[tmitchel2]

They emit methane as a by product of eating the grass, it's like chucking the grass up in the air and having to wait approx 20 years for it to fully come back down to be eaten again, it's still circular, it's still fully sustainable. Digging up fossil oils from deep ground and shuving it into the atmosphere and never ever putting it back deep into the ground is the elephant in the room here.

[cyberax]

Except that there isn't enough natural grass to keep all the cows fed.