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.”