Help rendering in layers with mental ray

So i have a scene with a still camera and still enviroment.
i wnat to render the enviroment as one image and just use it in the background and have my character rendered as an animation applied over the still background.
I know how to do it normally in maya but i'm using mental ray. Withouth the background my character isnt' lighted correctly becaues it no longer does the reflected light form the enviroment.
So how do i not have the enviroment visible to speed up render times but still have the reflected light from the enviroment.
Also, how do i not have the ground visible but have the shadows show up.
I need faster render times on this animation, 13 min is just too long.
any help would be nice
thanks alot
david