The following is an excerpt from Michael Pollan's (2008) book, In Defense of Food.  I highly recommend this book to anyone interested in feeding themselves or their dogs.

            With the rise of industrial agriculture, vast monocultures of a tiny group of plants, most of them cereal grains, have replaced the diversified farms that used to feed us.  A century ago, the typical Iowa farm raised more than a dozen different plant and animal species: cattle, chickens, corn, hogs, apples, hay, oats, potatoes, cherries, wheat, plums, grapes, and pears.  Now it raises only two: corn and soybeans.  This simplification of the agricultural landscape leads directly to the simplification of the diet, which is now to a remarkable extent dominated by - big surprise - corn and soybeans.  You may not think you eat a lot of corn and soybeans, but you do: 75 percent of the vegetable oils in your diet come from soy (representing 20 percent of your daily calories) and more than half of the sweeteners you consume come from corn (representing around 10 percent of daily calories).