Watch for nasal discharge and coughing, (heaves). I hate to say it, but many horses have had moldy hay on accident at some point in their lives. The mold spores are the cause for concern in the respiratory system. It can cause colic, so if you think there is an issue you want to listen for gut noises often. Next time you have moldy hay, get it OUT of your barn because it may be moldy from being wet - and that can cause combustion (it happened to a barn about 5 years ago where I am from, the whole barn burnt, horses died, etc). Give it to a cow farmer, cows and sheep can eat moldy hay and that way its not for waste.