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Herbicide resistance: An agricultural arms race

Written by Anita Küpper, a 2016-2017 Sustainability Leadership Fellow and Ph.D. Student in the Department of Bioagricultural Sciences and Pest Management.

The journey of a tumbleweed

Sometimes skipping, sometimes lurching, seemingly without a sense of direction, a tumbleweed rolls over the prairie in Eastern Colorado. The thick scrub made of scrawny branches is arched so much that it resembles the round shape of a ball. While the harsh and strong south wind sweeps over the treeless plains, it effortlessly propels the weed along its journey. A second tumbleweed joins the sheared plant, chased by the relentless gusts. The two are rolling away, sometimes snagged together, sometimes as distant travel partners. Soon the two of them are no longer alone and a whole flock of detached scrubs stagger over the shortgrass steppe. First in tens, then hundreds and finally thousands they steadily mill ahead, like an armada of withered skeletons. A thick row of snow fences emerges and suddenly interrupts the cheerful march of the tumbleweeds. But other tumbleweeds that have previously come this way already piled up in front of the obstacle, forming a convenient ramp for our shrub, enabling it to climb over and continue its journey over the Great Plains.

Long after our tumbleweed and his companions passed and the landscape had forgotten about their transit, the soil seedbank had not. While the seemingly dead plant jumped over the soil, little three-lobed seeds had detached from the bolls and landed in a wheat field. The following spring after the snow melts and spring temperatures cause seedlings to emerge from the soil by the score, Ken Hildebrandt, a farmer and aerial pesticide applicator in Eastern Colorado, gets ready for another busy season. The constant economic pressure on farmers to produce high yields at low prices on limited space forces them to spray herbicides to avoid weeds from competing with their crops for water, light and nutrients. This particular season was not any different from others except that the herbicide applied killed most of the unwanted plants but not all of them: Despite having been sprayed, a clearly visible trail of sprouts remained growing where our tumbleweed had passed a few months earlier. Why is it that these survived while others did not?

 

 

Our tumbleweed got lucky. It had a genetic mutation that allowed it to survive the herbicide. Continuous selection pressure due to the repeated usage of the same herbicide favors plants that evolved to survive an application. When this resistant tumbleweed’s offspring reproduces and the next generation gets exposed to the same herbicide again, the number of resistant plants in the population will increase and at some point replace the susceptible plants, rendering the chemical no longer effective at controlling the weeds. The problem is comparable to that of antibiotic resistance in bacteria: If we re-use the same antibiotic over and over again, we are selecting for the bacteria that have the genetic set-up to survive the application, ultimately leaving us with antibiotic-resistant bacteria only. Similarly, herbicide resistance is a numbers game: If a herbicide gets used very frequently over the course of several years without any alternative ways of weed management techniques it is not a question of if but a question of when resistance will evolve.

A farmer’s struggle

The case of resistant weeds is especially troubling because some plants are able to produce up to a million seeds. So, if the farmer does not detect resistance in his field in time or chooses to ignore it, then he will have a much more severe problem in the next year. “I can see it from the air,” Ken Hildebrandt explains. “At first I didn’t quite understand how everything was dead in the field except for this one trail. The next year I noticed it is three to four times worse than that and five years later the whole field had escaped weeds despite spraying. One plant is enough to start a problem. Some farmers think resistance came over from the neighbor’s field because he didn’t control his fields or only used one kind of chemistry. No one will get excited about one escape in the field, but once it starts to have a monetary consequence the farmers will start to do something about it.”

And the economic impacts resistant weeds can have on crop production are severe. “They can take over the field. Sometimes I can see weeds as high as the crop the farmers are trying to grow. We are having weeds that we can’t seem to get rid of. In the last few years the problem seems to have increased at an exponential rate.” Hildebrandt elaborates further. “Resistance is costing us. I remember the times when you spent a few dollars on chemistry and could clean up a whole field. Now it can cost over 30 dollars an acre because you have to use more expensive combinations to get adequate control. With the price of herbicide as high as it is, it drives down the profit. Production agriculture has to make the farmer money.”

Once weeds have become resistant to a particular herbicide, farmers need to resort to other options to continue producing at high yields. They can either change to a different herbicide mode of action that still works or switch the crop rotation. But if all these possibilities fail, the only option left is to use mechanical cultivation which might include tillage, plowing, disking or sweeping. These techniques have the disadvantage that they deteriorate and dry out the soil, are more labor- and time-intensive and therefore also more costly. Not being able to use herbicides anymore drives up the price of food production, making food less affordable. “It once was more cost-effective to spray a field than to mechanically work it. Now it can be the other way around when we don’t have anything that works, it forces us to go back to the farming practices of the old days. We have come full circle.” Hildebrandt describes. “Farmers are willing to adapt. They try out different things but at some point they are going to run out of options. Right now, farmers are hoping there are new chemistries coming out or some new technology is going to come to help with the problem.”

Ken Hildebrandt spraying a field with his air tractor (Video by Curtis Hildebrandt)

Developing ways to fight herbicide resistance

So what is it that we can do to counteract herbicide resistance? This is where the weed science laboratory at Colorado State University comes in. Here, we are trying to understand how the weeds evolved mechanisms of resistance and how these work. To me doing research on resistant weeds often feels like visiting an inverse crime scene because I study the individuals that survived instead of the ones that were killed. And the question is always: How did the survivors manage to pull this off? What is the strategy that makes them superior to their susceptible peers? Like a detective I am trying to hunt for clues that would give an idea of how the weed managed to survive with the final goal of finding the mutation that is responsible for resistance. Several possible mechanisms of resistance are already known: They range from reduced herbicide uptake or reduced translocation within the weed over plant-internal changes at the herbicide binding site to metabolic detoxification. Often enough weeds come up with mechanisms that we did not know were possible and have little knowledge about.

It is a fascinating area of research because despite us spending a lot of money, weeds seem to have an amazing ability to get around every effort to kill them. It is like a fast-paced arms race, like a game of Elmer Fudd and Bugs Bunny where each party is trying to outsmart the other by being a step ahead. And in the process, we learn a lot about basic science and plant evolution in general. Since herbicide resistance has only been an issue since the 60’s, its research is still in the early stages. But thanks to technological advances in the areas of molecular and genetic sciences many new methods are available to answer these pressing questions.

Ultimately, the aim of these studies is to provide better weed management strategies for farmers and aerial applicators like Ken Hildebrandt. This can be done by developing diagnostic markers for monitoring resistance occurrence and helping to provide the farmers with the knowledge of which herbicides still work before the spraying season starts. The evolution of herbicide resistance in weeds is a human-made problem that will continue to be a challenge because wherever there is selection pressure, individuals with evolved traits to survive the selection pressure will prevail. Therefore, it is questionable if herbicides are a sustainable solution to our problems with weeds as it is just a matter of time until weeds become resistant to a herbicide. However, understanding how herbicide resistance works is a crucial step in preventing further resistance development from happening. This will help prolong the usage of herbicides that are already on the market, keeping food production affordable for years to come.

For more information on weed management and herbicide resistance visit the blogs www.weedcontrolfreaks.com by the weed research group at the University of Wyoming and www.ahri.uwa.edu.au/blog/ by the Australian Herbicide Resistance Initiative (AHRI). Information, graphs and maps on the occurrence of resistant weeds can be found at www.weedscience.org.

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