Stehfest et al. published "Climate benefits of changing diet" [pdf] in Climatic Change (2009)
Abstract: The livestock sector... accounts for 18% of the greenhouse gas emissions and for 80% of total anthropogenic land use... we found a global food transition to less meat, or even a complete switch to plant-based protein food to have a dramatic effect on land use. Up to 2,700 Mha of pasture and 100 Mha of cropland could be abandoned, resulting in a large carbon uptake from regrowing vegetation. Additionally, methane and nitrous oxide emission would be reduced substantially. A global transition to a lowmeat-diet as recommended for health reasons would reduce the mitigation costs to achieve a 450 ppm CO2-eq. stabilisation target by about 50% in 2050 compared to the reference case.
Sounds like we've got to put a price on those cow farts.