What Does Ionophore-Free Mean?

You may have heard about “ionophores” and/or “ionophore-free” feeds when it come to choosing a horse feed for your horses. This is because the risk to your horse’s health and wellbeing is real if their feed is accidently cross contaminated with ionophores. There are multiple instances where horses have died throughout the United States after accidently ingesting ionophore contaminated horse feed. Thus, this is a topic relevant to any horse owner.

What Are Ionophores?

Ionophores are a class of antibiotics that are used as a feed additive in livestock and poultry feeds. While they are classified as an antibiotic, they are not used to treat disease. Rather, their purpose, when fed to cattle, is to encourage feed efficiency and weight gain. In poultry, ionophores help gut health and reduce coccidia (parasite) populations.  The types of ionophores used in cattle or poultry feed include monensin, narasin, salimnomycin, laidlomycin, among some others. 

The most important takeaway when discussing ionophores is that they should never be fed to horses. This is one of the main reasons it is never advisable to feed any other type of livestock feed to horses. There may be additives, like ionophores, included or the feed may have been mixed in the same equipment that could be contaminated with ionophores. Horses have a very low tolerance for ionophores and, if fed enough (i.e., more than 3 mg), it can and does lead to illness and even death. 

Ionophore Toxicity in Horses

Horses are one of the species that are the most susceptible to ionophore toxicity. The signs and symptoms of ionophore toxicity for horses will depend on how much they ingest, but they can include:

  • Cardiovascular issues. Horses may exhibit abnormal heart beats (arrhythmia) or even suffer cardiac arrest.
  • Digestive upset. Horses may experience severe colic or diarrhea.
  • Muscle impacts. Horses may experience muscle weakness, stiffness, trembling or difficulty walking.
  • The horse may be depressed or appear weak.
  • Respiratory issues. May experience trouble breathing.

In severe cases, ionophore toxicity can lead to sudden death. If your horse experiences any symptoms of ionophore toxicity, you should contact your veterinarian immediately. While there are no “cures” to ionophore toxicity, supportive care can help manage and stabilize your horse.

If you do suspect your horse feed might be contaminated by ionophores, you should take the following steps:

  • Stop feeding the suspected horse feed immediately and remove access to it from any horse.
  • Isolate the affected horse and contact your veterinarian immediately. You can explain the situation and receive guidance.
  • Monitor your horse for any further deterioration. Note any changes.
  • Sample the feed and packaging for potential testing.
  • Report your concerns to the horse feed manufacturer, as they should be interested in investigating the issues and take corrective actions.

The most important takeaway is to take a proactive approach to having your horse assessed by a veterinarian.

How Can You Prevent Ionophore Toxicity?

The ideal prevention measure is to only feed horse feeds from companies that ensure their horse feeds are produced in ionophore free facilities. This means that the horse feed being made in a manufacturing plant has little to no chance of being contaminated with ionophores. For example, Tribute feeds are made in a facility that is completely ionophore free and there is no risk of putting any horse’s health in jeopardy.

What can be concerning is some mills use sequencing or flushing to try and minimize any chance of crossover of ionophores in their horse feeds. Yet, this still can put horses at risk if the sequencing or flushing was not successful.

Take Home Message

The risk of ionophore toxicity to horses is real throughout the world. While it does lead to the death of some horses each year, owners can reduce this risk by purchasing horse feeds made in ionophore free facilities. You can read more about other ways horse feeds are safely made in our Is Your Horse Feed Safely Made article. It also serves as a reminder that feeds formulated for other species (i.e., cattle feed) should never be fed to your horse. If you have any concerns about what you are feeding your horse or need further information, please feel free to contact us.


Novilla, M.N. and Thomas, E.E. 2006. Ionophore toxicity in horses. Tech Talk: Scientific update from Elanco Animal Health. https://www.nutrenaworld.com/doc/1432088177808/elanco-techtalk-ionophores.pdf

Article By: Chris Mortensen, Ph.D.
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