Wired: Inconvenient Truths of the Environmental Movement:

the truth is that when it comes to greenhouse gases, organics can be part of the problem.
Take milk. Dairy cows raised on organic feed aren't pumped full of hormones. That means they produce less milk per Holstein — about 8 percent less than conventionally raised cattle. So it takes 25 organic cows to make as much milk as 23 industrial ones. More cows, more cow emissions. But that's just the beginning. A single organically raised cow puts out 16 percent more greenhouse gases than its counterpart. That double whammy — more cows and more emissions per cow — makes organic dairies a cog in the global warming machine.