The Collaborative International Dictionary of English v.0.48:

gramma \gram"ma\ n.
   A pasture grass of the plains of South America and western
   North America; same as grama grass, which see.

   Syn: grama, grama grass, gramma grass.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

gramma
    n 1: pasture grass of plains of South America and western North
         America [syn: grama, grama grass, gramma, gramma
         grass]