Question

I recently switched my cattle feed to a medicated formula that contains monensin. Though my horses and cattle are fed separately, the horses have access to the empty cattle troughs after feeding. Today, I found my horses licking the cattle trough, and I panicked. How can I make sure my horses didn’t ingest any monensin? What steps should I take next?

Answer

You have every right to be concerned for your horses’ health after possible ingestion of ionophores such as monensin sodium, often sold under the trade name Rumensin. Cattle feeds sometimes contain ionophores, antibiotic-like compounds that alter rumen fermentation and improve feed efficiency. However, these compounds are toxic to horses and can lead to death when consumed in sufficient quantities.

Horses would have to consume a lot of cattle feed to be affected. High-intake cattle feeds contain 3.3-5.5 ppm monensin, which means a 1,100-lb (500-kg) horse would have to consume a few hundred pounds of cattle feed for toxicity to occur.

One circumstance that could cause ionophore toxicity in horses consuming less cattle feed involves errors in feed manufacturing that result in higher concentrations of ionophores. Fortunately, your horses only had access to the empty trough and did not eat any feed. You should not have anything to worry about in this specific situation.

To avoid this situation in the future, I suggest completely separating the feeding locations and troughs of cattle and horses to ensure there is no cross-contamination, and use high-quality horse feeds that are produced in ionophore-free mills.