The animals burn up most of that food’s energy just to breathe and keep their bodies warm, so we end up with a small fraction – usually no more than one-third and sometimes as little as one-tenth – of the food value that we feed them. By contrast, cows grazing on pasture eat food that we cannot digest, which means that they add to the amount of food available to us.
赞成这种生产方式的人辩护说，这是应对人类日益增长的食物需求的一种方式，虽然令人遗憾，但却必不可少。恰恰相反，当我们把动物限制在工厂化农场中时，我们也必须为它们生产食物。只是呼吸和保持体温，这些动物就将它们所吃食物的能量消耗大半，因此我们最后只得到一小部分的能量，通常不超过饲喂食物能量的三分之一，有时甚至只有十分之一。与之相对照，在牧场放牧饲养的牛吃的是我们无法消化的食物，这就意味着增加了我们可用的食物数量。