I'll start answering your question by taking a shot at the first one that was addressed to me 15 minutes ago, and hopefully my response to yours won't take as long to formulate.
Part of the reason it took me so long to come up with an answer is that I'm still wracking my brain trying to figure out what sort of disease my five-year-old or seven-year-old daughter would get that I would use ionophores to treat. The drugs that are used in beef cattle for the promotion of growth and feed efficiency are ionophores and they are not used in human medicine at all.
I think that's a really important point to keep in mind, that removing this tool from the livestock industry, whether it's cattle or dairy or chickens or pigs, is not going to benefit human health at all, and it will negatively impact producers and it will impact society as a whole. Because when you're improving feed efficiency, what it means is that you're using less feed to produce the same amount of meat. So it's resource efficiency.
Now, what was your question? How long have ionophores been used? Decades.
But there was another question that I thought I'd have a go at.