Is there a risk of botulism when feeding silage to horses?

Botulism is caused by the bacterium Clostridium botulinum and is rarely associated with well-made silage. In cases where this disease has been linked to silage, it has always been because an animal carcase has been ensiled along with the forage. The rotting carcass provides a niche environment in which this disease organism can potentially thrive.  

Clostridia will not thrive in higher DM silage (> 30% DM) and as haylage is normally baled at > 50% DM there would not normally be an issue – just check there are no animal carcasses in the bale as you are feeding it. If there are, discard it.