There was a recent freakonomics podcast [1] from which I learned that greenhouse gas emissions mostly come from cattle breathing, not their manure (I guess the manure imagery that often goes along with describing livestock environmental impact is just used because it's evocative?). Also that livestock farming in the US only contributes 3% of the nation's greenhouse gas emissions [2], which I wouldn't call a particularly "impressive percentage".

[1]: http://freakonomics.com/podcast/meat/

[2]: https://www.epa.gov/ghgemissions/sources-greenhouse-gas-emis...