Something that is often forgotten is that not all land that is used for grazing animals is suitable for growing crops on. So it turns out that the most sustainable/efficient use of farmland averages out as mostly laid out to crops with a small amount of meat and dairy. (Far less than in a current meat eating diet). It would also no doubt be a lot healthier if we cut down on the dairy and meat. This is ignoring any ethical considerations, but it does seem that abolishing all meat and dairy in favour of plants is not the most economic way forwards.