The journey through the digestive tract

Almost no other topic commÜberschrift 1ands our attention like the feeding of our horses – which is good. No matter if your horse is a Shetland pony or a Shire, athletic partner or leisure companion, chestnut or black: the right feed will help them to enjoy a long and healthy life. To understand what “the right feed” means, we must first take a closer look at the equine digestive process.

First stop: The horse’s mouth

Our tour of the equine digestive tract begins at the mouth. The mouth is where food is taken in, chewed, and salivated. This process alerts the gastric juices, gastric acid, and pancreatic enzymes to the imminent arrival of chyme. The number of chews determines the quantity of saliva produced as well as effects the horse’s satiety: more chewing results in reduced hunger feelings. The horse’s teeth are evolved to chew grass with hard stalks and are adapted for constant abrasion. That is why it’s important that the feed ration contain enough structured crude fibre.

The way to the stomach

After being chewed, the food passes through the oesophagus into the stomach. There are a few places along the way where problems can arise if the horse swallows large feed elements that are not sufficiently broken down and salivated, increasing the risk of oesophageal obstruction.

The equine stomach: small but essential

With a capacity of 15 L in an average-sized horse, the stomach is rather small and is not able to expand significantly, which is why a slow and continual passage of small amounts of feed to the stomach is ideal. The stomach is where protein, fat and starch digestion begin. The stomach is not a storage organ; rather, it passes the pre-digested feed on into the small intestine. Fibrous feed that is heavily insalivated passes through the stomach more quickly. Large quantities of concentrate feed, especially cereals, take longer to pass through, so that their starches begin to ferment, producing lactic acid which has a negative effect on gastric health. Concentrate feeds should therefore be given in small quantities.

Let’s take an even closer look at the structure of the stomach: it is basically divided into two areas. In the glandless upper part, the bacteria living there break down easily digestible carbohydrates into short chain fatty acids. The glandular area is where the chyme mixes with gastric juices. The hydrochloric acid in the gastric juices kills off germs before they enter the gut. The enzyme pepsin starts digesting protein here as well.

The small intestine, the heart of enzymatic digestion

In the 20–25 metre-long small intestine, the chyme is now joined by pancreatic secretions and bile. Because horses don’t have gall bladders, their bile streams continually into the small intestine from the liver. Here, carbohydrates, proteins, and fats are digested by enzymes in the body. Amylase, the enzyme responsible for the digestion of starch, is not produced in large quantities in horses and does not easily adapt to different types and quantities of starch. Starchy feeds that have not been hydrothermally or mechanically processed are especially difficult for the equine gut to break down. The horse digests proteins in the small intestine with help from the enzymes trypsin and chymotrypsin. Lipases digest fats. The wall of the small intestine is equipped with projections, called villi, that help absorb nutrients effectively. Only the fibrous parts of forage and the nutrients which are bound to it pass largely undigested to the large intestine.

The horse – an efficient hindgut fermenter

The digestion of crude fibre is carried out by microbes in the more expansive large intestine, which consists of the caecum, colon, and rectum. This includes cellulose, hemicellulose, and other undigested carbohydrates. The gut flora breaks down crude fibre into glucose and volatile fatty acids. Overall, the small intestine is responsible for the mixing of the chyme, the breakdown and absorption of nutrients, and for the further transport and removal of the feed mixture. It synthesises water-soluble vitamins and is involved to a large extent in the body’s water and electrolyte balance. The feed’s passage in the large intestine can take up to 45 hours, becoming increasingly thicker until the horse eliminates it in the form of horse droppings.

What does digestive physiology mean for the equine diet?

Forage, the basis for every ration
Forage – hay, haylage, straw, etc. – provides not only energy. It also gives the horse something to do, satisfies its need for chewing, and is especially important for problem-free digestion. The horse eats forage slowly and with thorough salivation, providing the optimum conditions for a healthy filling and emptying of the stomach. The microorganisms in the gut also need a continual supply of crude fibre to ensure effective digestive processes.
  • The rule of thumb is 1.5 to 2 kg of hay per 100 kg of body weight per day
  • Because haylage contains more water, the quantity here must be raised correspondingly.
  • You can find more on the topic of forage in our article “The necessity of base feed”.

Use caution with cereals
Cereals provide lots of energy and can be good energy sources in small amounts – as long as there are no pre-existing conditions. Because the enzyme that breaks down starch has only limited capacities, the horse can no longer completely digest large amounts of starch. The starch that is not broken down then passes into the large intestine where it is fermented by bacteria. This process leads to the formation of lactic acid and gases, lower pH value, and digestion imbalances.
  • The rule of thumb is maximum 1 g of starch per 1 kg of body weight per meal and maximum 2 g of starch per 1 kg of body weight per day
  • What this means in practice: The maximum oat intake for a 500 kg horse, for example, would be 1.25 kg per meal and 2.5 kg per day. To take strain off the digestive tract, one should use a cereal with high digestibility or even replace the cereal with other energy sources where possible. This is important, especially for horses that already suffer from metabolic diseases or have digestive tract issues.
  • You’ll find more on the topic of cereals, starch, and cereal-free diet in our articles “Starch in horse feeds” and “Why cereal-free feeds?”.

High energy density in the ration through fats/oils
Using fats and oils provides a high amount of energy through the feeding of relatively small quantities. Vegetable oils especially offer high palatability and digestibility. Oils should be introduced to the horse’s diet slowly in order to avoid digestive problems. Ideally, divide up an oil ration over several small meals. Because horses have no gallbladders, they cannot make optimal use of fats, and undigested fats disturb the gut flora.
  • The rule of thumb is maximum 0.5 g of fat per 1 kg of body weight per meal, maximum 1 g of fat per 1 kg of body weight per day
  • You’ll find more on the topic of fat and energy in our article “Sources of energy in horse feeds”.

Proteins – important building blocks for muscle metabolism
Proteins not only serve to generate energy – they are also essential for developing body tissue for building and maintaining muscle. The amount of protein in the ration should nevertheless not be too high: unutilised protein must be broken down into urea and this puts additional strain on the horse’s liver and kidneys. Instead of quantity, focus on quality: A horse can only benefit from protein sources that can be digested before they reach the large intestine. High protein quality means a large share of pre-caecal digestible protein in the feed’s total protein content. Foals, young horses, and lactating mares have increased protein requirements.
  • The rule of thumb is that protein in the ration should be a scant 10%; avoid more than 14%.
  • You’ll find more on the topic of proteins in our article “Protein in horse feeds”.
Janina Beule, BSc. Organic Agriculture
March 2022, © Agrobs GmbH

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