Once the feed is masticated (chewed), the bolus passes through the oesophagus and enters the stomach, as highlighted in the 3D illustration below
The stomach itself is made up of 2 sections, please see diagram below:
The stomach has what we call a non-glandular and a glandular region. The non-glandular region is at the top of the stomach. That cardiac opening is where the feed comes in from the mouth via the oesophagus. The area which we refer to as the fundus, is an area of the stomach which has no protection from the acids and enzymes found within the stomach.
Down the bottom of the stomach is the body and the pyloric region. This is where a lot of what’s commonly referred to as the digestive juices are retained. These are the acids and enzymes designed to start breaking down the food chemically. The pH in that area is about 2 meaning it is extremely acidic. These acids are designed to break down most of the feed components.
The body and pyloric regions are very much protected against these acids and enzymes. In fact, you can see that the wall of the stomach is very different in the body and the pyloric compared to the fundus. The fundus has quite large areas of what we call raised villi and they are very, very sensitive to acids and enzymes. Equine gastric ulcers often occur in the fundus, whether it be through stress, restricted feeding, or other causes because acids rise up into that area, literally burning a hole through the stomach lining, causing a lot of pain and discomfort.
So, the stomach can be divided into two compartments being the non-glandular or squamous region, and the glandular region being made up of the body and the pyloric.
The horse has a very small stomach with a capacity of only about 10-20 litres meaning feed passes through rather quickly. It holds only about 8.5% of the total volume of digesta in the gastrointestinal tract. If it is a slowly digested feed, such as hay, then it will sit in the stomach for longer. This is a good thing because it can use up a lot of these acids and enzymes, protecting the squamous/ fundus area. You can promote this effect by feeding your horse a lot of fibre.
The stomach has a strong influence on eating and feeding behaviour. Being rather small, a horse’s stomach fills very quickly. Horses are what we call trickle feeders which means that they must constantly graze for up to 18 hours a day, usually lasting between 30 minutes and 4 hours at a time only broken up with periods of rest, disturbance (primarily by us humans) and drinking. You’ll find in a 24-hour period the balance of the time is generally sleeping so they don’t spend an enormous amount of time asleep. Peak grazing times are early in the morning, late in the afternoon, and night. Night time grazing is punctuated with dozing and sleeping. They don’t sleep continuously like humans, for an average of around 7 to 8 hours. They might sleep for 2 or 3 hours and then they’ll graze for a bit, and then they’ll sleep again and then they’ll graze for a bit and they will do that throughout the night.