Article: Breaking Down Digestion
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Article: Breaking Down Digestion: How Your Horse Digests Its Feed
Article Info: October 7, 2009: Medical Intuitive, Lizzy Meyer explains the process of feed digestion in horses with an example: Digesting a Meal of Grain and Hay.
Feed Keywords: Digestion, Fiber, Stomach, Intestine, Enzymes, Bicarbonate, Volatile Fatty Acids
By Lizzy Meyer
October 7, 2009

From the moment a horse chews feed, his body is primed for an intensive activity: digestion. In order to support your horse’s gut, it’s vital to understand the basics of how their system is capable of digesting certain feeds. Horses were originally designed to eat forage and small amounts of seeds. Their stomach can only hold about two gallons at a time. Here, we are going to look at the way a horse digests a typical grain and hay meal.

Digesting a Meal of Grain and Hay

The basic ingredients here are starch (carbohydrates), protein, fat, and fiber. As the grain is chewed, amylase (a digestive enzyme) in the saliva begins breaking down starch. This saliva is alkaline since it contains bicarbonate, which will buffer acid in the stomach. As the food bolus nears the esophagus, it must pass through the cardia, which is the muscular sphincter on the top of the stomach. This muscle is responsible for the reason horses are unable vomit.

As the stomach begins to fill, the stretch receptors trigger the G cells to make Gastrin. This hormone is then released into the blood. Gastrin causes the top part of the stomach to relax so more food can fit inside. The Parietal cells in the stomach then are triggered by Gastrin. After these hormones are activated, the most powerful part of digestion can begin, secretion of Hydrochloric acid (HCL). As a specific adaptation to grazing, horses secrete Hydrochloric acid HCL from the glandular part of the stomach at all times. Secretion of hydrochloric acid involves the actions of hormones, chemicals in the gut, and various stimuli. Following HCL secretion, a protein-digesting enzyme called Pepsin is activated and proteins are broken down into smaller particles.

After this series of complex chemical reactions, the grain is now reduced into emulsified fats and very small proteins. This slurry is now called chyme. The small intestine accepts the downward flow of gastric juices and chyme. The acidic chyme causes the pH to drop to an acidic level in the intestine. Secretin is a hormone that is sensitive to acid changes and it tells the pancreas to secrete bicarbonate to buffer the acid. CCK is a hormone released by the body in the presence of fats and protein in the small intestine. This hormone’s action slows down the peristalsis in the stomach and alerts the stomach not to send additional food down to the intestines.

The small intestine is now at work digesting additional protein in the grain. Because the stomach began protein digestion with the enzyme, pepsin; the pancreatic enzymes can take over the rest of the job. The body then releases bicarbonate by the pancreas to alkalize the environment in the small intestine. Messages go through hormone pathways to the stomach to tell it not to send any more food to the small intestine, so it can absorb what has just arrived. After a long chain of enzyme activities, the final products are now amino acids and are fully absorbable.

Next, the breakdown of fats begins. The pancreas secretes additional amylase and especially lipase enzymes to digest fat. As lipid globules flow further down the intestine bile acids are secreted by the liver. Horses do not have a gallbladder to store bile. Due to their grazing nature, they constantly trickle bile into the small intestine to break down small food particles at a time.

Starch passes through the small intestine with marginal absorption. As the starch continues to seek a place where it can be digested, it crosses into the large intestine. The large intestine is not designed to ferment starch and creates microbial fermentation by-products, which can cause harm. These microbes are only designed to digest fiber. Starch is very rich and causes the microbes to over-produce and then upset the delicate balance of the hindgut.

Now that the grain has been digested after this lengthy process, the fiber that has flowed into the hindgut can finally be processed. It has gone along for the ride with the grain, but the body really has put no effort into digesting it yet. Fermentation, water/electrolyte absorption all takes place in the hindgut. The fermentation vat of the cecum takes over with hungry microbes that break down fiber into usable energy in the form of Volatile Fatty Acids (VFA’s). This is the main source of energy for horses. Not only is this section of the gut responsible for fiber digestion, but it uses bicarbonate to buffer acid and it uses prostaglandins to maintain the health of the mucous producing cells in the gut.

Fiber is the easiest feed for a horse to digest. It does not stress the stomach, the small intestine, or the large intestine. It basically travels straight through the gut and becomes valuable “microbe food” here and feeds the rest of the body VFA’s. The grain, or starch component requires more steps overall to be digested and many more enzymatic processes, more chemical processes, and more coordination with various hormones.

Feeding small amounts of whole grains is not a problem for most horses. But, emphasizing a high fiber and low fat diet will yield a healthier gut in the long-term. A diet that respects the physiology of a horse’s gut, one that includes minimally processed whole foods is one of the kindest things we can do to help the horse’s gut function optimally.

Lizzy Meyer
Illustrative Videos
  • Left Video shows the functionality of proton pump inhibitors like Ranitidine, Omeprazole (Gastro Guard), and Cimetidine. This functionality is the same in both humans and horses.
  • Right Video is courtesy of our friends at Succeed® and illustrates digestive action in the hindgut.



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