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?
If I remember later I can try to find a chart on this that I might have bookmarked. But the chart basically shows that the amount of calories a cow has to eat to product, say, 100g of meat is 11 times as much. It was a pretty big eye opener for me when I was first going over to veganism.