The trouble with yard clipping is that they're fibrous and tough. They're difficult to break down enough for enzymes to work on them. That's why cows have 4 stomachs and spend all day chewing.
An alternative would be to produce ethanol from sugarcane or corn. There's a reason that those crops are the primary feedstocks for biofuels right now.
"I can't believe how many people think Air Lock is pronounced Hydrometer." -BigKahuna
"If you gave me a beer with placenta in it without telling me I would kick you in the nuts." -ODaniel
"We be in a big hurry for dope beer with much alcamahol and flavor, quality, balance, and aroma don't matter. We just wantz to be druck, u know?" -Yooper