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QUICK FACTS: Fiber Fermentation

Horses are adapted to eat a fiber-based diet, and are very efficient at doing so! This is due to a symbiotic relationship with microbes in their hindgut. While mammals lack the ability to break down the structural cell wall components of plants (also called insoluble fibers), microbes in the horse’s hindgut can!

These microbes breakdown these fibers into short-chain fatty acids, and the primary ones produced in the equine hindgut on a forage diet are (Raspa et al, 2022):

  • Acetate (71%)

  • Propionate (17%)

  • Butyrate (6%)

These fatty acids are absorbed in the hindgut and the horse can use them for energy. Horses can actually get a decent amount of energy from fiber as well - a moderate quality grass hay will provide ~2.0 Mcal/kg - to represent this in a way that’s more comparable to human nutrition, 1 kg of grass hay will provide 2000 cals of digestible energy! Higher quality or hay containing legumes can have higher energy content too!

Comparing this to high-calorie human foods:

  • 1 kg of plain chicken will provide ~1100 cals

  • 1 kg of white rice will provide ~1300 cals

  • 1 kg of potatoes will provide ~800 cals

  • 1 kg of pasta will provide ~1300 cals

Energy requirements of 500 kg horse at maintenance, to moderate work (school horses, show horses, ranching, low level polo) generally range from 15.2 to 23.3 Mcals/day, and as horse will voluntarily consume around 2.0-2.4% of their BW in forage, it’s easy to see how a horse consuming 10-12 kg of hay can meet most of their energy requirements, if not exceed them in some cases, on forage alone!

While quality of the hay is key, and horses with increased requirements may require additional energy sources, it’s important that we don’t underestimate the nutritive value that fiber has for horses!

Horses are incredibly well adapted for doing well on fiber-based diets, and due to the supportive effects of high-fiber diets on gut health and behaviour, these diets are an incredibly valuable nutrition strategy when considering whole-animal health optimization!


Raspa et al, 2022. A high-starch vs high-fiber diet: effects on the gut environment of the different intestinal compartments of the horse digestive tract. BMC Vet Res 18(2022):187.


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