Your horse will benefit from prebiotics. Check out the research:
Prebiotic compounds, such as short-chain fructo-oligosaccharides (scFOS), have been shown to improve health, welfare, or both in several species, but few studies have been conducted in horses, despite the sensitivity of their hindgut microflora. We hypothesized that prebiotic oligosaccharides, known to be able to stabilize the intestinal microflora in other species, would be of great importance in horses.
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https://www.ncbi.nlm.nih.gov/pubmed/17940163