The entire scene was far too large to be composed in one pass, so each frame was rendered twice with only half the grass each time. Afterwards the two rendered halves were composited together to give the end product.

And yes, I did create the grass by hand. Well... I made 3 different strands, and replicated each 15,000 times with introduced slight variations. Easy enough. :)

3 types of grass, 15,000 replication each per panel [total 45,000 per panel]
21 panels x 45,000 = 945,000 grass strands in scene.