There are a lot of those fields around growing just seed. Some are Bayer owned, others are regular farmers who contract with Bayer to grow seed as their crop. One of the most common jobs for young farm kids is "detasseling" where they go around to seed fields to cut the tassel (male part of the corn plant) off some of the plants. Normally you plant 1 row of male plants, and 4 rows of female plants, since corn plants are both male and female, they need to cut off the male parts of the female plants. (the male plants produce corn on the female parts that is not used for the seed). You can just assume that every rural kid between 15 and 18 does the above job for a couple summers, which gives you an idea of the scale.