If you want solid evidence you can read a book on the history of animal husbandry. Roman sources include Cato the Elder, Columella, and Varro describe how they used supplemental grains to get cows through the winter and provide oxen enough energy to work (and to feed cavalry which would have been completely impossible without them). Humanity has been feeding grains to cattle for thousands of years, likely prehistorically.

Then in the first half of 1800s a bunch of American farmers with an abundance of corn independently discovered that they could grow bigger cows for slaughter in half the time if they fed them grains instead of roughage like hay or grass. That idea quickly spread to Europe and by the time the green revolution and globalization rolled around in the second half of the 20th century, almost every body started doing it.

This isn’t some new phenomenon. It predates the globalization of agriculture and if you were to ask a random farmer around the world whether they feed their cows a ton of grain they’d look at you like you were asking a very stupid question.

It’d be like asking “do plants need fertilizer?” Yes. If you want to feed the world, yes they do.

where is the mythical land of people that prefer gamey metallic beef?