Animals are lossy at converting plant calories into meat calories (varies for different kinds of livestock obviously). I don’t think it’s an obvious conclusion that more agricultural land would be required.
In comparison, animal rearing is feasible in pretty desolate places as long as you can transport feed and water.
Just claiming it's a lossy energy conversion is pretty irrelevant. Not all the 'calories' here are created equal.
To answer your question, almost anything will do as feed. Byproducts of agriculture, grasslands, processed/filtered organic waste. Animals will thrive on rotten meat too -- famously leading to the mad cow disease.