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?
That's an orthogonal injustice though, no? Collectively, our species massively overproduce food, so I would think the fact that there is a prior reason to be trying to cultivate land like this, which ought to be managed for native flora/fauna is a separate and solvable large scale land allocation problem, the solution to which frees whatever livestock use the argument excuses.