Is Rumensin (sodium monensin) harmful to my horse?

Yes, yes, yes! Rumensin is a feed additive (ionophore) for cattle. Horses should not consume any feed that contains Rumensin as it is often lethal. Rumensin affects the heart muscle of the horse leading to changes that cause heart failure. Horses that do survive after ingesting feed containing Rumensin are impaired for life due to the damage to the heart. You should always read feed labels before turning horses in with cattle.