“About 2,000 pounds/907 Kg of grains must be supplied to livestock in order to produce enough meat and other livestock products to support a person for a year, whereas 400 pounds/180Kg of grain eaten directly will support a person for a year. Thus, a given quantity of grain eaten directly will feed 5 times as many people as it will if it is eaten indirectly by humans in the form of livestock products.…”
M.E. Ensminger, PhD
Consuming a plant-based diet also has significant benefits for our environment. For example, the amount of potatoes we can produce from an acre of land is about 40,000 lbs/18.1 tons compared to only 250 lbs/113 Kg of cow flesh supported from the same acre. When we feed plants to animals in order to eat the animal, we waste an enormous amount of food. For only one pound of cow flesh, 16 lbs/7.3Kg of plant food and 5,000 gallons/18,927 liters of water are required.
Compare that to 25 gallons/95 liters of water for a pound of wheat. Animal agriculture is also responsible for most of the food-borne illness epidemics from water runoff. Our use of animals contributes significantly to greenhouse gas emissions as well as the destruction of forests for grazing. More than 250 million acres of forests are cleared every year in the US alone for this purpose.
Source: Why Vegan?