Between the feed they require and the methane they produce, beef production takes the hardest toll on the earth of any food available. You see, cows are ruminants, which means they are among the few species that can obtain nutrients from plant-based foods by fermenting it first.
This fermentation process produces a lot of methane, a greenhouse gas that is 25-times stronger than carbon dioxide (CO2). Thus, cows create the highest food carbon footprint. In fact, one pound of beef translates to 57 pounds of CO2, the equivalent of burning 2.6 gallons of fuel in your car.
Cattle also require more land and resources than almost any other farm animal. For every pound of beef that makes it to market, 10 to 14 pounds of feed is consumed. Not only is this inefficient, but it has resulted in deforestation, as farmers make way to plant more corn and soy to feed the cattle.
Another ruminant, sheep cause many of the same problems as cows. They are often overlooked, though, simply because of their smaller size. In reality, lamb have only a slightly lower carbon footprint at 55 pounds of CO2 per pound of meat.