A natural guild seems to be forming here in this field of soybeans and amaranth! Unfortunately for the GMO soybean farmer, amaranth is (and was the first known species to be) glyphosate resistant.
Aside: Maybe I'm taking a leap here, but it seems the smart thing is to grow and harvest them both! I'm aware of the difficulties farmers have with 'weeds' interfering and damaging harvesting equipment, but I'm betting that some one, some where could find a solution?!
Anyway, back to the title of the post. My intuition is bringing out some interesting features of this situation, like... how did the amaranth become so quickly resistant? is it the soil biota? did the roots from the soybeans transfer genetic or epigenetic material to the amaranth? It seems a strong possibility that these two species 'want' to coexist - mutually dependent, mutually beneficial. Is there a higher function at work - nitrogen fixers guilding with nitrate accumulators, for instance? I've been googling for research about this possibility but haven't found anything yet. Hmm.