The Collaborative International Dictionary of English v.0.48:

Hayrack \Hay"rack`\ (h[=a]"r[a^]k`), n.
   A frame mounted on the running gear of a wagon, and used in
   hauling hay, straw, sheaves, etc.; -- called also hay
   rigging and hay rig.
   [1913 Webster]

WordNet (r) 3.0 (2006):

hayrack
    n 1: a rack that holds hay for feeding livestock
    2: a frame attached to a wagon to increase the amount of hay it
       can carry [syn: hayrack, hayrig]