i would render a closeup of the grass, if they are like long pipe grass looking ones.

Then create an alpha channel in photoshop from the grass, then import the alpha channel and collor map back to maya as a background.

Then from that image use it as a sort of matte background behind the 3d grass, this will lower your render time.

Mostly you try to do as much as possible in matte paintings as possible.

This saves time. in production and rendering.