Targeting weeds at the genetic level aids battle against herbicide resistance

| | September 22, 2017

To a farmer, weeds might be the ultimate enemy. For as long as farmers have been cultivating crops, weeds have always been there, choking off crops and causing untold damage. Weeds are powerful adversaries: they adapt quickly, changing their genetic codes to survive in the fields.

How do they do that? It seems like the weeds always have the next trick up their sleeves; to beat them, we would have to be able to predict how changes in the weeds’ genomes occur that confer resistance. In this Modern Farmer article, Alexa Kurzius describes how weed genomics can help scientists figure out how and why herbicide resistance develops and how weeds become more invasive.

Put simply, a genome is the blueprint for an organism’s bloodline — the traits passed down from parent to offspring. It’s usually encoded in the DNA. Using different DNA sequencing techniques, some have partially or fully mapped various crop weed genomes.


Crop weed scientist Patrick Tranel’s research group from the University of Illinois at Urbana-Champaign began sequencing part of the waterhemp’s genome in 2009. Waterhemp has remarkable genetic diversity, giving it “a quick and cunning ability to adapt to its surroundings,” including herbicide resistance. Tranel was looking for genes that might be involved in herbicide resistance; today, his lab has begun testing specific gene mutations in waterhemp that provide resistance. Another plant scientist, Neal Stewart from the University of Tennessee has focused his efforts on sequencing the entire horseweed genome. Horseweed will be the first weed to have its entire genome sequenced, and the genetic information is expected to be released to the public later this year.

“By studying resistance mechanisms we’re hoping to understand these kinds of weaknesses for the farmers’ benefit,” says Doug Sammons, a chemist at Monsanto who has partnered with Stewart for the last ten years. The horseweed genomic information will likely help determine the best methods to manage the weed rather than just new herbicide discoveries.

That doesn’t mean that the scientists are planning to genetically modify the weeds. Instead, the information would ideally be used to first help scientists understand how resistance develops in a weed, and then design herbicides that target specific genes in the weed.

The dream? That work done by his lab and other scientists will help chemical companies develop herbicides that target specific genes, akin to designer drug development in the pharmaceutical industry.

“That’s our hope in the long term,” says (USDA research plant scientist David) Horvath.

This new understanding of weeds should give farmers a distinct advantage when combined with current methods for combating weeds: mechanical weeding, herbicides, biotech crops and crop rotations. And while many have blamed genetically modified crops for today’s herbicide-resistant weeds, genetic and genomic technology – pairing biotech crops with designer herbicides – plus integrated pest management might be exactly what farmers need to truly fight weeds.

Related article:  Opportunity lost: Has Uganda paid a price for not embracing GMOs, biotechnology?

XiaoZhi Lim is a freelance journalist and former GLP editor and writer.

4 thoughts on “Targeting weeds at the genetic level aids battle against herbicide resistance”

  1. Mr. Lim, you state, “Weeds are powerful adversaries: they adapt quickly, changing their genetic codes to survive in the fields.” I understand that you are writing for an educated but not academic audience. You’ve misstated the phenomenon of resistance as if weeds are changing their genes to survive. Thus, you’re continuing a misunderstanding of evolution. Those mutations in genes associated with enzymes and/or cell receptors that are specifically targeted by various herbicide biochemical classes already exist in the population. The mutations may be simple amino acid changes in key parts of the enzyme, for example, that make binding and thus inhibiting the enzyme less likely unless the dosage is elevated significantly over what would normally be applied. The change that you are talking about is actually the change in frequency in the population of resistant alleles. But the key point is resistant alleles, i.e., forms of the gene inherited from one parent or the other, already have these mutations and repeated use of the same selecting agent (i.e., herbicide) just changes the prevalence in the next generation. Of course all this is entirely preventable…simply don’t let your weeds have sex will do it. Thus, proper integrated pest management applied to weed control is post spraying surveillance where weed escapes are rogued out of the field. Takes time and money, but that form of management reduces the likelihood of resistance.

    • Good clarification of this key technical point, Allan. All too often we have seen fear propagated about “superweeds” based on some presumption that the weeds are somehow smart, wily things with virtually human intelligence. You are entirely correct that there is no such thing as a superweed in the sense that is traditionally intended as an attention grabber. Herbicide resistant weeds are simply selected by intensive use of a particular herbicide from the pre-existing population. Same with insects and pathogens.

  2. As I look over the many plots of GMO soybeans in Wisconsin, some fields appear to be clean of weeds but there are many others that are loaded with weeds. Seems to me to delay the resistance, why not hire school kids or people who need work, to walk the fields and yank weeds or, if too hard, use a big pair of scissors and cut at the dirt line. also, along the edge of the fields, do the same before the weeds go to seed. seems like the farmers are not doing their job to get after the weeds. with gmo corn, could do the same before the corn gets too big. again, yank/cut the weeds in the plot and along the field edges. corn gets big, will shade out the weeds and should not get seeds!

    • Putting school kids in agricultural fields would be a great concept,.starting in late June in Wisconsin and ending at Labor Day. One small problem– weeds germinate in March and they continue growing until frost, which in Wisconsin these days will be about the end of November. The weeds don’t adhere to school schedules.

      Not long ago in our society kids actually did work in agriculture instead of playing video games. Children actually were expected to work (and some were actually forced to work) when the U.S. was predominately an agrarian society and small self sufficient farms constituted a majority of the population. That was over 100 years ago. Even 60 years ago you could still find a few school kids to work on farms pulling weeds, at least in the northern and central states. There even used to be a “Fresh air kids” program sponsored by the government where urban children with adjustment issues were bused to farms and introduced to the concept of field work. In the south, it was mostly low income women (the “ladies”) who desperately needed the work, and who actually knew the difference between a weed and a cotton or soybean plant. Hoeing weeds was all back breaking work in hot, dusty fields, all the while exposing a disadvantaged subset of our people to the risks of continuous sun exposure. Is that really what you are advocating?

      All things considered, I would not recommend going back to that. The advent of modern herbicides in the 1950s was a good step forward, and coupled with biotechnology the targeted use of herbicides in agriculture continues to improve human life. Today most of us in the industrialized world view our surroundings through rosy glasses. Things didn’t appear quite so pleasant when those glasses were drenched in sweat.

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