John Phipps quotes a rant about farmers misusing genetic technology here.
It caused me to wonder about this: suppose farmers start using compound A on weeds. Over the years the weeds develop resistance to compound A. So our marvelous chemical industry develops compound B, which farmers start using in place of compound A. Now if I understand how it works, because weeds are now growing and reproducing in an environment where the gene(s) providing resistance to compound A no longer provide a competitive advantage, those genes should eventually be lost from the weeds' genetic makeup. (While the weeds are busily gaining resistance to compound B.)
What I wonder, assuming I've got the biology roughly correct, is how long it takes to lose the resistance: 2 generations, 12 generations, 22 generations, 122 generations?