Summary of news story: An ABC news story from the cotton fields of Arkansas:at least one strain of pig weed (amaranth family) has developed a tolerance for herbicides, and can no longer be controlled by chemical spray. Like the giant ragweed that's currently growing in abundance on my land, pig weed grows large, fibrous stems within a few weeks. The stems "stop a combine in its tracks" (according to a farmer who was interviewed), preventing harvest by machine. Some farmers are paying laborers to chop out the pig weed; others will have to hire labor to harvest the cotton by hand. The chemical companies say they're working on the problem but do not expect marketable results for 6 to 8 years.
My comments: I do not know of any herbicide-resistant GM crops in the amaranth family. It looks as though pig weed probably developed the herbicide resistance on its own, in the same general way that certain bacteria develop resistance to antibiotics. Researchers have observed cross-species and even cross-family sharing of nutrients and genes. I recently read an article about the danger that antibiotic-resistant strains of staphylococcus will pass their resistance to other kinds of bacteria. If cross-family sharing of genes is possible, then it's possible that pig weed could have "picked up" genes for herbicide resistance from GM crops, but far as I know, it's not likely.
It's interesting to consider the affects that herbicide resistant weeds and insecticide resistant insects could have on the economy. One possibility is that unemployed people could find work on farms chopping weeds and harvesting cotton. I would not expect many U.S. residents to take this sort of work, and the general mood in the U.S. seems to be against allowing in immigrants. More intelligent farm machines could be be very useful for chopping out weeds before they get large, but it will be several (many?) years before machines of this sort are available commerically. Most of the focus right now seems to be on robots for harvesting, because until recently, the least expensive means of controlling weeds has been chemical sprays. Another possibility would be a move toward more elegant farm management, using permaculture techniques. I would like to see a combination of intelligent machines and elegant management techniques.
See: Precision Ag Works (progress on crop machinery robotics) and Vision Robotics