Ionophores have been safely utilized in the beef industry for a long time. If fed according to the recommended rates, ionophores are considered safe and effective. Ionophores are feed additives used in cattle diets to increase feed efficiency and body weight gain. In addition, ionophores can decrease the incidence of bloat and coccidiosis.  Ionophores can be fed to cattle in several different supplemental packages from liquid feeds, cakes, pellets, and loose minerals. The classification of the animal (ie., lactating cow vs stocker) can dictate how ionophores get delivered according to the label.  Ionophores approved for use in cattle include monensin (Rumensin®), lasalocid (Bovatec®) and laidlomycin propionate (Cattlyst®). 

They are compounds that alter rumen fermentation and fermentation end products. Ionophores function by negatively altering the metabolism of gram-positive bacteria in the rumen. These affected gram-positive bacteria are those that decrease efficient rumen digestive physiology and the energy supplied from the ruminal digestion of feedstuffs. By controlling certain microbes in the rumen, less waste products or methane are generated, and ruminal protein breakdown is decreased. The shift in ruminal bacteria population allows beneficial bacteria to be more efficient through an increase in the amount of propionate and a decrease in the production of acetate. Overall, ionophores can increase the energy status and use feed resources more efficiently.

From a performance standpoint, grazing trials using steers and heifers have shown that formulated intakes of 155 mg/day of monensin result in an improvement in average daily gain of 0.18 lb per day or a 13.5% increase compared to cattle receiving no monensin. Increasing monensin intake up to 200 mg/day, cattle gained an additional 0.20 lb per day or a 16% improvement compared to cattle without monensin.

Although all ionophores can be toxic, this article will focus on monensin, due to the amount of information available and use in the cattle industry since the mid-1970s. Proper management, avoidance of overdosing, and reading label recommendations will help prevent the occurrence of adverse effects associated with ionophores. Always follow the label instructions when feeding medicated minerals or feeds as overfeeding or incorrectly feeding ionophores can lead to toxicities.

One consistent mistake made by many producers is offering a medicated mixing mineral to their cattle free-choice and not properly managing intakes. For managing mineral costs, mineral consumption of the herd needs to be monitored.  Rumensin, for instance, reduces palatability of the mineral supplement which can result in decreased overall mineral intake.  Salt is a key driver of mineral intake. If overconsumption is an issue, adding more salt to the mineral can help regulate intake.  

The normal safe range of monensin used in stocker calves for increased body weight gain and prevention of coccidiosis is 50 to 200 mg/head/day.  Lethal dose of monensin that would cause death of 1% of animals (LD1) is estimated to be 2.5 mg/lb of body weight. Acute oral LD50 or median lethal dose for monensin is 12 mg/lb of body weight.  For a 700 lb stocker calf, daily intakes of 1,700 to 8,400 mg/day would result in 1 to 50% of exposed animals to die. Although it may seem difficult to achieve intakes that are 8.5 to 42 times greater than the safe range, it can be an easy mistake at the time of mixing due to the small amount of monensin used in a ton of feed. Drug interactions, such as macrolide antibiotics and sulfonamides, potentiates toxicosis.

The clinical signs of monensin toxicity can be difficult to assess and will vary based on exposure. The first symptoms affect the digestive system, with anorexia occurring approximately twelve hours post ingestion, followed by diarrhea.   Death loss begins three days after exposure, with some animals having no clinical symptoms prior to death.  Peak death loss occurs between five to 10 days.  Heart and skeletal muscle damage may also occur, which presents as weakness, incoordination, difficulty breathing, and nasal discharge.

Diagnosing monensin toxicity requires a review of the entire system – feeding and mixing history, feed intake, clinical signs of sick and healthy calves, and necropsy samples. Diagnosis is challenging due to when symptoms occur and the similarities to different disease processes.  Because death isn’t observed until day three post-ingestion, sampling rations for ionophore levels is unrewarding.  The current ration may not be diagnostic if exposure occurred following a one-time mistake in mixing.  Rumen contents are unreliable because the animal is often anorexic and monensin may not be detected. Samples of cardiac and skeletal muscle may also be evaluated for damage during necropsy. 

There is no treatment for monensin toxicity other than supportive care.  Long term prognosis to affected animals is guarded due to heart muscle damage, weakness, and poor feed efficiency. 

Overall, incorporating ionophores into supplement strategies has a positive response on increasing growing cattle performance. Livestock producers should consider using ionophores to increase calf gain and gain efficiency in a cost-effective manner. Feeding ionophore in beef cattle diets is a safe, cost-effective strategy; however, properly feeding these compounds is crucial.

Interviews with the authors of BeefWatch newsletter articles become available throughout the month of publication and are accessible at https://go.unl.edu/podcast.