Prebiotics are non-digestible fibers and oligosaccharides that selectively feed the beneficial bacteria in your horse's hindgut. While probiotics add live organisms, prebiotics nourish the microbes already living there — think of prebiotics as fertilizer for the good bacteria in your horse's gut garden. The most common equine prebiotics include fructo-oligosaccharides (FOS), mannan-oligosaccharides (MOS), and inulin. FOS and inulin are fermented by beneficial Lactobacillus and Bifidobacterium species, promoting their growth and crowding out pathogenic bacteria. MOS works differently — it binds to harmful bacteria like Salmonella and E. coli, preventing them from attaching to the gut wall. Prebiotics can also enhance volatile fatty acid production, improving energy extraction from forage.
Typical supplemental doses are 5 to 20 grams per day of a prebiotic blend for a 500kg horse — about one to two tablespoons of powder. The best natural prebiotic for your horse is simply adequate long-stem forage: the structural fibers in hay are slowly fermented by hindgut bacteria and naturally support microbial diversity. Supplemental prebiotics are most useful during transitions, stress, or when forage quality is poor.
Since prebiotics are not a required nutrient, there are no deficiency signs per se. However, a forage-poor, grain-heavy diet lacks the natural prebiotic fibers that maintain a healthy hindgut microbiome. This can contribute to poor fermentation efficiency, soft or irregular manure, reduced nutrient absorption, and an increased risk of hindgut acidosis.
Large amounts of rapidly fermentable prebiotics (especially FOS or inulin) introduced suddenly can cause gas, bloating, and loose manure by overwhelming the hindgut with fermentable substrate. This is the same mechanism behind pasture-associated laminitis when horses gorge on fructan-rich grass. Always introduce prebiotic supplements gradually.
