The digestive system in action
“Your horse’s lips and sensitive muzzle mark the start of his digestive system. His lips, tongue and teeth are designed to grasp food, which they do effectively,” explains Sara Fleck MRCVS. “Horses can move their jaws from side to side, back and forth, and also have very moveable tongues and lips — to the point where they could just about form words.” Once food is in your horse’s mouth, his teeth break it down by crushing and grinding, which releases nutrients from inside the plant cells. “The food becomes a soft pulp and is made into a bolus [ball] by the tongue,” says Sara. “When your horse swallows, the trachea is blocked by the epiglottis to make sure food goes into the oesophagus and not the windpipe.” The oesophagus is a muscular tube about 1.5m long that transports food from the mouth to the stomach by rhythmic contractions known as peristalsis. The bolus then passes through the cardiac sphincter and into the lumen of the stomach. The sphincter acts as a one-way valve, which explains why horses can’t vomit.
Spotlight on the stomach
The stomach is a muscular organ that’s about the size of a rugby ball (between nine and 15 litres), which is very small. “This is because horses are designed to trickle feed, eating little and often, and the majority of digestion occurs in the hindgut,” explains Sara. The stomach contains gastric juices, pepsin and hydrochloric acid, which break down food into a semi-digested liquid (chime). It is made up of two types of tissue and the lower section is lined with glandular mucosa, which secretes acid to help digestion. The stomach is divided by a line called the margo plicatus. “This area also produces mucus and has protective mechanisms to ensure it’s not damaged. The upper section is lined with squamous mucosa, which doesn’t have this protection so is therefore vulnerable if it comes into contact with stomach acid.” Once processed by the stomach, semidigested food passes through a valve (the pyloric sphincter) into the small intestine. “Horses don’t have multicompartmented stomachs like cattle, sheep and goats do,” explains Sara. “Instead, they have a simple stomach similar to a human’s. Preliminary digestion occurs first in the foregut (mouth, stomach, small intestine), before fermentation takes place in the hindgut (caecum). Horses are therefore known as hindgut fermenters.” The stomach empties when two thirds full, which means continuous foraging or several small feeds are preferable to large ones.
Investigating intestines
Around 30m in length, the intestines are where a lot of the essential work is done. “The small intestine consists of the duodenum, jejunum and ileum, with the duodenum around 1m long,” says Sara. “Enzymes secreted from the pancreas and liver break down food into basic nutrients here. Bile is also secreted direct from the liver.” The jejunum is roughly 19m long and the chemical breakdown of food is finished in this part of the small intestine, with nutrients absorbed into the bloodstream to be used by the body or stored in the liver. “The ileum, around 1m long, is the final part of the small intestine. It continues the absorption of nutrients and controls the passage of partially digested food into the caecum,” explains Sara. “The junction between the small intestine and the caecum is called the illeo-caecal junction and is a prime spot for tapeworms.” The caecum itself is 1 ½ to 2m long and holds up to 30 litres of fibrous food and fluid. “Contraction of the caecal muscles results in mixing of the food with microbes that digest tough plant cells via fermentation. This fibre breakdown produces substances called volatile fatty acids, which can then be absorbed and used by your horse for energy,” explains Sara. Finally, from the caecum, the digesta enters the large intestine, almost 8m in length. This consists of the large colon, transverse colon and small colon, and is where most of the water absorption takes place and the remaining food matter is turned into faeces. The diameter of the large intestine varies from five to 50cm. The pelvic flexure, only 8cm, is where the large colon ‘turns’ to change direction — a common site for impactions. The small colon is where faeces form into balls and are evacuated.
“The stomach is small, and horses are designed to be trickle feeders”
“The stomach empties when two thirds full, which means continuous foraging or several small feeds per day are preferable to one or two large ones”