There are a lot of misconceptions about so-called “superweeds”. So much so, in fact, that the Weed Science Society of America has recently written a fact sheet about this topic:
“Misconception: Herbicide use is creating a new breed of herbicide-resistant superweeds unlike anything we’ve ever seen before.
“Reality: The costly issue of herbicide resistance isn’t new – and neither are the competitive characteristics of weeds. Although the number of acres affected by resistant weeds has increased over the last decade as more growers have come to rely solely on herbicides with a single mechanism of action for weed control, weeds have exhibited resistance to many types of herbicides over the past 40 years. Many weed populations have even evolved resistance to multiple herbicide mechanisms of action.”
It is also important to keep in mind that when weeds evolve resistance to herbicides, they don't become any more aggressive. Part of the confusion about “superweeds” is due to the fact that there are many different definitions of this word. As I wrote in a post on my own blog:
“A superweed can be a ‘really weedy weed,’ or it could be a ‘herbicide resistant weed,’ or it could be ‘a weed that hybridized with a GMO crop.’ The word superweed might mean different things depending on who’s writing the article; and it could be interpreted to mean different things depending on who’s reading the article. Certainly, this isn’t ideal. But there are many words in the English language that can mean different things in different contexts, so why is this a big deal?
“The problem arises when the connotations become conflated. Most uses of the word in the media relate to herbicide resistance. But because many people associate superweeds with extra aggressive or vigorous characteristics, there is a real possibility for some readers to think that the weeds are extra aggressive or vigorous because they are herbicide-resistant. But this is simply not the case. That 7-foot tall glyphosate-resistant superweed you just read about would be seven feet tall even if it weren’t herbicide resistant. Likewise, there are many weed species that produce hundreds of thousands of seeds per plant, even if they’re not resistant to herbicides. Herbicide resistance doesn’t confer any additional super-abilities; it just makes the weeds more difficult to kill with herbicides.”
So how do weeds actually become herbicide resistant? Dr. Brad Hanson, a weed scientist at University of California at Davis explains:
“Herbicide resistance in weeds is an evolutionary process. A common misconception is that herbicides ‘cause’ resistance; however, herbicide-resistance is usually due to naturally occurring mutations. On a population level, organisms occasionally have slight genetic mutations; some of these are lethal (i.e. a plant that does not produce chlorophyll), some are beneficial (i.e. drought tolerance) and some are neutral (no observable effect). Occasionally, a one of these chance mutations affects the target site of an herbicide such that the herbicide does not affect the new biotype. Similarly, mutations can affect other plant processes in a way that reduces the plant’s exposure to the herbicide due to reduced uptake or translocation or through more rapid detoxification. Whatever the cause, under continued ‘selection pressure’ with the herbicide, resistant plants are not controlled and their progeny can build up in the population.”
So herbicides don't "create" superweeds; rather, repeated use of the same herbicide selects for naturally concurring plants within the population that can survive the herbicide. DNA evidence has shown that one particular herbicide resistance trait in the weed blackgrass actually pre-dates the discovery of herbicides by several decades.
And finally, although an increasing number of weeds are becoming resistant to the herbicide glyphosate in Roundup Ready crops, there is evidence that widespread use of this technology may have decreased the rate of herbicide resistance to other types of herbicides.
If you consider the total problem of herbicide resistance, Roundup Ready crops haven't necessarily made the problem better or worse. We've simply seen much more resistance to glyphosate, and less resistance to other herbicides. The problem of herbicide resistance is a serious issue, but it is one that will exist with or without GMO crops.