Now this is fascinating: the the hidden danger of vegetarianism

It turns out that animal protein is the only effective, concentrated store of excess vegetable matter during record harvests.  Without animal protein (milk and eggs don't count), the surpluses would almost certainly go to waste leading farmers to produce less which would be catastrophic in lean years.  Hat tip Marginal Revolution.

The overriding advantage of meat is that demand for it is elastic.  People don't need it but they like it, and up to a point, however much you produce, they'll keep on buying it.  The demand for cereals for human consumption, on the other hand, tends to be inelastic.  People need their pound of grain a day, but they don't need much more, and they won't buy any more unless they have sufficient wealth to invest the grain in animals, either to produce higher value food, or else to keep it "on the hoof" for a rainy day (or a drought).

The existence of meat means that a farmer can sow wheat, barley, oats, beans, maize, and so on with reasonable confidence that, in the event of a good harvest, someone will buy it, because even if everybody has sufficient, it can be fed to animals.  This dynamic is not restricted to a money economy.  It works exactly the same for Melanesian subsistence farmers who can sow enough sweet potato and manioc to cover a bad year knowing that it is not a waste of effort, because in a good year the surplus can be fed to pigs.

Take the animals, the elastic element, out of the equation and the business of sowing grain suddenly become far more risky...This elementary matter of the need for a feed buffer fails to feature in most of the literature that is written about meat-eating and vegetarianism...

Could this explain why India was the last major nation that despite being free of war and having civil order experienced a major famine (1943). Most Indians, being Hindus, won't eat meat, therefore there was very little incentive to over plant for the lean years.