By Suzie Middlebrook BSc. | Cavalor Nutrition Specialist
When it comes to horses, there is quite a lot that happens between meal time and mucking time. The digestive tract of the horse is incredibly efficient at obtaining vast amounts of energy from forage that would be almost unusable to many other animals such as humans or pigs. To achieve this, the horse’s digestive tract developed some unique adaptations and compromises to best take advantage of this plentiful resource. By learning more about how the horse’s digestive tract operates, horse owners are better able to determine the best feed for their equine partners.
As one may have expected, digestion starts in the mouth. The mouth, or headgut, is comprised of lips, a tongue, teeth, and an oral cavity. The lips of the horse are prehensile, meaning that they are very flexible and able to pick up small objects, such as a blades of grass or a peppermint from your hand. After the lips have determined the targeted food, the teeth are what then grip the food. The horse has three main types of teeth: the incisors, the premolars, and the molars. The incisors grip and tear while the premolars and molars help further break down the food. This mechanical breakdown of the food is important to reduce fiber length and break open any husks, such as those found on whole grains and seeds. The tongue works to mix the food in the mouth with the saliva secreted. Once the food is thoroughly mixed with the saliva and reduced in size, it is swallowed, travels down the esophagus, and into the stomach.
In some cases, a horse may seem to be gradually losing weight and become a pickier eater. Often, the cause of this is that the horse’s teeth may begin to develop painful hooks and points on their teeth. These hooks can impede the horse’s ability to properly chew their food and can even cause sore and inflamed ulcers to develop in the cheeks and tongue. These ulcers can be so big that a finger can easily slip into them when checking a horse’s teeth. While these hooks and points develop naturally overtime in many herbivorous animals, we can manage them easily in the horse by having a licensed dentist or veterinarian maintain the teeth using a combination of hand and power tools. Maintaining your horse’s teeth on an annual basis can improve your horse’s weight along with their performance under saddle since accepting the bit is no longer a painful ordeal.
The stomach is the sack-like structure that collects all ingested food and where both chemical and enzymatic digestion begins. The lower portion of the stomach has a special acid-resistant lining that helps contain the acidic contents of the stomach. This same lining is what releases a potent chemical called hydrochloric acid and an enzyme called pepsin. The hydrochloric acid helps denature or unfold proteins, kill harmful bacteria, and activates some digestive enzymes. In the digestive tract, enzymes are essentially proteins that open and embed themselves into a nutrient to disassemble a variety of different chemical structures. Pepsin is an enzyme that helps begin the breakdown process of the unfolded proteins. Along with protein breakdown, the stomach also helps regulate the release of ingested food into the small intestine.
Gastric ulcers are a common problem in horses and part of it may be due to the type of hay and grain you’re feeding. Since acid is constantly secreted into the stomach, it is important that the horse has some sort of forage in front of them as often as possible. In cases when obesity is a concern, incorporating straw into the diet is a great way to help fight boredom or an empty stomach without excess calories. Also, since the stomach is intended to digest proteins, incorporating a high-protein forage in the horse’s diet, such as alfalfa, can be another way to help reduce the risk of ulcers without pharmaceuticals. For example, one could feed a flake of alfalfa daily for lunch or incorporate a feed that contains long-stem alfalfa fibers into your horse’s daily grain ration.
The small intestine is where the majority of nutrient breakdown and absorption occurs. The small intestine is lined with finger-like structures called villi. These villi are like little bristles that help increase the amount of surface area within the small intestine. In the small intestine, more surface area means more area to absorb nutrients. The same principle is used with microfiber towels, whose small and densely packed fibers help increase the amount of surface area to help speedily absorb excess liquid like when drying a car.
There are three main regions of the small intestine: the duodenum, the jejunum, and the ileum. The first section right after the stomach is called the duodenum. The duodenum receives digestive juices from its nearby partners, the liver and the pancreas. The liver produces a green liquid called bile that helps emulsify or break up larger globules of fat into smaller droplets. While bile is stored in a gallbladder in most animals, the horse’s liver secretes the bile directly into the duodenum. In the wild, the gall bladder’s absence is inconsequential since horses are naturally all-day grazers. However, in a domestic setting, it means that horses are ill equipped to able large amounts of fat that are sometimes introduced into their diets by well-meaning owners. Studies suggest that long-term feeding of high fat feeds can cause a phenomenon called duodenal reflux to occur due to the stimulated over productive of bile. Duodenal reflux is when the caustic bile in the small intestine back splashes into the stomach. This bile can then aggravate the sensitive stomach lining and exasperate any gastric ulcers present.
In addition to the liver, the duodenum also receives secretions from the pancreas. The pancreatic juices are the primary source for a variety of chemicals and enzymes which help further breakdown macronutrients. The main component of these pancreatic juices is a chemical called bicarbonate which helps reduce the acidity of the ingested material. While the stomach can handle very acidic contents well, the small intestine is not meant to handle such caustic substances. The bicarbonate helps neutralize the acid so that it does not injure the lining of the small intestine. In addition, this neutral environment allows the enzymes that are also found in the pancreatic juices to work most efficiently. These enzymes help breakdown macronutrients such as proteins, fats, and easily-digested carbohydrates. The more complex, fiber-based carbohydrates are digested later on in the hindgut.
After the duodenum, comes the jejunum and then the ileum. Both of these portions of the small intestine are primary locations for nutrient breakdown and absorption for starches, sugars, proteins, and fats along with minerals and some vitamins such as vitamin E.
