Rise of the Superweeds

One of the arguments against GMOs has been that they caused the evolution of so-called “superweeds”, an epidemic that apparently has the world headed for a crisis. Some claim that we will soon be unable to sustain our modern agriculture techniques, and GMOs are at fault.

Fortunately, the reality is quite different. Superweeds are indeed a problem, and their evolution is causing significant problems for farmers. However, we do have options to control them, and emerging technology is going to help us out.

Superweeds – What Are They?

Superweeds really aren’t that super. They aren’t bigger, faster growing, or more competitive than “normal” weeds. All a superweed actually is is a weed that is resistant to a certain type of control. Becoming resistant to Roundup (glyphosate) doesn’t automatically make a weed more competitive, it just makes it more difficult for us to control them. It eliminates one of the tools we have in our toolbox.

A common misconception is that superweeds are only resistant to glyphosate, but that is actually not accurate. Weeds have been evolving resistance to many kinds of control for as long as agriculture has been around. In fact, in India, farmers used to hand weed barnyard grass out of rice. Barnyard grass was very similar in appearance to rice, but it had a distinctive red stem. As farmers removed those weeds year after year, a different strain of barnyard grass with a green stem became more prevalent. Eventually, farmers could no longer tell the difference between barnyard grass and rice; they had selected for green stems (read more here). Essentially, you could call that particular strain of barnyard grass a superweed. 

What Causes Superweeds to Develop?

Like the example of the red-stemmed barnyard grass, evolution of resistance to glyphosate and other chemicals is really rather simple. Continuous application of the same chemistry to the same fields year after year will allow that one weed with natural resistance to proliferate. One year, there’s one of them. That weed produces dozens or – believe it or not – millions of offspring. In year two, depending on how many germinate, survive and reproduce, an exponential increase in resistant populations begins.

All that particular weed has is a mutation that allows it to survive the chemical. Sometimes, it can only survive a lower rate, which is why proper application rates are
so important. As that weed begins to spread, it finds its way into other fields and other farms. Combines do a great job of blasting weed seeds hundreds of feet through the air, and with the right wind, weed seeds can even be blown into adjacent fields.

Whose Fault Is It?

Are farmers to blame for this? In a word, yes. We are responsible for understanding the chemicals we apply on our own fields. Don’t get me wrong; I’m not trying to throw myself and my colleagues under a bus here. But there have been many farmers that simply grew Roundup Ready crops over and over for over a decade – applying incredible selection pressure to their weeds. Could pesticide companies do more to educate farmers on this subject? Yes, but they have vastly improved their education efforts. A fantastic example is Bayer Crop Science’s Mix It Up campaign, which informs farmers how their products can improve resistance management (read more here). I think the key going forward is that assigning blame gets us nowhere. Let’s instead focus on solutions.

When Roundup was first released, it was a novel herbicide that was considered to have “Low” susceptibility to resistance. Because of its mode of action, weeds would have to develop a complex resistance mechanism. Sadly, this assumption led to an overapplication of glyphosate, much of it at rates too low to be totally effective, and has thereby resulted in the loss of one of the great inventions of the modern age to many regions.

How Do GMOs Fit In?

The introduction of Roundup Ready crops in the 1990’s caused a surge in the use of glyphosate. It was cheap, safe, very effective on weeds and easy to apply. Despite what has been coming out in the media about glyphosate lately (read more here), it is actually a very safe product with no known health effects in humans or animals. It was a breakthrough in agriculture, one of the greatest of our time. It is because of this that its overuse was simply inevitable.

It is, however, important to distinguish that although Roundup Ready crops contributed to the overuse of glyphosate, they did not cause resistance themselves. Glyphosate is not the only product with resistance problems. Other chemical groups also have issues, such as sulfonylureas, imidazolinones, PPO inhibitors, plant growth regulators and so on have all led to the proliferation of their own resistant weeds. None of these products are tied to glyphosate resistant crops. In fact, even insects and plant diseases have evolved pesticide resistance. In Europe, for example, flea beetles have become resistant to pyrethroid insecticides. Canada thistle became a problem in the prairies largely because of cultivation, which allowed their roots to be spread all over the fields.

Any type of pest control can and will cause pest resistance if the selection pressure is high enough. Unfortunately for RR crops in the United States, the selection pressure was simply too high.

What Are We Doing About It?

Every problem has a solution, and superweeds are no different. The way to fix this problem is actually pretty simple – rotation. Using a variety of crops combined with a variety of chemicals prevents weeds from building resistance. Take that Roundup-resistant weed and hit it with something else. Maybe a different chemical, maybe a little bit of strategic tillage, maybe even just a more competitive crop.

There are some incredibly exciting developments in the world of crop protection. RNA interference technology may just be the new frontier in weed management. Palmer amaranth, a particularly problematic weed in the US, resists glyphosate by producing extra copies of EPSPS, an enzyme required for amino acid synthesis that glyphosate binds to and prevents growth, eventually causing death. It overcomes the glyphosate application by simply producing so many copies of EPSPS enzyme that it overwhelms the glyphosate molecules. To stop this from happening, RNAi prevents the production of the EPSPS enzyme. With less of it produced, glyphosate is once again effective (read more here and here).

While RNAi is very new and is probably years away from production, it is promising to see innovations like this on the horizon. In Australia, where weed resistance is a major problem, some farmers are using the Harrington Seed Destructor to destroy weeds as they leave the back of the combine, which is proving to be incredible effective. Robotic weed destroyers are prototypes today, but could be a game changer in the near future. But in the meantime, we must protect what we have. Careful rotations and proper application techniques will go far to secure the usefulness of the chemicals we have for the foreseeable future.

Superweeds are nothing new. Weeds always have and always will evolve survival mechanisms against our strategies to control them. GMOs are not the culprit here; a lack of discipline in the use of our most valuable chemistries is the reason we are having the problems we are having. Panic by local governments and knee-jerk reactions will not help us deal with weed resistance. Education, research and new ideas are what we need to combat this problem. Weeds have always been one of farmers’ greatest challenges, and we will have to continue to be innovative and determined to stay ahead of them.