Hasta La Motion!

i rendered out a single pass with my object over a plane that had a background shader and my scene in the image plane. i exported out into nuke and realised that the image plane was rendered to. no prob. my questions is, say i did not render my image plane out, would my shadows still be present. should i do render layers passes, so my image plane can be just thrown out. i tryed this on a previous project, and my background shader never showed when i comped in nuke. really don’t know if i explained this right, but any advice would by highley appreciated:D