In your country. In my country, we don't advertise beef as "grass fed" as a market segmentation tactic, because all our beef is grass fed, that's the only way we feed stock raised for beef.

Once again, it's not just a simple case of "all animal husbandry is unsustainable", but we need to pick apart why the unsustainable practices have been developed into what they are now.

My gut feeling is that Government subsidies of corn in the US create an oversupply of it, so sure, why not feed it to cattle?

It's not like the American beef industry was predicated on grain feeding, it started with ranchers grazing them on prairies.

So what changed?

No, it is the worldwide use. In my country it is around 80% for lifestock and 10% for human food.

Averages are terrible across widely disparate data points.

It is not a average, it is the total global use of farming. Just because a few countries does better does not mean that it is okay. And that does not also change the fact that the 1 cow vs. 30 mice sounds like nonsense. And that if everyone ate less meat we would need less space for farming.