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 FDA-approved 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...

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