Looks like the polycount is the problem then?
DoF needs an additional view for the depth wich should double the triangles, while shade-c may also create some more extra views for reflections and other things, so that might also add additional polygons.
How do you remove grass that is further away? Are you using gamestudios LOD system or something else? Because with those additional views it is possible that some settings aren´t correctly copied from the camera or something.
Some screenshot with debug panel open would probably also help.