Reducing the number of animals required to produce a fixed level of output can yield significant efficiency gains. For example, a 28 percent overall reduction in annual methane emissions in the United Kingdom between 1990 and 1999 can be attributed largely to reductions in cattle numbers and the increased productivity of dairy cows (DEFRA, 2001). Strong disparities in resource-use efficiency and GHG emission intensity still exist between animal farming systems and across regions (Herrero et al.,