I think there is a big difference between intensive animal farming (growing animal feed like corn and soy to then grow animals with) and letting animals graze on natural grassland. For example grazing works in non-arable land (eg hills / mountains / rocky areas) and is therefore (I'm guessing) a more efficient form of food production for these areas than whatever else could be done there. On the other hand, growing plants on farms specifically to feed livestock, probably not very energy efficient compared to growing plants for direct human consumption on the same farms.