Using Ionophores with Grazing Beef Cattle

What is an ionophore?

An ionophore is a feed additive used in beef cattle rations to improve feed efficiency and animal gains. Ionophores improve fermentation characteristics in the rumen, which leads to improved production efficiency. These additives are also known for their ability to help with the control of bloat, coccidiosis and acidosis. The primary forms of ionophores used for beef cattle on pasture include monensin and lasalocid.

Ionophores select against gram-positive bacteria and protozoa in the rumen. When these bacteria are controlled, the rumen fermentation environment becomes more efficient because less waste products, such as methane, are produced. This also creates a favorable environment for more desirable bacteria to grow that produce fermentation products that can improve the overall energy status of the animal, resulting in greater feed efficiency.

How can they be fed?

Most often ionophores are added into dry or liquid commercially manufactured feeds, or in mineral mixtures. Ionophores are included in small amounts when mixed into commercially formulated feeds.

Observations with Grazing Cattle

Growing cattle (i.e. stockers) and mature cows can benefit from the consumption of ionophores. Mature beef cattle grazing medium- to high-quality pasture have been observed to have increased weight gain and feed efficiency when provided an ionophore compared to non-supplemented cattle (Sprott et al., 1988). In a summary of 24 grazing trials, Potter et al. (1986) reported that growing cattle receiving 200 mg of monensin per head per day gained 0.20 pounds more per head per day compared to those that were not supplemented.

Will these additives be affected by changing FDA regulations regarding the use of antimicrobials in feeds?

Beginning in December 2016, all antimicrobials administered through livestock feed will require a Veterinary Feed Directive (VFD). A VFD is like a prescription written by a veterinarian for administering feed-through antimicrobials. However, only antimicrobial drugs that are medically important for treating human disease that are currently FDA-approved for production purposes will be impacted. Examples of drugs that will be affected include, but are not limited to, neomycin, tylosin, virginiamycin, chlortetracycline, and oxytetracycline. Ionophores such as lasalocid and monensin will NOT require a VFD unless used in combination with a medically important antimicrobial.

For additional information on the Veterinary Feed Directive guidance from the FDA, visit:



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