Vegetarians sometimes argue** that we have to abandon meat-eating because meat is an inefficient way of generating calories: if we all scaled up to Western diets, there wouldn't be enough land to grow food for everybody unless we switched over to vegetarian diets.
That argument is wrong on two fronts.
First, some land does most efficiently generate calories by producing grass to be eaten by animals. Like New Zealand high country sheep farming. There's not enough water in the country to irrigate that land to make it suitable for cropping. But it'll support low-density pastoral agriculture. Further, some grain crops wind up being of low value for human consumption but still suitable as supplement for animals. So the optimum is unlikely to involve everyone switching to vegetarianism if the objective is maximizing total available calories for human consumption (though if the developed world's bigger problem is obesity, I'm not entirely sure why we ought to take this as maximand anyway).
Second, it's entirely a partial equilibrium story. What happens as folks in the third world get richer and start moving from inferior goods like lentils and rice to superior goods like beef and lamb? The price of the latter get bid up. And what happens when relative prices change? People change their consumption bundles. And so I find the Washington Post reporting that Americans are eating less meat as meat prices increase.
Why is this happening? The Daily Livestock Report blames rising meat prices in the United States. As countries like China and India get richer, they’re eating more meat, which is helping to drive up U.S. exports and making beef, pork, and chicken more expensive here at home. Ethanol also plays a role: Nowadays, American farmers divert bushels and bushels of corn to make fuel, which drives up feed prices and, again, makes meat pricier.There's no need for a moral imperative to reduce meat-eating. Get rid of subsidies in the agricultural sector, make sure effluent externalities are properly priced or regulated, then let relative price adjustments take care of the rest. The optimal amount of meat will be eaten, so long as we keep waving our hands about the moral questions.
* You can maybe square this by saying people get massive disutility from being slaves while animals don't know that they're slaves. But humanity has a long history of slavery, and most slaves, as best I'm aware, didn't commit suicide. So by revealed preference, lifetime utility was likely still positive. And then we're stuck again.
**This was somewhat sparked by an argument on Google Plus linked here (thanks Ryan!)