Animal & Veterinary

Questions and Answers on the Proper Use of Monensin in Dairy Cows

The Food and Drug Administration approved the drug monensin for increased milk production efficiency in dairy cows in total mixed ration systems in October 2004, and in component feeding systems (including top dress) in December 2005. The purpose of this document is to provide a list of answers to some of the most frequently asked questions concerning the use of monensin in feed for dairy cows.


Total Mixed Ration (TMR)
Component Feeding (including top dress)
Other Issues


Total Mixed Ration (TMR)

Q: The monensin approval in Title 21 Code of Federal Regulations (21 CFR), Part 558.311(f)(3)(xiii) states that the drug is to be used in a “total mixed ration”; how does FDA define the term TMR?

A: Generally, the term TMR is defined as the feeding system in which the roughage and the concentrate ingredients, including the protein, mineral, vitamin, and other ingredients are all fed as one mixture. This means that the composition of a TMR is such that it provides a balanced diet in each mouthful an animal consumes. However, dairy producers frequently provide their cows free-choice buffers, salt, and trace minerals in addition to the TMR. FDA considers the free-choice availability of buffers, salt, and trace minerals to be allowable, as long as these free-choice products are not significant sources of energy and protein. Monensin may not be provided in these free-choice products.

Q: Does this mean that monensin can be offered to dairy cows only in a TMR?

A: No. Monensin has also been approved for use in component feed (including top dress) and therefore does not have to be offered to dairy cows only in a TMR. However, when offered in a TMR, all conditions of the TMR use would apply.

Q: What if part of the cow's diet includes grass consumed while grazing and, at the same time, the cow receives TMR that contains monensin?

A: This use is not covered by the approved use of monensin in TMR systems because the “TMR” (sometimes called “partial TMR”) does not provide all feed ingredients to cows as a single mixture. Rather, this is an example of a component feeding system where cows derive nutrients from the Type C medicated feed with monensin and grazed forage from pastures (See section below on component feeding (including top dress)).

Q: According to monensin labels for a TMR use, it appears that the approved monensin concentration of 11-22 g/ton in Type C medicated feed is correct only when expressed on a 100% dry matter basis; is this correct?

A: That is correct. The monensin concentration in the final Type C medicated TMR feed for dairy cows has to be adjusted for the dry matter content in the ration, so that the final monensin concentration is between 11 and 22 g/ton on a 100% dry matter basis.

Q: Medicated feeds are guaranteed on an “as is” basis so why require the use of a 100% dry matter basis in this particular approval?

A: While it is true that medicated feeds are in principle guaranteed/regulated on an “as is” basis, which in practice means that dry feeds contain approximately 90% dry matter, this situation is different because the dry matter levels of a TMR could vary over a wide range. The purpose of the 100% dry matter basis is to ensure that dairy cows receiving TMRs of variable dry matter levels, receive the amounts of the drug that are safe and effective.

Q: What about the Type B medicated feed containing monensin; do monensin levels in the Type B medicated feed have to be expressed on a 100% dry matter basis?

A: No. The Type B medicated feed containing monensin for use in dairy cattle continues to express the drug concentration on an “as is” basis, just like any other medicated feed.

Q: What are currently approved monensin levels in Type B medicated feed for use in dairy cows?

A: According to the currently approved labels, the approved monensin levels are 23 to 80,000 g/ton and 40 to 1,440 g/ton in the dry and liquid Type B medicated feed, respectively.

Q: The Blue Bird Type B Medicated Feed Label includes a table to formulate TMRs with two dry matter levels (50% and 60%); do manufacturers of Type B medicated feed have to use these two dry matter levels in their labeling?

A: No. The 50% and 60% dry matter levels listed on the monensin blue bird labels are only examples when formulating TMRs. The manufacturers of the Type B medicated feed can provide examples with different levels of dry matter that better reflect the situation in the particular region they serve. Alternative examples can be developed using the formula that appears in the footnote to the table on both sample Type B medicated feed blue bird labels.

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Component Feeding (including top dress)

Q: The monensin approval in 21 CFR 558.311(f)(3)(xiv) states that the drug is to be used in a “component feeding system (including top dress)”; what is component feeding and what is top dress feeding?

A: Component feeding is a feeding system in which animals receive feed in other ways than in a TMR (e.g., feeding forage and concentrate as separate components). Top dress is an example of component feeding where animals generally receive monensin in a small amount of feed added on top of the animals’ usual ration.

Q: What is the approved monensin concentration in Type C medicated feed intended for dairy cow component feeding (including top dress)?

A: The approved monensin concentration in Type C medicated feed is between 11 and 400 g/ton (as is basis).

Q: The regulation also states that the Type C medicated feed must be fed in a minimum of 1 lb of feed to provide 185 to 660 mg/head/day monensin to lactating cows or 115 to 410 mg/head/day monensin to dry cows; does this mean that one chooses whether to follow the g/ton or the mg/head/day requirement?

A: No. The label must carry, and the user must comply with, both the g/ton (11 to 400 g/ton) and mg/head/day (185 to 660 mg/head/day monensin for lactating cows or 115 to 410 mg/head/day monensin for dry cows) stipulations. Feed intake and stage of lactation (e.g., lactating or dry) will determine the actual monensin concentration in the Type C medicated feed. Also, the specified drug amount cannot be fed to dairy cows in less than one pound of feed daily. In summary, the label for component feeding of monensin has to include mg/head/day, g/ton, and the one pound of feed minimum specifications.

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Other Issues

Q: Monensin is approved for continuous use in dairy cows; how does FDA define the term continuous use?

A: The term continuous use means that an animal receives medicated feed daily for at least 14 consecutive days.

Q: Would it be okay to label feed containing monensin for use in dairy cows with an amount of the drug that an animal needs to receive (e.g., 300 mg/head/day) instead of stating the concentration in g/ton of feed?

A: This situation is not allowed in TMR feeding systems where the label must carry the g/ton concentration (11 to 22 g/ton).  For component feeding systems (including top dress), the label must carry both the g/ton (11 to 400 g/ton) and mg/head/day (185 to 660 mg/head/day monensin for lactating cows or 115 to 410 mg/head/day monensin for dry cows) stipulations.

Q: Would it be okay to label a feed containing monensin for use in dairy cows with additional claims, e.g., preventing coccidiosis?

A: No. Additional claims, including those that were separately approved, cannot be combined without FDA approval.

Q: Is it true that feeding a monensin product labeled for prevention and control of coccidiosis for the purpose of increasing milk production in a dairy cow is an illegal extra-label drug use unless it is reviewed by a veterinarian?

A: The Animal Medicinal Drug Use Clarification Act prohibits extra-label use of an approved new animal drug or human drug in or on animal feed. Therefore, it is illegal to feed monensin to dairy cows for prevention and control of coccidiosis and increased milk production regardless of a veterinarian's involvement, as these are not approved claims for dairy cows.

Q: Would it be okay to feed dairy cows the drug poloxalene concurrently with monensin?

A: No. While the drugs poloxalene and monensin are approved for separate use in dairy cows, there is no approved combination of the two drugs that could be legally fed.

Q: If poloxalene is offered free-choice as a block in addition to a TMR containing monensin, would it then be okay?

A: No. The two drugs, even if physically separate at feeding, would still constitute an unapproved drug combination.

Q: Do labels for the products containing monensin for use in cattle have to include the warning statement advising against feeding to lactating dairy cows?

A: No. The lactating dairy cows warning statement is no longer required to appear on medicated feed labels of products containing monensin for use in cattle.

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Page Last Updated: 08/09/2013
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