The last part of the small intestine, the ileum, leads to a fork-in-the-road between the cecum and the colon. These two structures are collectively known as the hindgut and is where the majority of the microbial “magic” happens in the horse’s digestive tract. The cecum is a huge sack containing entire microbial communities comprised of innumerable different bacterial, fungal, and protozoan species. The entire cecum is a fermentation vat dedicated to the digestion and processing of fibers found in a horse’s normal diet by these microbial populations. The microbes are able to digest the tough, fibrous components found in forages such as hay and grass. An important thing to note about the hindgut is all the enzymes and chemicals that assist in the breakdown down of complex carbohydrates in this portion of the horse’s digestive tract are solely produced by the microbes and not by the horse. Realistically, one can look at the fiber portion of a horse’s diet as what feeds the microbes while the non-fiber portion is what feeds the horse. The only contribution made by the horse in this region of gastrointestinal tract is mucus secreted by the lining of the large intestine for lubrication purposes. The cecum itself has several sacs and interior pockets in order to increase surface area and slow down the movement of ingested material. The slower the rate of passage within the cecum, the greater amount of fermentation can occur.
Microbial fermentation has several benefits for the horse. The most obvious advantage is that the horse is able to have access to the energy locked up in the fiber that would otherwise be unavailable. The microbes are able to digest fibers that make up tough cell walls found in plant and convert them into volatile fatty acids, or VFAs. These VFAs are essentially fats that the horse is able to absorb in the large intestine and utilize as a valuable source of energy. It is believed that as much as 60% of the horse’s daily energy requirements are met by these VFAs. These microbes also create a plethora of different vitamins such as vitamin K and the various B vitamins.
After circulating within the cecum, the material will then leave the cecum and continue into the colon. Like the cecum, the colon lined with shallow pocket-like structures to help facilitate the additional microbial fermentation. The colon is commonly referred to as the large intestine and is comprised of a large tube-like structure that meanders throughout the rear part of the horse’s body cavity. It is important to note that the colon makes several hard U-turns on its way to the exit of the horse. These U-turns, or flexures, are potential sites of impactions since they act can cause a bottleneck effect if the horse’s digestive ability is compromised in some way.
The colon is primarily responsible to the absorption of water along with the uptake of vitamins, VFAs, and proteins that are a result of the microbial fermentation. The proteins that are found in the colon are actually the result of sloughed microbes and intestinal lining, both of which are constantly turned over and renewed. By reabsorbing these proteins, the horse is able to take advantage of high quality protein sources that would otherwise go to waste. If you have a horse that seems to have particularly odorous feces, this could be a sign that absorption of nutrients in the colon is impeded for some reason. The distinctive aroma is caused by excess VFAs, which are very smelly chemicals, being excreted with the feces. In a healthy horse, the VFA’s are absorbed and utilized as an energy source. Some potential causes of the poor nutrient absorption could be poor diversity of microbes in the hindgut, harmful mold or fungi exposure, or hindgut ulcers. In many cases, adding a high quality probiotic supplement is enough to address the issue.
Near the end of the colon, the remaining material is primarily comprised of microbes, indigestible fiber, and water. The colon will perform involuntary contractions and relaxations of the muscles to help remove additional water from the feces prior to defecation. These muscle movements also help form the fecal balls or “apples” that one expects to see from a well-fed horse.
By understanding how a healthy digestive tract should function, a horse owner is better able to understand when and why something may be not quite right as well as a potential solution. For example, if the horse seems prone to developing gastric ulcers – even though the horse is correctly managed otherwise – they could benefit from a diet higher in protein through alfalfa hay or higher quality grain. Or, if the horse is prone to producing liquid with normal fecal balls, then one can narrow down the source of the cause to the large intestine, which is responsible for absorbing excess water from the feces. The best way to identify when something is amiss with your horse’s digestion is to understand and be able to identify what is normal and healthy.
Berschneider, H. M., A. T. Blikslager, and M. C. Roberts. 1999. Role of duodenal reflux on nonglandular gastric ulcer disease of the mature horse. Equine Veterinary Journal – Supplement. (29):24-9. PMID 10696289
Collinder, E., A. Lindholm, T. Midtvedt, and E. Norin. 2000. Six intestinal microflora-associated characteristics in sport horses. Equine Veterinary Journal. 32(3):222-7. PMID: 10836477
Edwards, M. (2009, October). The Gastrointestinal Tract. Equine Nutrition. Lecture conducted from California Polytechnic State University, San Luis Obispo, CA.
Jurgens, M. H. and K. Bregendahl. Animal Feeding and Nutrition. 10th ed. Dubuque: Kendall/Hunt, 2007. Print.
National Research Council of the National Academies. Nutrient Requirements of Horses. 6th Ed. Washington, DC: National Academies, 2007. Print.
About Suzie Middlebrook, BSc | Cavalor Nutrition Specialist
Suzie Middlebrook is a budding equine nutritionist and lifelong equestrian. She graduated from California Polytechnic State University in San Luis Obispo, CA with her bachelor’s degree in animal science where she focused on equine and wildlife nutrition. While there, Suzie was instrumental in a number of feeding trials and experiments that looked at the fiber and mineral digestion in a number of different herbivorous species, including the red panda, the leopard tortoise, and the horse. Her personal project looked into developing an accurate and practical digestion model for the endangered red panda which she presented as an undergraduate at the Association of Zoos and Aquariums/Nutrition Advisory Group conference on Zoo and Wildlife Nutrition in 2011. Suzie loves talking and teaching about the peculiar nuances of equine nutrition and is able to do so as the Nutritional Specialist for Cavalor North America. When not “nerding out” on nutrition, Suzie enjoys competing her two horses in dressage and eventing, trail running, and playing with her boxer dog.