“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.