According to an article in the latest Scientific American,
…weeds have begun to become resistant to glyphosate, the key ingredient in the widely used Roundup and a chemical that the biggest cash crops have been genetically engineered to withstand. Agricultural scientists must now seek out new strategies to protect plantings. Meanwhile some critics argue that reliance on genetic engineering should be reassessed. -(article summary)
Here’s another idea for you. Has anybody tried to genetically engineer the so-called “superweeds” to be profitable cash crops themselves? Can pigweed, ragweed, etc., be used to produce foods or biofuels? You know the saying: “if you can’t beat them, join them” (or, in this case, make them join you).
Of course, scientists have to be very careful because any intervention of this kind can have unexpected side effects. Increasing the nutritional content of a plant that spreads as well as these weeds apparently do, means introducing a new food source into the environment. Who knows which critters will find in it a new environmental niche where they can thrive? And if the GM version hybridizes with the non-GM version, how do you keep track of which is which, so that only the useable version finds its way into processing and production? And, would making the weed more useful for us, make it less useful for the other inhabitants of the environment that use the unaltered weed as their own food supply?
That said, in principle I am not opposed to GMOs, as some people are. Humanity has been making GMOs since the beginning of agriculture and the domestication of animals, by means of selection and breeding. Nowadays it can be a lot faster and more controlled, by directly modifying the genetic material in the lab, but the principle is the same. The important thing is that it be done carefully and responsibly.
Also, I think it would be wrong to do it to fully sentient life forms (like human beings), at least without their consent. Even with their consent, it becomes an ethical minefield, which has been amply explored by science fiction authors and ethicists already, but which remains controversial and dangerous territory.