Perhaps I should have elaborated. Rumensin actually is the only Ionophore approved for use in breeding stock. You are correct that it, Bovatec etc. aren't implants and don't work as such. We feed Ionophores in various forms to a whole slug of feeder cattle with mostly good results.
What I did mean though is that Ionophores really are growing cattle for a short term purpose such as finishing or developing a big shiny sale bull etc.
I'm not a fan of taking a heifer who I'm going to ask to go back into the herd for 10 years and bring a calf to the weaning pens each of those years and use products that falsely alters their energy to turbo charge developmental growth. Some do, and it may work very well for them, its just not how we like to grow replacement heifers.