Nurbs are not adapted for this sort of thing ;)
ELse you must draw each blade of grass! Ask a random position of each blades!
Better is apply a texture of grass on a volume with different technic, Bump mapping, particules, fur&grass plugin etc...:)
It's more polygons modeling
http://www.google.fr/images?hl=fr&q=texture%20of%20grass&um=1&ie=UTF-8&source=og&sa=N&tab=wi&biw=1024&bih=603

http://www.pxleyes.com/video-tutorial/blender/8221/Making-Grass-with-Particles.html

A crazzy one Plugin by tak2hata in Sketchup! (it's free! ) It's amazing!
http://forums.sketchucation.com/viewtopic.php?f=323&t=28092
http://sketchupdate.blogspot.com/2010/05/having-spent-last-three-hours-fiddling.html
By Rico91