You can get a 5 pack of CDS cells at radioshack for 3 bucks, so if the tubes from John Deere are somewhat expensive, you might be able to save some money with just opaque PVC or metal conduit tubes. Since you most likely won't be planting at night, you might even be able to avoid the LEDs by just drilling a hole in the tube across from where the CDS cell is. You might have enough natural light coming through that hole, but drill it with a 5mm bit just in case you do need to stick an LED in there.
Just in case you don't know, a CDS cell is a photoresistor. It works like a resistor that decreases in resistance as more light is applied to it. Basically, it's what they put on street lights to make them turn on automatically at dusk. The idea I'm thinking is a simple CDS cell inside an opaque tube with a light source directly across from it. Each time a seed falls between the light source and the CDS cell, the resistance will spike, working somewhat like a switch that goes on and off really fast, but would still be detectable by the Arduino.
John Deere makes some really nice machines, they are a lot of things, but cheap is not one of them. If you order them from JD, you might end up spending a lot more for one tube than it would cost you to build enough for the whole machine.
One nice thing about using your Arduino rather than buying a monitoring system from JD (aside from the price, and the fun of building it) is that you could even add in a safeguard to the coding where if a CDS cell is blocked for more than a fraction of a second (say one second) that could automatically set off an alarm saying that you have a clog in the tube.
With corn, you might even be able to get away with lever switches in the tubes where the seeds hit the lever and activate a switch. This might be easier for a beginner to work with than the CDS cells (but not by much) but any seed smaller than a corn kernel might not set it off. A small CDS cell on the other hand can be as small as 1/3" and should be able to detect enough of a drop in the light as long as the seed passes directly in front of it (using a half-inch diameter tube should solve that problem).