I swear there was another reason why the practice was considered bad. Something about nitrogen and sustainability. I never really understood that part, hence the question.
I think that one was the cows burp methane, which is a greenhouse gas. So if you apportion the greenhouse gas emissions over the delicious hamburgers, you make more climate change by making a cow burger than a veggie burger. So we should cease the production of cows as part of our attempt to not make our planet terrible. And buying cow burgers to eat is contrary to the goal of ceasing cow production.