The Collaborative International Dictionary of English v.0.48:

broomcorn \broom"corn\, broom corn \broom" corn`\ (Bot.)
   A tall variety of grass (Sorghum vulgare technicum), having
   a joined stem, like maize, rising to the height of eight or
   ten feet, and bearing its seeds on a panicle with long stiff
   branches, of which brooms are made.
   [1913 Webster]

WordNet (r) 3.0 (2006):

broomcorn
    n 1: tall grasses grown for the elongated stiff-branched panicle
         used for brooms and brushes [syn: broomcorn, Sorghum
         vulgare technicum]