But wet only enough hay for one feeding. Straw bedding contains higher levels of small mold spores than other bedding, such as wood shavings or paper. Mold can occur if the stable is poorly ventilated or stalls aren't routinely cleaned. Ammonia produced by bacteria that break down organic matter, urine and manure can harm a horse's airways. Ammonia levels can get quite high in stalls that aren't routinely cleaned.
Ammonia is especially harmful to young, newborn foals.
Make sure your stables are well ventilated to cut down on mold spores, bacteria and noxious gases. Use dust-free hay or wet the hay. And don't use straw for bedding. Bedding should be dust-free. And don't let deep bedding accumulate.