The ionophore monensin is a class of antibiotics that can improve animal health, maintain body condition and increase feed efficiency, yet limit feed intake. This supplement may be beneficial during dry spells when ranchers scramble for additional feedstuffs.

Freelance Writer
Gilda V. Bryant is a freelance writer based in Texas.

Doses, benefits and increased energy

Monensin increases ruminal efficiency by reducing gram-positive bacteria in beef cattle, says Paul Beck, Ph.D., an Oklahoma State University (OSU) extension specialist for beef nutrition. This increases the production of propionic acid in the rumen, which is used to produce glucose to boost the energy available from feeds.

“Monensin, approved by the Federal Drug Administration (FDA) for growing calves, is labeled for 50 milligrams per day up to 200 milligrams per day, or producers can feed it every other day at 400 milligrams per feeding,” Beck says. “For beef cows, that dosage level is cleared at 200 milligrams daily for hand-fed products.”

When feeding monensin, ranchers should strictly follow FDA guidelines and label directions to give precise doses to beef cattle.

“Provide it in the concentrated supplement with [range] cubes to allow equal access for all the cows,” Beck says. “We do this when we supplement them in winter or drought situations. That is an adequate way to ensure we get as close to an even dose for all the cows. Within a group, there’s always variation on intake, but the average should target 200 milligrams per head per day.”


During drought, most producers feed forages or hay daily instead of every other day. That works well in winter when adding monensin, meeting FDA clearance guidelines.

“During the summer, unless we’re in a drought situation, we normally do not feed a lot of concentrated supplements,” Beck says. “It’s hard to adapt these programs to include monensin because of the product’s restrictive clearances for feeding beef cows.”

Beck and his colleagues, David Lalman of OSU and Shane Gadberry of the University of Arkansas (UA), analyzed over 20 hay and grazing studies that involved feeding monensin.

Data showed a 10% decrease in cows’ forage intake. There was a slight increase in milk yield, thanks to increased cow efficiency, and their body condition scores (BCS) did not change.

Because of FDA label restrictions, monensin works best when feeding cows in a drylot or nearby pasture.

“We’ve seen it work well when a producer utilized a sacrificed paddock, where he was limit-feeding hay,” Beck says. “The producer could decrease hay amounts, saving 10 percent in feed. He fed the hay every day at the same time. Making that work and saving pastures for better drought recovery later is a key benefit.”

Positive effects on thin cows and heifers

Jason Warner, Ph.D., an extension beef cow-calf specialist at Kansas State University (KSU), reports that the active drug is the compound, monensin sodium. Two different brands of monensin are available under the trade names Rumensin and the generic form Monovet90.

Methods of providing monensin include adding it to energy or protein supplements, such as range cubes or pellets delivered to animals on pasture. Operators can thoroughly mix this product into a total mixed ration (TMR) if feedbunks are available.

“During drought situations, we have limited forage or high-priced feedstuffs,” Warner says. “Think about [monensin] from a rate of return standpoint. Most data suggests that when feeding cows monensin, there is a 10 to 15 percent improvement in feed efficiency. If feed costs 2 dollars per cow per day, and if we can save 10 percent on feed, that saves 20 cents per cow per day. Monensin runs around 3 cents per cow per day, saving 17 cents per cow per day.”

Ranchers often manage horses, mules, donkeys and cattle together in the same pasture, pen or trap.

“Monensin is fatal to horses and other equines,” Warner says. “That’s a hurdle for many operators who want to use monensin from a management standpoint. Producers must use caution to avoid unintended losses if equines consume cattle feed formulated with monensin.”

Monensin positively affects beef cattle weight and BCS, Warner says. For instance, thin cows with a lower BCS might be more nutritionally challenged in the last 60-90 days before calving. Monensin can increase energy utilization by the animal, particularly when third-trimester and calving requirements ramp up. Since physically challenged animals are potentially at greater risk for coccidiosis, monensin helps them stay healthy.

“Producers need to work with a feed service provider who can help them navigate the right way to provide monensin to their cow herd,” Warner says. “A beef nutritionist, veterinarian or extension expert can help them select options. First, look at the type of cattle you’re feeding and understand what your goals are. Look at your choices and understand the best logistical way to provide monensin and what times of the year are most strategic for you to feed it. We often think about supplying monensin when we have low-quality forage in the fall or wintertime to maximize the use of lower-quality forage. Work with folks who can help you navigate how to deliver it in a cube, pellets or TMR.”

Coccidiosis prevention

Monensin does not require a Veterinary Feed Directive (VFD), says Clinton Roof, DVM, assistant professor at the Texas Tech Veterinary School of Medicine. Although this product does not require supervision by a veterinarian, Roof warns producers to make sure it is appropriately mixed in feed and not left out for free-choice consumption.

“Unfortunately, with free choice, we risk cattle reaching toxic levels that could cause detrimental effects to their hearts and other muscles,” Roof says.

The most efficient method of providing monensin to cattle on pasture is when mixed with range cubes and delivered daily. Roof suggests producers talk to representatives at large feed companies who can prepare custom orders. For instance, they can add monensin to range cubes, ensuring cattle receive the correct dosage per head per day. It is often more affordable if a number of area producers ask for the same product.

Providing monensin to a herd in drought-prone areas also prevents coccidiosis. It prevents coccidiosis spores from damaging the intestine, cecum and colon linings, decreasing spore growth. Symptoms include acute diarrhea and decreased appetite. In severe infections, animals show dehydration, pale mucous membranes and weight loss.

Monensin increases energetic efficiency. “That’s unique because it’s the only antibiotic labeled for an increased rate of weight gain,” Roof says. “It works and does what it’s supposed to do. During drought, our animals don’t lose condition, and ranchers save money on additional feedstuffs. Monensin also encourages reproductive cycling in heifers and cows, allowing them to cycle sooner because they maintain their caloric needs.”

The disadvantages of providing monensin include feeding it every day at the same time, which increases fuel and labor expenses. Roof stresses that operators should not feed it every other day or wait three to four days and then feed it. The benefits to the rumen decline if ranchers skip regular feedings.

“There are no commercial feed products that contain monensin,” Roof says. “Monensin is an additional expense. The idea is to buy it as inexpensively and [feed it as] consistently as we can. Working with a beef nutritionist would be beneficial too ... it is within our ability to feed it to cattle. Just do it safely.”