[Murph]

Most of the grass fields in the AAA version of the maps did not have detailed grass- I remember asking about it at the time the map was released. Cannon told me that it was a compromise that he made in order to have the map run with acceptable frame rates.
A few of the grass fields, such as Hawking, have textured grass.
Just a thought; Is it possible that some of the fields have grass surfaces on the early war version of the map and concrete on the later war version? It would make sense, as some bases were improved to allow heavy bombers to take off and land.