Extensively farmed beef emits three to four times as many GHGs than intensively farmed beef
"Production of 1 kg of extensively farmed beef results in roughly three to four times as many greenhouse gas emissions as the equivalent amount of intensively farmed beef. Differences are mainly caused by differences in farming system. In intensive systems the nutrients in the feed are relatively efficiently ‘transformed’ into meat and dairy because the animals do not have to (or cannot) walk much about to find their food."
"Land use related to ruminant meat is relatively great, particularly for meat from extensive grassland farming, since these grasslands are less productive than arable lands, and because cattle have a slow reproductive cycle and a relatively low feed conversion."
"For cattle rearing in extensive systems, a further intensification could affect semi-natural ecosystems. Given the rising global demand for meat, the increase in yield however could alleviate the pressure on the remaining natural areas, especially in South America. Beef production in Brazil is currently relatively extensive, but is becoming much more intensive. Between 1970 and 2007, beef yield per hectare quadrupled and the carbon footprint per kilogram of beef has more than halved Because grazing animals have been around for a very long time in such original grasslands, nature has adapted to grazing, leading to a richness in species. Extensive grazing systems currently often preserve this biodiversity, albeit mostly by newly introduced species of grazers. The balance tips when the grazing gets too intensive and the grasslands may deteriorate (Blanco-Canqui and Lal, 2008). Carbon footprints of beef from extensive production may seem very high compared to beef from intensive systems, however this is at least partly dependent on assumptions regarding allocation and soil carbon sequestration (Flysjö et al., 2012)."
Durk Nijdam, Trudy Rood, and Henk Westhoek, "The price of protein: Review of land use and carbon footprints from life cycle assessments of animal food products and their substitutes," Food Policy 37, (2012): 760-770, http://dx.doi.org/10.1016/j.foodpol.2012.08.002.