Besides the big amount of grains used for livestock, what is often overlooked is the water consumption involved in raising livestock. According to Robbins, author of Diet for a new America:

Livestock use approximately 50% of all the water in the US.

Livestock produce twenty times the excrement as the human population of the US. This increases the nitrate/nitrite water pollution.

It requires 60-100 times more water to produce one pound of beef than a pound of wheat.

Livestock require excessive water usage because the land needed to grow grain for livestock takes up about 80% of the grain produced, and because water is needed for the animals.

When one considers the water needed for this extra grain and for the care of the livestock, a flesh-food diet creates a need for 4.500 gallons per day per meat-eater as compared to 300 gallons per day for a vegan.  A vegan saves approx. 1'500'000 gallons per year as compared to a flesh- and diary eater.