Guts - Understanding your Horse's Digestive System

Why do we bang on about forage being the basis of your horse’s diet? Because that is how their physiology is designed to operate.

Here’s a brief summary of how it works.

Animals are generally herbivores (eat grass), carnivores (eat meat), or omnivores (eat both). Horses are herbivores.

Most herbivores fall into one of two digestive categories: monogastric (one stomach) or ruminant (multi-chambered stomach). Horses are monogastric.

Other monogastric herbivores include: rabbits, guinea pigs, elephants, zebras and donkeys.

These animals don’t chew cud like ruminants but instead rely on microbial fermentation in the cecum and colon (hindgut) to break down cellulose and produce volatile fatty acids (VFAs) for energy.

Volatile fatty acids are short-chain organic acids produced by microbial fermentation of fibre in the horse’s hindgut. The three primary VFAs are:

Acetate

Used for fat synthesis and muscle energy

Propionate

Converted to glucose in the liver (gluconeogenesis)

Butyrate

Fuels colonocytes and supports gut lining health

Together, these VFAs supply up to 70% of a horse’s daily energy needs. VFAs are absorbed through the colon wall and used for maintenance, growth, and performance. Microbes also synthesise B-vitamins and some amino acids, contributing to overall health.

The other 30% is synthesised in the foregut - sugar and starch (glucose for immediate energy or stored in the muscles and liver), fat (converted into fatty acids), and protein (used for muscle development and tissue repair).

The Importance of Gut Flora

Bacteria - Break down fibre, produce VFAs

Protozoa - Aid in cellulose and pectin digestion

Fungi - Help degrade tough plant cell walls

Archaea - Support methane production and fibre digestion

Sudden diet changes, stress, medications, or high-starch meals - or breaking into the feed shed and gorging on concentrates - can shift microbial populations.

Effects of phenylbutazone alone or in combination with a nutritional therapeutic on gastric ulcers, intestinal permeability, and fecal microbiota in horses - PubMed

A drop in pH (<6.0) leads to microbial die-off, reduced fermentation efficiency, and increased risk of endotoxin release, which causes colic and is a well-established trigger for acute laminitis.

The bacterial community of the horse gastrointestinal tract and its relation to fermentative acidosis, laminitis, colic, and stomach ulcers - PubMed

Roughage vs Concentrates

Microbial Impacts

The equine digestive system is designed for continuous grazing, with microbial fermentation occurring primarily in the hindgut. Here's how feed types shape that microbial ecosystem:

Roughage (Forage-Based Diets)

High in fibre (cellulose, hemicellulose, pectin) — ideal substrates for fibre-digesting microbes.

Promotes stable pH (~6.4–6.7), supporting cellulolytic bacteria like Fibrobacter and Ruminococcus.

Encourages diverse microbial populations, including beneficial protozoa and fungi.

Results in slow, steady fermentation, producing volatile fatty acids (VFAs) — key energy sources for horses.

Concentrates (Grain-Based Feeds)

Rich in starch and sugars, which are ideally digested in the foregut.

If fed in excess or improperly timed, undigested starch spills into the hindgut.

The effect of current grain feeding practices on hindgut starch fermentation and acidosis in the Australian racing Thoroughbred - PubMed

This favors lactic acid-producing bacteria (e.g., Streptococcus bovis), lowering pH and risking hindgut acidosis.

Disrupts microbial balance, potentially killing fibre-digesting microbes and triggering issues like colic, laminitis, and behavioral change.

Changes of the hindgut microbiota due to high-starch diet can be associated with behavioral stress response in horses - PubMed

Common fibre sources include:

Fresh pasture

Hay 

Chaff (ideally lucerne)

Beet pulp

Soy hulls

Fibre is classified into two key types:

Soluble fibre: Found in the soft, non-woody parts of plants—pectin, gums, resins, beta-glucans. These are easily fermented and nourish beneficial bacteria.

Insoluble fibre: Made up of structural plant components like lignin, cellulose, and hemicellulose. Most are fermentable except lignin, which resists breakdown and is excreted.

Fermentable fibre fuels microbial populations in the hindgut, helping to stabilise gut pH, prevent disease, and maintain optimal digestive function.

In short, providing your horse with a forage-based diet will ensure they get enough fibre to maintain a healthy hindgut microbiome.

Forage as hay or pasture should comprise at least 50% of the diet and be fed at a minimum rate of 1.5% to 2% of the horse’s body weight per day.

Ideally forage will supply the bulk of your horse's diet and hard feeds and supplements only make up what is missing from forage, which is predictable.

If you want to support the hindgut you can introduce prebiotics and probiotics to populate the gut with healthy bacteria, and feed the healthy bacteria that already exist.

Effect of live yeast culture supplementation on hindgut microbial communities and their polysaccharidase and glycoside hydrolase activities in horses fed a high-fiber or high-starch diet - PubMed

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