Wheat and oat straw may cause impaction colic in horses. These materials contain highly non-digestible fiber that do not ferment in the hindgut. Unlike cattle, horses can't digest these forages.
Bermudagrass hay has been implicated in ileal impactions, but good quality bermudagrass cut and baled by a reputable source, is safe.
Bermuda hay has a low moisture content and fine fibers that may form obstructive masses so access to clean, fresh drinking water is crucial. Horses do not chew this hay as thoroughly as other, more coarse-fibered grass hay and may overeat if fed free choice. For pasture grass, seed with Alicia, Coastal, Tifton 85, and Jiggs varieties.
Consider adding a live yeast supplement like PROBIMAX EquiColic Colic Prevention Aid to prevent the pH drop in the hindgut that can lead to colic.