Here are 3 populations with 3 different colours of grass (using 3 different PF to populate).
For all populations, I put in Rotation minimum Y rotation = 0 and maximum Y rotation = 0
It means that they all seems identical from the POV of the camera.
That another reason I use 3 different colours to reduce the impression of "same thing everywhere".
For all 3 grass objects, I then animate Rotate Z starting at 0 at frame 1 to 30 at frame 50 and back at 0 at frame 100.
My next test will be with several areas (between 3 and 5), each starting the rotation at a different angle in order to give an illusion of windy wave on grass.
I'll post the result.
David