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.
The carbon, nitrogen, etc. contained in that grass, waste, and straw should be buried in/returned to the soil to grow plants instead of being farted into our atmosphere. Assuming the land in question is arable in the first place (which I think is valid if it's producing enough plant matter for grazing to be viable), if managed at all would produce more calories of human-compatible nutrition per calorie invested than harvesting of grazing animals on said land would.