Understanding how the equine digestive system works to help reduce the risk of health problems

What does it mean to be a large herbivore, what are the two most major digestive issues commonly observed in horses and how can owners minimise the risk? Dr David Marlin tells us more.

As all horse owners know, the horse is a herbivore. More specifically we describe the horse as a non-ruminant herbivore as it has a small, simple single compartment stomach. In contrast, ruminants, such as sheep, goats, cattle, deer, giraffe and llamas, have a stomach which consists of four chambers: the rumen, the reticulum, the omasum and the abomasum.

The rumen is the largest of the four chambers and contains bacteria, protozoa and fungi which break down food particles by fermentation. Ruminant herbivores also regurgitate partially digested food for remastication (chewing). In contrast, no significant fermentation takes place in the horses’ stomach and the cardiac sphincter (the valve that connects the oesophagus to the stomach), does not permit regurgitation or vomiting in horses. Furthermore, the horses stomach fluid pH in the lower part of the stomach is typically 2-3 (very acidic) compared with 6-7 (slightly acidic to neutral) in ruminants. The acidity of the horses’ stomach is important for maintaining stomach hygiene by killing off bacteria, protozoa, fungi and viruses ingested with food as well as initiating protein digestion. Whereas protein, fat and starch are digested in the small intestine, fibre can only be digested in the large intestine (hindgut).

The horses’ stomach is also relatively small and suited to constant slow intake of forage as opposed to large meals of concentrated energy such as cereals. The horse also has a psychological need to chew. Chewing also causes a continuous release of saliva which helps to buffer stomach acid.

What does it mean to be a large herbivore?

In the first place, plants that horses eat in the wild are generally low in energy. This means that in order to obtain sufficient energy, horses typically need to spend at least around 12h a day eating and in some cases, up to 18h. Compare this for example to a dog, also a mammal but a carnivore; anyone with a dog knows that eating only takes a few minutes per day!

When it comes to what horses eat, there is a big difference between domesticated horses and wild horses. Whilst the wild horse is predominantly a grazer, they will generally be grazing grasses that are low to moderate in protein, low in starch and of low-moderate digestibility. But they may be grazing a large variety of species of grass. In addition, wild horses will also browse many other plants. In contrast, the domesticated horse is often grazed or fed more highly digestible grass or forages with higher protein and energy. At the same time, the domesticated horse usually does not have the opportunity to browse other vegetation. For this reason, whilst wild horses may achieve a balanced or adequate nutrient intake, the domesticated horse that is on a forage only diet will almost certainly not be receiving a balanced or adequate intake of nutrients.

Secondly, herbivores have a much larger digestive tract than carnivores or omnivores. This is because the hindgut, which consists of the caecum, large colon and small colon, where digestion of fibre by bacteria takes place to release energy has to be large to accommodate a large amount of food, fluid and bacteria for efficient digestion. In a person the total digestive tract weighs around 6% of the total bodyweight. However, in a horse the digestive tract weighs around 12% of the total bodyweight. If the horse were a carnivore it would look more like a giant greyhound and be able to run even faster!

What are the two major digestive problems?

The two major digestive problems that we most commonly observe in horses are gastric ulcers and colic. Both of these are often related to nutrition. Gastric ulcers (ulcers in the stomach) are described as either squamous or glandular. The glandular region of the stomach is the lower part of the stomach where acid is secreted and where the gastric fluid sits. Ulcers that develop here are considered to be due to breakdown of the mucosal defences – the lining of the stomach. Squamous ulcers occur in the upper part of the stomach and are considered to be due to damage to the stomach surface by excess acid.

Some of the factors that appear to be associated with an increased risk of ulcers are: Hard/long exercise, especially on an empty stomach, long periods without forage, large meals, high starch intake, NSAIDS (e.g. Bute), colic/hindgut discomfort, low forage intake, poor quality forage (e.g. straw), low water intake, limited pasture turnout, high electrolyte intake and stress. Whilst gastric ulcers can lead to poor performance, altered behaviour, reduced appetite, weight loss and loss of condition it is rarely fatal or a cause for euthanasia. This is in marked contrast to colic which is causes more deaths of horses than any other disease.

So, what are some basic guidelines for minimising the risk of digestive problems in horses?

The horse has a physical and psychological need for good quality forage. Whilst low quality forage such as straw or poor hay may be seen as an option for weight reduction, this can lead to other digestive problems. Hay or haylage should also be of a high level of hygiene with minimal dust and micro-organisms (yeast, bacteria, fungi, etc). This is also critical for good respiratory health and high temperature steaming is an excellent option to better ensure good hygiene and minimise the risk of respiratory disease. Where possible providing ad libitum forage will also reduce the risk of digestive problems.

The horse has a small stomach and this is not designed to cope with large high energy, especially starch, meals. Concentrate feeds should be small and frequent and ideally always given after forage. Forage slows down the passage of concentrate feeds and reduces the risk of undigested starch reaching the hindgut and leading to hindgut disturbance. It also goes without saying that clean water should always be available. Water intake is related to forage dry matter intake. So horses eating lush grass will drink less than horses eating haylage and horses eating haylage will drink less than horses eating dry hay. Finally, follow other basic rules such as regular worming, introducing new feeds slowly (over weeks not days) and regular dental check-ups.