgut fermentation. These animals are also herbivores, but unlike ruminants, the plant-based material is passed through the acidic stomach before undergoing bacterial fermentation in the cecum.
Advantages and Disadvantages with Ruminants Vs. Non-Ruminants
A horse’s stomach and digestive system have several advantages and disadvantages when compared with a ruminant’s:
- A horse’s stomach is smaller and lighter than a typical ruminant, allowing them to be much faster than many ruminants when running away from predators.
- A horse’s digestive tract can process food faster than a ruminant, allowing large quantities of food to be quickly passed through the digestive system. Thus, horses can maintain their weight better than cows when fed on poorer quality grazing.
- Ruminants are much better at extracting nutrients from plant material than horses. Thus, ruminants don’t need as much food as horses to maintain their weight when fed high-quality fodder.
- Cows and other ruminants take longer to digest food and thus can afford to spend longer periods not actively grazing as they spend part of their time “chewing cud.” Horses cannot go longer than four hours without food before they start being at risk of developing digestive problems.
- The gastrointestinal bacteria in both cows and horses are very sensitive. They must be managed carefully to avoid developing nutritional problems.
The Anatomy of a Horse’s Digestive System
A horse’s digestive system is complex and very long. The horse’s gastrointestinal tract is comprised of the:
- Small intestines
- Large intestines
- Large colon
- Small colon
A horse’s lips and tongue are highly dexterous. They can easily sort through and selectively graze the juiciest morsels while leaving behind everything they don’t want to eat. Once the food enters the horse’s mouth, they begin chewing. Chewing allows the teeth to break down big pieces of vegetation into small pieces that are easily swallowed. Chewing mixes the food with saliva. Saliva performs two essential functions:
- Saliva contains important enzymes which begin chemically digesting carbohydrates.
- Saliva moistens the food, allowing it to easily slip down the horse’s throat and esophagus before entering the stomach.
The esophagus is a long flexible tube that connects the horse’s mouth to the stomach. Chewed food is rolled up into a ball, called a bolus, and moved down the esophagus by a powerful peristalsis action. The peristalsis action causes wave-like contractions to move down the esophagus, forcing the bolus to move down.
Food that is improperly chewed (if a horse eats too fast) or insufficiently moist can become stuck in the esophagus, causing a choke. A choke is a potentially life-threatening situation and needs to be urgently seen by a vet.
The esophagus empties into the stomach via a sphincter called the cardiac sphincter or gastroesophageal sphincter. A sphincter is a circular band of muscles that, when constricted, close off the entrance into the next part of the gastrointestinal tract. When relaxed, the sphincter allows movement of the gastrointestinal contents into the next portion of the gastrointestinal tract.
A horse’s stomach is the smallest stomach of any livestock or even domestic animal when compared to its size. An average horse’s stomach is only 2 to 4 gallons! The horse’s stomach continually produces hydrochloric acid.
This means that although the horse’s stomach cannot hold large quantities of food, the stomach must always have food in it. Otherwise, the acid produced by the stomach will begin destroying the stomach lining. Destruction of the stomach lining will result in gastric ulcers that are very painful for the horse. The horse’s stomach performs three functions:
- The acid secretions containing pepsin begin to chemically break down the food, particularly large protein chains. It also aids in neutralizing many harmful substances.
- The movement of the stomach assists in mechanically breaking down the food and mixing the acid into the stomach contents.
- It acts as a storage chamber and allows a steady trickle of food into the small intestines, thus preventing the small intestine from becoming overloaded with too much food. The transition of stomach contents into the small intestine from the stomach is controlled via the pyloric sphincter. The pyloric sphincter is found at the exit of the stomach and the entrance of the horse’s small intestines.
4. Small Intestines
The small intestines account for approximately 30% of the digestive tract and can be up to 70ft long! The food moves fairly quickly along the small intestines. The gastrointestinal contents can move from the stomach to the cecum at the beginning of the large intestines in as short a time as 45 minutes. The small intestines are the primary site of digestion and absorption for non-structural carbohydrates, proteins, and fats.
5. Large Intestines
The large intestines of the horse easily account for the biggest area of the gastrointestinal tract and have a few distinct features. At the junction of the small intestines and the large intestines, there is a blind end sac called a cecum.
The cecum is very important for a horse’s digestion as this is the primary site for hindgut fermentation. Bacteria in the cecum digest the cellulose (structural carbohydrate) and other nutrients found in vegetation. The process produces large quantities of volatile fatty acids and gas. The fatty acids are the primary source of energy for a horse whose diet is comprised of a high percentage of forage.
The cecum then passes to the large colon, small colon, and rectum. The hindgut is responsible for the fermentation of structural carbohydrates, absorption of volatile fatty acids, vitamin B, amino acids, and water reabsorption. The rectum stores and eliminates fecal matter (non-digestible material) via the rectum.
There are numerous changes in the diameter of the large intestines. The large intestines are the primary site for the majority of horse colics. Excess gas production related to fermentation can cause gas colic. Food may become impacted and form a blockage, especially in areas where the diameter of the colon becomes smaller. Lastly, the large intestines may become displaced or twisted as with a volvulus colic. All types of colic are excruciating and even life-threatening.
Can A Horse Vomit?
A horse cannot vomit, which means that everything a horse ingests must pass through the entire digestive tract before being eliminated via the anus. A horse cannot vomit because:
- The gastroesophageal sphincter is extremely strong and highly resistant to opening under backflow pressure.
- The gastroesophageal sphincter in horses is positioned much lower than in animals that can vomit. When the stomach is enlarged and bloated by gas or the stomach contents, the stomach “folds” against the gastroesophageal sphincter preventing it from opening.
- The horse’s stomach is located deep in the abdominal cavity and experiences minimal squeezing by the abdominal muscles.
- Horses have a weak gag reflex and cannot reverse the peristalsis of their esophageal muscles.
The Best Way to Feed Your Horse: The General Rules
Horses can be fed safely when following a few fundamental principles:
- Horses are trickle feeders and need to have continual access to food. A horse should never go more than four hours without food.
- Grains and concentrates should be fed in small quantities, divided into 2 to 4 meals per day.
- The bulk of a horse’s food should be made up of high-quality roughage. A horse should eat 1.5% to 3% of the body weight per day in roughage. Good doers may need to be fed less and poor doers more, depending on their body condition.
- It is best to feed horses from the ground as this is their natural grazing position and causes the least number of problems for the horse.
- Dusty and dry concentrates and grass should be dampened before being fed to the horse. Dry foods can cause the horse to choke, and dusty food can cause the horse to develop respiratory issues. However, the food should not be made so wet that it resembles soup. A soupy food can cause the horse to swallow air, causing gas colic as they attempt to “bite” the liquid food.
- Always change food slowly over a period of a few weeks to allows the horse’s gut flora to adapt to the new food. A sudden change in food can cause the bacteria to die off, causing colic.
- Never feed a horse urea. Urea is very dangerous to horses and can be life-threatening if ingested.
- Never feed horses directly off a sand floor as it can cause the horse to develop sand colic.
A horse is a non-ruminant monogastric herbivore; in short, they have a single-chambered stomach. At only 2 to 4 gallon capacity, the horse has the smallest stomach comparative to the size of any domestic animal. The inability to vomit is a significant vulnerability for horses; if there is an excess build-up of fluid, gas, or other material in the stomach, the stomach is at risk of rupturing.
However, the stomach needs a constant supply of small feed quantities to prevent the development of gastric ulcers. It is essential when feeding a horse to always consider their physiological and anatomical needs and limitations with regards to their diet.