Mason katie
Extension Beef Cattle Specialist / University of Tennessee Institute of Agriculture

What are ionophores?

Ionophores are a class of antibiotic-like compounds that do not include drugs that are considered medically important for humans. Commercially available ionophores in the U.S. include monensin and lasalocid. These feed additives do not fall under the Veterinary Feed Directive (VFD) status of other feed-grade antibiotics and, therefore, can be purchased without a prescription. However, they are still classified as a medication and may only be purchased premixed. Ionophores increase feed efficiency by altering the fermentation process. They also decrease potential for bloat and acidosis. In feedlot cattle, ionophores are primarily used to improve weight gain and decrease coccidiosis.

How do they work?

As feeds enter the rumen, microbes break feed particles down and through biochemical reactions, create volatile fatty acids (VFAs). These VFAs are absorbed by the rumen and serve as the main energy source for cattle. The major VFAs produced in the rumen in order of relative quantity are acetate, propionate and butyrate. Ionophores target gram-positive bacteria that produce acetate, giving an advantage to the bacteria that produce propionate. Propionate production is more energy efficient, so this overall process is what improves the energy status of the animal. Additionally, less methane is created as the acetate-to-propionate ratio shifts.

Feeding considerations

While the majority of ionophores are fed to finishing cattle, there is opportunity to create improved performance in pasture-based systems. Research indicates that cattle supplemented with ionophores have improved average daily gain (ADG), feed efficiency and body condition scores compared with cattle that aren't supplemented. The quality of the diet, specifically forages, will impact the degree to which ionophores improve performance. Cattle grazing high-quality forages will likely show a lesser response to ionophore inclusion.

Typically, ionophores are provided in dry or liquid supplements or included in mineral mixtures. Always follow label instructions when feeding medicated feeds, as incorrect use may lead to toxicities. Monitor mineral feeders closely to ensure they stay full to encourage consistent intake. Be aware that the doses of ionophores used in cattle feeds are highly toxic to horses. Care should be taken to ensure that horses do not have access to feeds or mineral mixes containing ionophores. There is no withdrawal time for ionophores, meaning cattle can consume them up to time of slaughter.


As the beef industry looks for ways to remain sustainable, it is important to embrace the proven technologies available. Ionophores create greater production efficiency in pasture-based systems and can be incorporated into most beef cattle operations.