this is a typical Maya PFX problem. Im surprised its still not been addressed. Its basically Maya not being able to composite the grass at the post processing point in the render.

You can either split the objects in to different render layers and leave the grass on the ground plane object and then comp it all together in PS unless its an animation then do it in After effects or something.

Also it could be because its against that wire fence too (is it against a transparency mapped fence?) if so remember PFX needs depth map shadows not raytraced so rendering in separate layers with different render setting could be the way to go which can often happen.

Also there could be a maya software issue with regards to memory. if you have other apps open - close them.

Also try rebooting your machine, as sometimes all Maya needs is a quick kick to clear out the temp files etc that it allocates here and there.

Ive come across each and every one of these issues in production so give it a whirl

Cheers
Jay