http://jas.fass.org/cgi/content/abstract/74/6/1406 [Broken]The point is that they consume the amount of grain (primarily, corn) that could've satisfied caloric requirements of 800 million people, IN ADDITION to grass. BTW, the use of land as pasture to grow grass is still inefficient. We could use that land to grow vegetables and feed even more people. And corn is quite fit for human consumption. If we intended to feed it to humans, maybe we would've grown different varieties or used somewhat lower plant density, but that does not change the arithmetics substantially.
Ultimately, it's simple conservation of energy. Plants collect sunlight and turn it into chemical energy. We could either eat plants directly, or we could pass that energy through an intermediate step (feed it to animals, and then eat those animals). We know that the intermediate step is very inefficient: <10% with cattle, 20%, tops, with rabbits.
Not all grains are of a quality usable by humans and not all land can be used for producing vegetable foods for humans. Distilleries even sell their byproduct grains for feed. As well, cattle can consume a larger portion of the plant than a human.
Much of the corn produced by the US is exported as feed (we apparently have much more than we need).
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