This topic is often very poorly portrayed. The biggest problem with livestock isn't really the livestock. And lets not forget that 6 barrels of oil go into each cow. The real problem is the fossil fuels used to create the fertilizer that makes the animal feed and drive the tractors. A radio assay of carbon isotopes proves this beyond a shadow of a doubt. Nitrous oxide also comes from feed production and a little from waste. But those are very easily taken care of.
If we implement
http://www.e3biofuels.com/ then that solves the GHG's from waste and produces fuel that can be used to create fertilizer or run our cars. E3 biofuels is working profitably right now.
Livestock GHG's are a serious problem but not one can't be easily remedied. People just have to care enough to act. Honestly I think this is one problem that will solve itself as the price of fossil fuels go up.