According to Graham Harvey's book (Graham Harvey is agricultural adviser to the Archers and a prominent pagan incidentally) The Carbon Fields,  cows fed on meadow grass actually reduce greenhouse gases rather than adding to them.  This is because permanent meadow grass grows long roots that act as a carbon sink and more than compensates for the methane tthat the cows produce, which is in any case much less than that from cows fed on soya-based concentrates.  Thomas