The biomass required to raise a cow far outways the biomass that would have to be raised to substitute for the sustenance that cow would provide. A study at cornell claims that "For every kilogram of high-quality animal protein produced, livestock are fed nearly 6 kg of plant protein."
Or sometimes the figure used is in acreage, as in 1lb beef = 20 acres of arable land used. I'm not arguing with what you're saying and I take your point, but it wasn't exactly what I was going for in what you responded to. I'm not arguing for meat either, I'm just not convinced that at the present time converting to an entirely plant-based diet would be as seamless and easy as the article implies it to be. Let's say that we did convert to an all-vegetable diet. The first thing that comes to mind is, where would the seed come from? Well, Monsanto, likely. After the loss of their revenues from corn and soy I'd be surprised if they didn't try to recoup that with sales of other seeds. Then there's the consideration of labor. It's one thing to harvest vast quantities of a grain crop, like corn, but an entirely different thing to harvest say, tomatoes. Basically, I don't know that converting to an all plant diet would save anything, at least in the short-term. The agriculture industry is largely set up to maintain our current system and I wonder how difficult it would be convert. Also, how would this impact farmers and the need for refrigerated trucks for shipping. Would there be any sort of impact in regard to food waste? Finally, what would happen to animals previously bred and developed to be large-yield food animals? They're not "natural." Would they go extinct? Obviously I'm taking this to an extreme, but changes on such a large scale are bound to have large impacts.