It's more than 6%. Cows eat a lot, so people cut forests to grow more feed (like in the Amazon). Deforestation contribute to global warming, not to mention the biodiversity damage.
If we care about deforestation then we definitive should also not use ethanol as a replacement fuel, since ethanol uses the exact same farming and plants as the one used for feeds. Animal feed and bio fuel farming are usually combined. The other main source of bio fuel is from the lumber industry.
If your arguments aren't believable, I'm buying extra steak this evening.