Cash crops use something known as hybrid vigor (https://en.wikipedia.org/wiki/Heterosis) which breeds together two distinct, albeit mediocre, blends into a single offspring that is sort of like a super plant. The catch though is that it only works for a single generation, and if you were to replant the crop's offspring you would end up with a highly undesirable crop. For this reason, almost no farmers replant using their harvest. This pre-dates the use of GMO seeds.

Every summer in the midwest you get armies of workers who do something called 'detasseling' where you walk through the seed fields and rip off the male genetalia of one of the two breeds, which forces the seeds to be pollinated in a specific way. The result is the seeds that are used to plant next years crop.

source: I grew up on a corn farm.