Ionophores are feed additives that improve ruminal efficiency by shifting fermentation toward greater propionate production and reducing energy lost as methane in the rumen. The ionophores monensin (marketed as Rumensin by Elanco Animal Health and as the generic Monovet by Huvepharma) and lasalocid (marketed as Bovatec by Zoetis) are approved for growing cattle on pasture or in confinement and are commonly used in heifer development programs.
Multiple studies show that both monensin and lasalocid increase weight gain of growing heifers; however, lasalocid has not been widely studied for reproductive performance. In contrast, there is a large body of research evaluating the pioneer monensin product, Rumensin, on heifer efficiency and reproductive development. A recent analysis pooled these studies to quantify growth and reproductive responses of beef replacement heifers fed monensin during development.
Feeding monensin to developing heifers increased average daily gain by 5% and reduced feed intake by 4.3%, resulting in a 14% improvement in feed efficiency. Beyond growth and efficiency, monensin reduced age at puberty by approximately 9 days and increased the proportion of heifers cycling prior to breeding by 16%.
Earlier puberty and increased cycling activity before the breeding season improve the likelihood of early conception. Cows that calve early in the calving season tend to have higher reproductive rates and greater longevity in the herd, making these responses highly valuable for improving lifetime productivity of replacement heifers.
A study from the early 1980s reported that lasalocid increased gain of replacement heifers by 0.2 pounds/day and reduced age at puberty; however, additional research evaluating reproductive responses to lasalocid is limited.
Importantly, feeding monensin did not increase mature size or weight at puberty and did not negatively affect pregnancy rate. Improvements in feed efficiency, earlier puberty and enhanced cyclicity make monensin a valuable tool for heifer development.
As the U.S. beef herd moves into expansion, the role of ionophores in improving efficiency, lowering development costs, and strengthening reproductive performance of developing heifers is increasingly important. — Paul Beck, Oklahoma State University Extension beef cattle nutrition specialist
