Now do: Animals which would have been killed if all forests were cut down to supply the agricultural needs of 7 billion vegetarians.
The sort of one dimensional thought process presented here is somewhat dangerous.
The sort of one dimensional thought process presented here is somewhat dangerous.
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.