The conventional wisdom is that it takes as much as 10 units of grain to produce one unit of meat with the equivalent nutritional value.
George Monbiot has recently pointed out that we should be comparing the amount of land and water required to grow meat with the land and required to grow plant products of the same nutritional value. And that when you do that you come up with radically different results.
Many of the world’s animals are fed on products which humans don’t eat and could not eat. These include residues and waste and straw and grass which is grown on fallows, rangelands and other places where there cannot be grain production. When you take that into account, the worldwide average conversion ratio of useful plant food into meat of about 1.4:1 – even though there are plenty of animals being fed on grain. Where animals are fed on exclusively waste and land which cannot be used for gain, the ration drops below one. That is, it can be more effecient to eat such meat than grain. The problem is with meat from grain-fed animals, not with meat per se.
As to water, it is commonly claimed that it takes something like 100,000 litres of water to produce one kilogram of meat. But this figure assumes that every drop of water which ever falls on a field on which a cow is grazed goes into that animal and never re-emerges. Simon Fairlie (former editor of The Ecologist magazine) has calculated that the true figure for beef is no more than about 400 litres of water per kilogram of meat..
Simon Fairlie concludes that "within any agricultural system there is a certain level of meat eating which comes more or less free because it is basically a byproduct of an agricultural economy which is mainly designed to produce grains and other vegetables. I couldn’t find a really good name for this but I just had to call it default meat, which is a term used by the Food and Agriculture Organisation."
If they are free range, cows, sheep and chickens, which have been bred over a very long time to be efficient producers of meat, are good sources of "default" meat; as are pigs which are fed on scraps and game animals, such as kangaroos in Australia, which grow on waste land.