Many in the equine industry have been saddened to hear of the extensive losses at Beutler and Son Rodeo Co. ranch in Oklahoma. Up to 70 horses have died after consuming horse feed contaminated with monensin. Monensin (or Rumensin) is an antibiotic often included in cattle feed to increase feed efficiency, resulting in higher growth rates and milk production. Monensin can be toxic to cattle, with an LD50 (a dose that kills 50% of the population) of 50-80 milligrams per kilograms (mg/kg) of body weight, and 90-200 mg/kg in poultry. In horses, however, the LD50 is 2-3 mg/kg body weight, which means a much smaller amount can kill a horse. Because of this high rate of toxicity, it is very important that horses never come into contact with monensin.

So, what happened in Oklahoma? The feed that was offered to the horses was contaminated with monensin; the company reports it likely occurred due to a combination of a failed cleanout procedure and a sensor malfunction.

While mistakes happen, owners can help prevent them from affecting their own horses.

Understand Where Horse Feed is Manufactured

One easy rule of thumb is to make sure that you only feed horse feed to horses.

It is also important to buy high-quality horse feed from a facility recognized as “ionophore-free” or “ionophore-safe.” Ionophore-free facilities are feed mills that only make horse feed and would not even have monensin or other medications on the property. Ionophore-safe facilities might make other livestock feed and have monensin on the property but take measures to reduce the risk of cross-contamination. These measures can vary, such as having a different part of the mill be equine only, while others might only flush the pipes also used for other feeds (and/or corn) to help remove any medicine residues. Corn helps remove product build-up within the pipes.

I recommend that my clients and all owners contact their feed companies to determine what type of facilities make their products because some companies might have products made at different types of facilities. Several companies are proudly equine-only feed mills, and feed manufacturers in the U.K. are safe because those countries don’t allow monensin. You can learn a lot about a company by how helpful their feed representatives are. If you find that your feed sales representative or dealer is not helpful or does not know (and won’t find out for you), it might be time to find a different feed company.

Observe Horses Closely for Illness

I also remind horse owners of the importance of noting their horses’ behavior. Just like you might notice a shoe missing by the sound of hooves on the concrete, owners and managers should notice if a horse (or several horses) refuse to eat their feed. In many cases, horses are quite good at avoiding toxic plants and feeds so, if you notice one horse turning up his nose at the latest bag or batch of feed, take it away from all horses immediately. I recommend taking a few samples of the feed for independent analysis and calling the feed company right away with the batch number (which should be on the feed tag). Also, call your veterinarian to give them a heads up—even about a single horse that might not be eating normally or out of concern for a potential toxicity (a researcher described the signs of monensin toxicity here).

Mistakes happen, so it is up to horse owners to ensure their horses eat safe, high-quality feed.