I hope people who oppose genetically modified crops are not going to embarrass themselves again as they did with the monarch butterfly nonsense. You'll remember that a study found that corn modified to produce Bt toxin (derived from Bt, the most common insecticide bacterium used on organic farms) produced pollen that was also toxic to Lepidoptera larvae, including monarch butterflies. Of course the scientists who did the study were quick to point out that the insectides typically used on cornfields were probably MORE toxic to monarch butterflies, but that seemed to make no difference. Frankenfood was a killer and that's where the thinking stopped. People forgot that the entire point of Bt corn is to kill Lepidoptera larvae, just in a less toxic and somewhat less effective way.
As I read the study it says that fields of plants modified to withstand a common - but not super-effective - herbicide will show more somewhat more weed mass if a more effective herbicide is typically used on them but less weed mass if the same herbicide is typically used on the fields. Less herbicide is used for weed control and used later in the season.
I think the study seems like a success for GM crops. The point of these herbicide-resistant crops is for farmers to be able to spray their fields later in the planting cycle and use less herbicide, while still killing weeds effectively. The reduced amount of weed seed in the soil is supposed to allow the farmer to till the soil less or not at all, conserving topsoil (this was not in the study). The study associates all the effects on the weed-dependent ecosystem with the more (or somewhat less, in the case of maize) effective weed control associated with the GM herbicide regime.
The worry that prompted the study - that some species of weeds would become herbicide resistant and being to dominate a field, was not observed in the study. The only remaining objection I can think of is that using less herbicide might give weeds a better chance to evolve and develop resistance, but the reduced seed rain tends to suggest that this will be less of a problem. Also, less herbicide would, it seems, mean that fewer plants are exposed to herbicide through a vector other than direct spraying. That should mean that plants in the verges of the farm field will have less opportunity to develop a resistance to the herbicide. I also like the fact that the GM herbicide regime creates more dead weed mass on top of the soil. I think that will be a positive going forward for topsoil retention and the rural ecosystem.
peace,
boddi