On top of only being 18-19% from the source you linked as well as another I found, I keep not seeing specific numbers on cows, but more on livestock. Also, not all cows that are livestock, and not all livestock itself , are being raised just to be eaten. I want to believe.

You should check out this discussion between people on this topic, a good debate that I feel brings out excellent points about that types of gas livestock would have versus a vehicle http://message.snopes.com/showthread.php?t=77718

I wish someone would just the update the image, because it is not like the information could not be presented in a way that is fun and informative but also be true.