Why would you want to use a vital part of soil production (manure) to create energy? That is an improper use of a resource. Cow manure belongs on and in the earth. 2% increase in soil on agricultural lands would bring CO2 levels back to preindustrial levels.

You better check Google before you write something the are more then 100000 Biogas Plant in the World for Manure and other Organic waste Methane goes not in the air like a normal open Pound Cowmanure still goes to Field after the Biogas Plant but you get Biogas ( Methane) for running Generator Set and get Eletric Power and get better Fertilizer it is a old Technic and many Farmers are do it