Fiber is the single most important nutrient in your horse's entire diet — it is quite literally what their digestive system was built for. Horses are hindgut fermenters, meaning their cecum and large colon house billions of microorganisms that break down plant fiber into volatile fatty acids, which supply roughly 60 to 70 percent of your horse's daily energy. This fermentation process is the engine that keeps everything running. Without a constant flow of long-stem fiber, that microbial population collapses, gas builds up, gut motility stalls, and the result is colic — the number one killer of domestic horses. Unlike ruminants such as cattle, horses cannot vomit, so anything that causes a blockage or gas pocket has no escape valve and can quickly become life-threatening. Fiber also keeps the gut physically moving through peristalsis: the sheer bulk of forage pushes material steadily through the 30-meter digestive tract, preventing dangerous impactions. Beyond digestion, the constant chewing required to process hay and pasture produces saliva — up to 40 liters per day — which buffers stomach acid and protects against gastric ulcers. A horse's stomach continuously secretes acid whether or not food is present, so long periods without forage leave the stomach lining exposed and vulnerable. This is why the golden rule of equine nutrition is forage first, forage always, and forage often.
Your horse should eat roughly 10 kilograms of hay per day — that is about the weight of a large bag of dog food, or roughly 2 percent of their body weight for a 500kg horse. This provides approximately 3 to 4 kilograms of crude fiber. Hay should make up at least 1.5 percent of body weight daily, and ideally 2 percent or more. The best approach is to keep hay available as much of the day as possible — slow-feed hay nets are excellent for extending eating time and mimicking natural grazing behavior.
7.45% of daily nutrient intake
Fiber makes up 7.45% of your horse's total daily nutritional requirements by weight.
Colic is the most dangerous consequence — your horse may paw at the ground, look at their flanks, roll repeatedly, refuse feed, or stand with a stretched-out posture. Other signs include weight loss despite adequate calories from grain, gastric ulcers (poor appetite, girthiness, dull coat), wood chewing or cribbing as the horse seeks fiber, loose or watery manure from disrupted fermentation, and a general decline in condition and temperament.
Excess fiber from hay is not a realistic concern for most horses — their digestive system is designed to process large volumes of forage continuously. The only caution is that very mature, stemmy hay with extremely high fiber and low digestibility can reduce overall nutrient intake in horses with high energy demands, such as lactating mares or horses in heavy work, simply because the horse fills up on bulk before meeting calorie needs.
| Life Stage | Size | Min | Max | Unit | Notes |
|---|---|---|---|---|---|
| Adult | — | 1.5 | 2.5 | % of diet | Minimum 1.5% of body weight as forage daily (7.5kg for a 500kg horse). Ideally 2% or more. Expressed as percent of body weight in forage. |
| Juvenile | — | 1 | 1.5 | % of diet | Growing foals and yearlings still need high-fiber forage as the diet foundation, but may eat proportionally less hay as they receive higher-energy concentrates for growth. Hay should still be available free-choice. |
| Pregnant / Nursing | — | 1.5 | 2 | % of diet | Pregnant mares need consistent forage intake. Late gestation reduces abdominal space, so smaller, more frequent hay meals may be needed. Quality forage remains essential. |
| Senior | — | 1.5 | 2 | % of diet | Senior horses may need soaked hay, hay cubes, or beet pulp if dental issues prevent adequate hay chewing. Maintaining fiber intake is essential for gut health regardless of age. |
| Working / Active | — | 1.5 | 2 | % of diet | Working horses need adequate forage despite higher energy demands from concentrates. Never drop below 1% body weight in forage even for hard-working horses. The hindgut must keep functioning. |
Source: NRC 2007, general veterinary consensus
The fiber-to-protein balance determines hindgut health. High-fiber, moderate-protein diets support healthy fermentation. Low-fiber, high-protein diets disrupt the hindgut microbiome, increase ammonia production, and raise colic risk.
What this means: Maintain forage as the diet foundation (at least 1.5% body weight daily) regardless of how much concentrate is needed for work or growth. Never sacrifice hay to make room for more grain — the hindgut must keep fermenting. If energy needs are high, add fat rather than more grain.
In the equine hindgut, rapidly fermentable sugars (especially fructans from lush pasture) compete with structural fiber for microbial attention. When sugar floods the hindgut, lactate-producing bacteria outcompete the fiber-fermenting species, crashing pH and potentially triggering laminitis. A fiber-rich, low-sugar diet keeps the microbial ecosystem balanced and stable.
What this means: Always maintain forage as the foundation of your horse's diet. When pasture fructan levels are high (sunny afternoons in spring and autumn), limit grazing time or use a muzzle. Never feed large grain meals that could overwhelm the small intestine's starch-digesting capacity.
