I don't really see the point of growing corn. I assume most folks are interested in the XP and not the cash. Per KillaSqirl, minimum completion time with Witch Marge is 54 days, at which time you get 9000 xp and $45,000. If you grew Silvertongue 2 times per day for 54 days, (75xp*2)*54, you would receive 8100 xp and $29,700 in cash. That's only a difference of 900 xp. If you grew it 3 times per day, which is certainly easier than using Witch Marge 4 times a day, you would get 12,150 xp and $44,500 cash, for a gain of 3150 xp. The gain is even greater for those without Witch Marge. At 90 days twice per day, you are getting 13,500 xp. Three times per day, a whopping 20,250 xp.
Of course, if you are really lazy, corn is certainly easier :)
The only reason to do it is for the handshake glitch. The problem of course is that if you are able to successfully grow it, there is a high probability that any handshakes you can acquire will soon be wiped out by an update, and you are back to square one. I doubt that there were many people who were able to take advantage of the glitch, except for those who already had corn growing before the glitch started. I, like many people, was very near to completion when my crop was wiped out by the VD event. At that point I said * it, and I will just stick with Channel 6, (or Ocho if you have it) from now on. At least it only takes 16 hours to rebuild if it gets wiped.