There is no way that analogy holds in this case. The farmer didn't cultivate their own RoundUp to spray, among other problems with it.
I could make up some convoluted argument but… the neighbor conducted the first sale so this should have extinguished any further patent claim. Of course, IANAL.
>A patent is a type of intellectual property that gives its owner the legal right to exclude others from making, using, or selling an invention for a limited period of time in exchange for publishing an enabling disclosure of the invention.
In this, letting the plant reproduce arguably falls under "making" (ie. manufacture) of the patented thing (the genetically modified plant).