The Collaborative International Dictionary of English v.0.48:

hayseed \hay"seed\ n.
   1. a rural, unsophisticated person; also used in an extended
      sense for one who is not very intelligent or uninterested
      in culture.

   Syn: yokel, rube, hick, yahoo, bumpkin, chawbacon.
        [WordNet 1.5]

   2. Seed from grass, especially that which falls out of hay.
      [PJC]

WordNet (r) 3.0 (2006):

hayseed
    n 1: a person who is not very intelligent or interested in
         culture [syn: yokel, rube, hick, yahoo, hayseed,
         bumpkin, chawbacon]

Moby Thesaurus II by Grady Ward, 1.0:

57 Moby Thesaurus words for "hayseed":
   acorn, berry, bird seed, boor, boorish, bucolic, bumpkin, churlish,
   clod, cloddish, clodhopper, clodhopping, clown, clownish,
   countrified, country bumpkin, country-born, country-bred, farmer,
   farmerish, flaxseed, from the sticks, fruit, gawk, gawky, grain,
   hick, hicky, hillbilly, hobnailed, inurbane, kernel, linseed,
   looby, lout, loutish, lubber, lummox, lumpen, lumpish, nut, oaf,
   pip, pit, provincial, rube, rustic, seed, stone, uncouth,
   uncultivated, uncultured, unpolished, unrefined, up-country, yokel,
   yokelish