“You can get more energy from the same feed, whether you’re feeding it to a lactating cow or developing a replacement heifer,” says Ron Randel, Faculty Fellow of Reproductive Physiology at Texas AgriLife. “The net result is better performance and better reproduction.”

Randel is talking about the proven opportunity that comes with supplementing replacements and mature cows with an ionophore. He and fellow researchers at Texas A&M University were among the first in 1977 to document the potential of feeding the ionophore compound, monensin, to heifers and cows. For the record, Rumensin® (monensin) is the only ionophore labeled for use in both growing heifers and mature beef cows.

If you’re unfamiliar with ionophores, they are antimicrobial compounds that inhibit the growth of rumen organisms that disrupt rumen fermentation and function; they help capture more feed energy and decrease protein erosion.

Depending on the research, ionophores in stocker diets increase average daily gain by 0.15 to 0.3 lbs./day and increase feed efficiency by 2-7%. That’s feeding at a rate of approximately 125-200 mg/day at a cost of pennies per day. Besides the gain, ionophores also reduce the incidence of coccidiosis, acidosis and bloat.

Increased feed efficiency means cattle attain the same performance on less feed or more performance with the same level of feed consumption.

“In all of the research we conducted, we didn’t find a single downside to the use of ionophores in either replacement heifers or lactating cows,” Randel says.

Enhancing heifer lifetime productivity

“Feeding an ionophore makes animals more energetically efficient,” says Dale Blasi, Kansas State University Extension beef cattle specialist. Thinking about what that can mean to developing replacement heifers, he explains, “It can reduce the age and weight at puberty. If you’re getting another 0.1 to 0.2 lbs./day of gain across a 150-day development period, you’re putting another 15-30 lbs. on heifers and that can be the difference in getting them bred.”

It’s more than energy efficiency and added weight at younger ages, though. Randel explains the ionophore positively affects ovary and pituitary function required for successful reproduction.

In that original study cited earlier, 92% of the heifers receiving monensin reached puberty during the trial vs. 58% of those not receiving it. In another study that included 590 heifers, heifers receiving 200 mg/day of monensin reached first estrus 13 days sooner than those not receiving the ionophore.

Though age of puberty revolves around multiple factors, including heifer age, breed and the plane of nutrition, Randel says the old rule of thumb still holds up: aim to get heifers weighing at least two-thirds of their mature body weight by the time they’re 14 months of age.

“You want heifers to be sexually mature by the time they’re 14 months old if they’re going to breed and calve on schedule as a two-year-old,” Randel says. “Use of an ionophore can decrease age of puberty in beef heifers by several weeks. Having an estrous cycle prior to breeding is important in getting them bred early in the season in order to calve early in the season. Heifers that breed and calve early tend to remain early breeders and calvers in subsequent breeding seasons and will produce more calves in their lifetime.”

So, decreasing the age and weight at puberty increases the potential lifetime productivity of heifers. Randel favors pounds of calf weaned per breeding exposure as the way to measure lifetime productivity. It gets at the number of calves produced as well as the weaning performance of the calves.

Randel points out using bulls with larger age-adjusted scrotal circumference (measured by the time bulls are 14 months of age) also sire daughter more likely to reach puberty sooner.

Manage cow body condition more cheaply

Cows receiving an ionophore glean similar benefit. In previous research, Dave Lalman, Oklahoma State University (OSU) Extension beef specialist, says Rumensin in cow rations reduced feed intake by about 10% without affecting performance. Generally speaking, its use increases feed efficiency 5-10%. Consequently, an adequate body condition score (BCS) can be achieved and maintained with less feed.

BCS, of course, has proven to be an extraordinarily accurate gauge of nutritional reproductive fitness.

Randel explains lactating cows partition energy consumed to maintaining life first, lactation second, their own growth and body condition next, and finally to reproduction. That’s why breeding back first-calf heifers can become such a reproductive bottleneck.

Cows in poorer than the optimum condition – BCS of 5-6 – experience longer postpartum intervals than their fleshier peers, meaning they conceive later, if at all, and calve later in the season.

In compiled research, cows receiving 200 mg/day of Rumensin conceived six days earlier, experienced a postpartum interval five days less, and posted an 11% higher calving percentage than cows not receiving the ionophore.

In a 2011 OSU study, cows receiving common prairie hay and 2.0 lbs./day of supplement (30% crude protein) with 200 mg/day of Rumensin gained 30 lbs., or about half of one BCS over 58 days. The cost of feeding the ionophore was 2¢/head/day.

Blasi likens feeding ionophores to beef cows and heifers with the suspension system on a pickup. “It gives you more cushion,” Blasi says. “Every little bump might be a weather insult or shock in forage quality. With ionophores, you get more energy from the feed available. Even in times of plenty, feeding an ionophore gives you more insurance.”

Rumensin in cow and heifer rations

For producers considering Rumensin in heifer and mature cow rations for the first time, there are some points worth keeping in mind.

First, Rumensin is toxic to horses. That means not only keeping them away from mineral or feed mixed with the ionophore, but making sure horses don’t have a chance to come across residual amounts when swapping pens and pastures.

Secondly, Blasi explains folks trying to mix appropriate levels of the ionophore into feed on their own are kidding themselves. Rumensin is mixed at 80-90 grams/ton. You’d be hard-pressed to find a cow-calf producer with the necessarily sophisticated technology to mix it so precisely.

So, Blasi and Randel stress that the ionophore should be added to feeds and mineral by reputable commercial firms.

Next, if you haven’t fed an ionophore previously, Blasi says you need to understand that it will reduce mineral, forage and feed consumption.

Reduced mineral intake saves money, of course, but Blasi says it bothers producers who mistakenly equate mineral quality with the level of mineral consumption. He believes an added benefit of Rumensin in mineral is that it prevents over-consumption of mineral mixes, which he believes is a common problem.

Finally, the quality of feed available needs to be considered.

“If forage quality is too poor, you will reduce forage intake to the point that the effect of using the ionophore can be negative,” Randel says. “If forage quality is less than 6% crude protein and digestibility is around 45-50%, using an ionophore is probably counter-indicated.”

At the very least, the amount of ionophore fed should be adjusted with ration quality, Blasi says. The higher the forage quality, for example, the more Rumensin that can be fed and benefits returned (Table 1).

“There’s something to be said for keeping Rumensin in front of cows through the mineral year round,” Blasi says. If that’s done, he’s comfortable with a rate of 100 mg/day. “At 2¢/head/day, you’re talking $7/cow/year.”

In round numbers, Randel says, “If you use ionophores within reason (according to recommended dosage), you should see a net decrease of about 10% in the cost of feed to develop heifers to puberty and pregnancy... Where there is the opportunity for daily supplementation, I advocate the use of ionophores. It should improve your bottom line unless companies are charging too much for the ionophore.”

As for mature cows, in that 2011 OSU study, those receiving common prairie hay and 2.0 lbs./day of supplement (30% crude protein) with 200 mg/day of Rumensin gained 30 lbs., or about a half BCS over 58 days as mentioned previously.         

“There are a lot of places where producers can spend their money,” Blasi says. “I believe providing ionophores to growing cattle and mature breeding cows is one of those things that offers producers a definite return.”