Herbicide Resistant Johnsongrass: Coming soon to a farm near you!

Written by Matt DiLeo

Pioneer and K State are jointly releasing a set of new herbicide resistant sorghum varieties, which will incorporate resistance to ALS and FOP herbicides. Ironically, these non-genetically modified varieties invoke one of the classic bogeymen of anti-GM thinkers – herbicide resistant weeds.

In the past, I’ve heard a couple pro-genetic engineering scientists give herbicide resistant sorghum as an example of a potentially irresponsible creation. This is because another Sorghum species is a notorious noxious weed throughout much of the world: Johnsongrass. Presumably, herbicide resistance alleles in a grain sorghum variety could quickly jump to Johnsongrass, where it would subsequently burn through the rural landscape, making the associated herbicides useless. Johnsongrass is without a doubt one of the most strikingly common weeds in roadside ditches and farm fields here in rural West Virginia and Maryland.* This new potential threat of herbicide resistant Johnsongrass is ironic not only because the resistant grain sorghum varieties were created by non-transgenic methods, but also because these varieties were developed specifically as a control for Johnsongrass!
A Pioneer website states that grain sorghum x Johnsongrass hybrids rarely occur and are almost always sterile. I don’t doubt this, but the extreme abundance of this weed easily beats any numbers game – especially with the intense selection of regular herbicide applications. I don’t know the agronomy of this system or whether the total cultivation/pesticide strategy does anything special to mitigate the risk of ALS/FOP resistant Johnsongrass emergence, but I’m going to assume it’s an absolute sure thing unless someone has data to the contrary.
The non-transgenic status of these plants in no way lessens or ameliorates this risk. The method of genetic change is completely inconsequential. I’m usually a big fan of herbicide resistant crops, but this just sounds like a terrible idea to me all around. Anyone know otherwise?
h/t: Jesse Bussard @cowgirljesse (who wrote about Jgrass here)
* I know of one house where it has apparently replaced the ornamental grasses in the center of the homeowner’s garden.

Written by Guest Expert

Matt DiLeo has a PhD in Plant Pathology from UC, Davis. During his postdoctoral research at Boyce Thompson Institute, he researched unintentional effects of genetic engineering. Matt builds R&D teams and biotech platforms: genome editing, gene discovery, microbials, and controlled environment agriculture.


  1. Heh. Yeah, a lot of people have been led to believe that the only way to get resistance is GM. Alas.
    But I was once in a discussion of roadside weed resistance with someone, and they said to me: Well, unless it’s resistant to flamethrowers, it won’t be a problem for us.
    I laughed–as far as I know that’s not the case.

  2. That only applies so long as everyone deals with the roadside weeds before they set seed – if you have a population which can proliferate seed far and wide then clearly its an issue for whoever utilizes the herbicide of choice – because while a flamethrower (or even a mower) can deal with any herbicide resistance it encounters – one probably doesn’t want to utilize either in a production field.

  3. It does sound like there’s more potential for resistant weeds with this than with roundup ready. However, I would imagine that the greatest risk of herbicide resistance comes from the proliferation of naturally resistant mutants in the face of a strong selective pressure, rather than by vertical transfer. Thus it seems to me that the best tactic for beating herbicide resistance would be to mix herbicide resistance in a variety, i.e. stack resistance genes (e.g. ALS resistance, and glyphosate resistance). Using this technology it would be possible to spray (together or in seperate terms) two or more different herbicides eliminating individuals resistant to one herbicide. Surely the chance of one individual containing mutations conferring resistance to >1 herbicide is vanishingly small. Another advantage of this is that one could choose to use two particularly environmentally friendly herbicides (e.g. glyphosate), using less of each and thus their individual effects on the environment are reduced further.

  4. This is a good topic, and it has been approached in a variety of ways. How should we treat herbicide tolerance bred into crop plants through traditional means? Some anti-GE activists in Europe decided it would be best to start targeting these as “hidden GMOs” and uprooted some mutagenized sunflowers that were supposed to be herbicide tolerant. Other groups, such as the center for food safety in the US, don’t even think about it and are pushing for the USDA to regulated only GE-derived herbicide tolerance. Tolerance derived through breeding doesn’t enter into the equation for them.
    There are two essential courses of action – one is to consider it by the method used to derive the trait, and the other, by what the trait and its potential effects are. I favor the latter, personally. (Although I don’t think uprooting everything that is herbicide tolerant is a rational way to go!)
    How do we manage the evolution of resistant weeds? One thing to keep in mind is that every plant is resistant/tolerant to some herbicide somewhere. So while you could regulate the use of new herbicide tolerance traits, you still have the tolerances that the plants already have, which can be used and abused just the same. I wonder if, on a regulatory basis, we should focus on regulating the usage of herbicides to prevent resistance, rather than the traits in the plants? (Or will that doom some farmers to get overgrown by weeds because they already sprayed their limit?) Hmmm.

  5. One complication is that in some (most?) regions/markets, farmers have tremendous faith in the agchem companies to continue coming up with technological fixes to problems like herbicide resistant weeds – so they don’t necessarily see it as within their own interest (even long term) to put effort into slowing the development of resistance.

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