Introduction

Since its first domestication, believed to be around 5000 BC, the role of the horse in society has evolved with human needs — from transport and warfare to sport and leisure. Horses are fundamentally nonruminant herbivores, adapted to high-fibre diets and continual microbial fermentation within the caecum and colon. Domestication, however, introduced high-energy cereal-based feeds to meet performance demands, bringing both benefits and potential digestive challenges.

The optimal feeding of horses blends art and science: science provides the understanding of digestion, metabolism, and nutrient needs; art applies that knowledge to individual horses and conditions. While good nutrition cannot enhance a horse’s innate ability, poor nutrition can limit performance. Understanding digestion helps minimize gastrointestinal disturbances from inappropriate feeding practices.

An Overview of the Horse’s Digestive System

The horse’s digestive system functions in two parts:

• Precaecal section – similar to that of monogastric animals like humans or pigs.
• Hindgut (caecum and colon) – more like the rumen of cattle, where microbial fermentation occurs.

This dual structure makes the horse unique and requires careful feeding management.

Precaecal: Monogastric-like Section

Prehension

Horses grasp food using their lips, tongue, and teeth. Their sensitive lips allow selective feeding, reducing ingestion of foreign matter but sometimes leading to picky eating or selective grazing.

Mastication

Chewing movements are both lateral and vertical. Horses naturally chew approximately 60,000 times a day while grazing, making them trickle feeders. Feed type influences chewing: hay requires around 3,400 chews/kg (about 40 minutes), while oats take around 850 chews/kg (about 10 minutes). Feeding more concentrate shortens eating time, possibly leading to boredom and stereotypic behaviours.

Poor mastication reduces feed efficiency. Regular dental care and feeding management (like adding chopped fibre) are essential to maintain proper chewing and digestion.

Salivation

Horses produce saliva in response to chewing, not in anticipation of food. The more a horse chews, the more saliva is produced. On average, a 500 kg horse produces around 12 litres/day on a mixed diet, and up to 100 litres/day when eating dry hay. Saliva lubricates feed but contains minimal enzymes, so digestion doesn’t begin in the mouth.

Stomach

The horse’s stomach holds 9–15 litres and empties rapidly (within 20 minutes per meal). It is inelastic, and vomiting is extremely rare. Feeding large, infrequent meals leads to faster gastric emptying, reducing digestion efficiency. The stomach has two regions:

• Nonglandular section – where bacterial fermentation begins.
• Glandular section – where acids and enzymes (e.g., pepsinogen) start protein breakdown.

Large, dry concentrate meals risk dysfermentation due to incomplete mixing with gastric juices, potentially causing digestive upsets.

Small Intestine

Digestion primarily occurs in the small intestine through enzymatic and pancreatic secretions. Bile (secreted continuously) aids fat digestion. Major dietary energy sources include:

1. Hydrolysable carbohydrates (starch)
2. Non-starch polysaccharides (fibre)
3. Fats (up to 10% of diet)
4. Proteins (a less efficient energy source)

Horses have limited amylase activity, restricting starch digestion to about 2 g/kg bodyweight per meal. Excess starch passes undigested into the hindgut, increasing colic and laminitis risk.

Fluid Shifts

Adult horses secrete over 100 litres of fluid daily into the pre-caecal tract. Large, infrequent grain meals can cause temporary dehydration (up to 15–24% plasma loss). Feeding smaller, frequent meals helps prevent this and stabilizes digestion.

• Blood glucose peaks 1–3 hours after feeding, followed by an insulin rise.
• Exercising soon after feeding can lead to low glucose and reduced fatty acid release.
• A full gut can limit lung expansion and reduce blood flow to muscles.

Caecum and Colon: Rumen-like Section

Feed reaches the caecum within 45 minutes of eating and is largely present within 3 hours. Fermentation occurs here through microbial activity producing volatile fatty acids (VFAs), used as an energy source. Water absorption also occurs here.

However, excessive undigested starch entering the hindgut lowers pH, disrupting microflora and causing gas production, endotoxin release, and possible colic, diarrhoea, or laminitis.

Hindgut Fermentation and Starch Digestion

When too much starch reaches the hindgut, fermentation increases acidity. The table below (Willard et al., 1977) illustrates how caecal pH changes with different diets.

Hay-based diets maintain stable pH, while concentrate-rich diets can drop caecal pH sharply, increasing acidosis risk. This supports the growing popularity of supplemental fat as a non-starch energy source.

Feed Processing and Starch Digestibility

The table below shows how grain processing affects pre-ileal starch digestibility:

Although corn contains more starch than oats, whole corn is less digestible, meaning more starch reaches the hindgut — increasing acidosis risk. Oats, being more digestible, are a safer starch source for horses.

Conclusion

A sound understanding of equine digestive physiology helps optimise feeding strategies and reduce the risk of digestive disturbances. Feeding smaller, more frequent meals, using appropriate feed processing, and maintaining fibre intake can greatly improve digestive health and performance.

References

1. Meyer, H. et al. (1993). Investigations of preileal digestion of oats, corn and barley starch in relation to grain processing. 13th Equine Nutrition and Physiology Symposium, Gainesville, Florida.
2. Pagan, J.D. (1999). Vitamins, trace elements and electrolytes for the performance horse. BEVA Specialist Days on Behaviour and Nutrition, Equine Veterinary Journal, UK.
3. Radicke, S. et al. (1991). Preileal apparent digestibility of oats and corn starch and consequences for cecal metabolism. 12th Equine Nutrition Physiology Symposium, Calgary, Canada.
4. Willard, J.G. et al. (1977). Effect of diet on cecal pH and feeding behaviour of horses. Journal of Animal Science, 45, 87.