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I'm an intelligent slashdot crowd, and I'm vegan you insensitive clod!

But in essence, I agree (I mean read my handle). So I want to rephrase your question a little.... Ok given our current cow-poo management policies, it seems like it would make sense to get electricity from it, but if people we to stop eating cows, drinking, milk, dedicating land to cows, the land and water that is used for raising cattle (and growing grain to feel the cattle), could be used much more effectively. I think in order to be fair, we need to compare a cow-poop scheme to a growing mustard, and producing biodiesel from it scheme.

Of course given that we are engaged in this wasteful misuse/abuse of animals, I don't see anything wrong with using the poop. I would surmise that the energy generate from cow-poop is less than the energy needed to run the tractors to feed the cows. But I'm not anti-poop digestion at all. I think humanure is probably one of the greatest sources of untapped energy.... (insert human poop jokes here) But seriously, why aren't we looking at running data centers (or at least their generators) off humanure? Centralized poop collection must become culturally acceptable!

I agree, but it is much easier to increase the efficiency of farming than it is to make humans give up its products. I'm a vegetarian, but I'd need a much more convincing argument than that to give up cheese. By using power generated from byproducts of farming (which, by the way, a lot of farms do already, so this isn't really news), we can increase the efficiency of farming and reduce the environmental impact of meat and dairy consumption now, rather than in a hundred years once we've convinced everyone to become vegan.

I'll have to have a word with my intelligent designer about that, and ask why he gave me these canine teeth that look like they're designed for tearing meat apart...

You must be terribly disappointed with the responses; one agree, one "agree, but...". For my part I'd agree that it's clear that growing plants then eating them is going to be more efficient than growing plants to feed to animals that we then eat, but I think it's more realistic to seek to increase the efficiency of farming animals than just give up on animal products.

The unnatural part is that there's so many of us, which is enabled primarily by agriculture. Before we started farming, hunting (and the implied eating of meat) was a major source of energy-dense food and we're 'supposed' to be omnivores; we didn't fuel the massive expansion in the size of our brains by eating grass/leaves, bark and roots like the other apes. Fruit has more energy, but it's not the most dependable food if you're foraging rather than farming.

Now that we do have agriculture, maybe we could survive quite well on plants alone, but that's the result of a few thousand years of improving farming methods and selective breeding. The hundreds of thousands or millions of years of evolution prior to that have set us up as omnivores, not vegans, and therein lies the flaw in your plans - generally speaking, any idea to improve society that requires people to act against their nature isn't going to work out, no matter how much sense it might make that things would be better if they did. See also communism ("Let's share" is nice enough, but people don't work that way en masse).

Hell, even while agreeing that it would be more efficient to just farm plants for food, I'm thinking that I don't want to give up meat... it tastes really good and I like eating it. That in itself is something of a demonstration that we're designed to eat the stuff; if we weren't built that way then why would so many of us want to carry on eating it?

We are not supposed to be carnivores.

There is nobody supposing us to be anything. Biologically, we are not carnivores because we can digest things that are not meat. We are also able to digest meat. That makes us omnivores.

You make a good case that meat production has a higher per-calorie cost than crops, but when you then go on to say what kind of organism we are "supposed to be", it kind of damages your point.

Also,

you cannot have 6 billion carnivores or omnivores, of the size of human beings, living on a planet the size of Earth

Interesting. You refer to sustainability in the long term, of course, since there are over six billion omnivores the size of humans (and several larger alpha predators such as tigers) living on Earth. Like, right now.

What simulation models and parameters did you use? What per-calory post is the limit for sustainability? What potential technologies, such as artificial protein cultures, new sources of energy, etc. affect this?

Or is that "no six billion omnivores on Earth" an article of faith?