Conservation of mass?
Do they understand that producing energy and fertilizer using the bodies of animals is less efficient than producing the same number of calories or mass of nitrates from plants+less-energy-than-is-required-to-raise-the-animal-in-question?
There’s an element of truth here, in that parts of the world have a system where farm animals eat stuff humans can’t, such as wild grass, kitchen waste and straw. But modern ‘factory farms’ do use insane amounts of growth su0lements and medicines, which are just as bad.