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.
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This work is supported in part by New Technologies for Agriculture Extension grant no. 2020-41595-30123 from the USDA National Institute of Food and Agriculture. Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture.
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