The Collaborative International Dictionary of English v.0.48:

Dehorn \De*horn"\, v. t. [imp. & p. p. Dehorned; p. pr. & vb.
   n. Dehorning.]
   To deprive of horns; to prevent the growth of the horns of
   (cattle) by burning their ends soon after they start. See
   Dishorn. "Dehorning cattle." --Farm Journal (1886).
   [1913 Webster]

WordNet (r) 3.0 (2006):

dehorn
    v 1: prevent the growth of horns of certain animals
    2: take the horns off (an animal)