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Herbicide Resistance: Looking Back and Forward

Posted by Blythe Howell | November 17, 2020

As I watch the snow falling outside my basement window – COVID-19 has driven me from my university office to my basement – I find myself not only reflecting back on the 2020 crop season but back across my now 30 years as an Extension specialist. When I started as the Extension Dryland Cropping Systems Specialist at the University of Nebraska-Lincoln Panhandle Research and Extension Center in Scottsbluff in 1990, the winter annual grass weed triumvirate of downy brome, jointed goatgrass, and feral rye (a.k.a. volunteer rye) were the bane of every wheat grower. This was before Clearfield or CoAXium wheat production systems existed and before Maverick herbicide was released. I remember telling growers that it was highly unlikely that they would ever have an herbicide that would selectively control these winter annual grass weeds in winter wheat. I thought this was particularly true for jointed goatgrass, which is closely related to wheat, sharing the D genome in common.
My focus at the time was to get growers to diversify and intensify their winter wheat-fallow cropping systems to control these winter annual grasses. I promoted the addition of summer crops such as proso millet and sunflower to the rotation. Dryland corn was added later as breeding efforts in corn resulted in more drought-tolerant hybrids with shorter maturities. The addition of these summer crops, particularly corn, required a reduction in tillage to conserve soil water.
The addition of one summer crop – it didn’t really matter which one – to the winter wheat-fallow cropping system made a huge difference in the fight against these troublesome winter annual grass weeds. Staying out of winter wheat for two years rather than one resulted in a significant decline in the  soil weed seedbank. The 1996 Farm Bill, which decoupled government support payments from base acres planted, allowed growers to add summer crops to their rotations without risking the loss of government payments. This resulted in an increase in adoption of summer crops in the rotation. In 2000, generic glyphosate became available, which made no-till an economically attractive option that resulted in even greater adoption of summer crops. Growers were managing their winter annual grass weeds and reducing the amount of fallow in their operations. I felt good about my educational efforts on managing winter annual grass weeds in wheat. The problem was not solved, but things had become manageable.

However, it was not long before the Clearfield wheat production system was introduced. This was quickly followed by the introduction of Maverick and Olympus herbicides. The ability to selectively control these winter annual weeds, which I had been saying was very unlikely, had come to pass. Growers could now control these weeds in wheat without the need of crop rotation. Many of the growers who had adopted the winter wheat-summer crop-fallow rotation primarily for weed control, returned to the winter wheat-fallow rotation. For nearly 20 years, this worked for them.

In 2012, I was lured to WSU by an Endowed Chair position funded by the Washington Grain Commission. I was welcomed by the same three winter annual grass weeds that I had worked on in Nebraska, along with rattail fescue, Italian ryegrass, and wild oat. I also found a very wheat-centric cropping system with very little crop diversity. This worried me because there were already signs that the herbicides we had been relying on for annual grass control in wheat for more than a decade were beginning to lose their efficacy.

My colleague, Dr. Ian Burke, has been screening weeds for herbicide resistance for several years with the financial assistance of the Washington Grain Commission. The results of his screening can be found on the Herbicide Resistant Weeds Map. Downy brome biotypes resistant to all of the Group 2 (ALS inhibitors) herbicides have been identified in Washington. Downy brome biotypes resistant to glyphosate have also been identified. A jointed goatgrass biotype resistant to Beyond has been identified in Washington and similar findings are being reported in other states. As I discussed in a recent Weeders of the West Blog post, wild oat biotypes resistant to many of the group 1 (ACCase inhibitors) and group 2 herbicides are becoming prevalent across eastern Washington.
It seems our dalliance with herbicides in lieu of crop rotation and other integrated weed management approaches is about to return us to the place where I started my career; a place where we have few, if any, effective herbicide options for the selective control of winter annual and other grass weeds in wheat. I am pretty certain that if we had used these herbicides and herbicide-resistant crops in partnership with crop rotation and diversity, many of these technologies would still be effective. While that horse is out of the barn, as they say, I do think it is a good lesson to keep in mind as new technologies come to the marketplace, as I discuss in this previous Timely Topic titled “A Word of Caution About Two New Weed Control Technologies“. When it comes to weed control, simple is often not sustainable.

Visit the Herbicide Resistance Resources page for more information on how to manage weeds in the era of widespread herbicide-resistant weeds.

Drew Lyon.
For questions or comments, contact Drew Lyon by email at or by phone at 509-335-2961.

10 thoughts on "Herbicide Resistance: Looking Back and Forward"

  1. Елена says:

    I was talking to a farmer once about herbicide resistance in weeds. He told me that his understanding of herbicide resistance was that if a reduced rate of a herbicide was used, a weed might only be half killed and could come back stronger (resistant) to the herbicide. I can see how that might make sense to people, but it s just not how herbicide resistance happens. I tried to explain to him how resistance developed, but I don t think he ever really got it. Hopefully I can do a better job in this article.

    1. Drew Lyon says:

      The application of a sublethal dose of an herbicide can eventually lead to herbicide resistance by killing or reducing seed production in the most susceptible individuals and allowing those individuals with slightly more tolerance to the herbicide to survive and produce more seed. With repeated application of sublethal doses, the population will shift to be predominantly more tolerant individuals and eventually the population may be classified as resistant to the herbicide. We have seen this in Russian thistle where 32 oz/A of glyphosate used to do a nice job of controlling Russian thistle in fallow, but but over time the effective rate increased to 40 oz/A, then 48 oz/A, then 56 oz/A then 64/ oz/A. We now have a few populations of Russian thistle that require two gallons/A or more of glyphosate for effective control. Another way of developing resistance is to put out a very effective dose of herbicide and quickly selecting for that rare individual that can survive that effective dose. Again, with repeated applications of the same herbicide, the population will eventually shift to being all resistant individuals. We have seen that in happen with chlorsulfuron (Glean) in Russian thistle and prickly lettuce. The rate at which this happens depends on a number of factors, including weed biology and cultural practices. The common denominator in both approaches is the repeated application of the same herbicide. This is what needs to be avoided to reduce the risk of developing herbicide resistance. The problem is that it is human nature to want to use what works for you, but if you do so too often, you will create a problem.

  2. Jerry Snyder says:

    Winter peas seem to be a rotation that is viable
    Requires good management when very little stubble is available

    1. Drew Lyon says:

      Jerry, I’m excited by the opportunity that winter peas, a.k.a. fall-sown peas, provide for diversifying the winter wheat-fallow cropping system. As you mention, they do come with a few concerns, but what crop doesn’t? Getting a broadleaf crop into this rotation opens the door for using different herbicide mechanisms of action. I would suggest not using the CoAXium wheat production system if you plan to grow winter peas. You don’t want to use quizalofop, the active ingredient in both Aggressor and Assure II herbicides, in both wheat and peas.

      1. Bob Venera says:

        Any herbicides for winter peas you do suggest?

        1. Drew Lyon says:

          Bob, effective postemergence grass control in winter peas can be achieved with clethodim (Select Max and other trade names), pronamide (Kerb), quizalofop (Assure II), and sethoxydim (Poast). That is the advantage of having a broadleaf crop in the rotation with wheat. The winter broadleaf weeds are a bit more of a challenge, but in a preliminary study conducted at the Lind Field Station in 2019, we saw excellent control of tansy mustard, flixweed, and tumble mustard with premeregnce applications of sulfentrazone + S-metolachlor (Broadaxe XC), imazethapyr (Pursuit and other trade names), saflufenacil (Sharpen) + metribuzin + linuron (Lorox), and flumioxazin (Valor) + S-metolachlor (Dual Magnum). We repeated this study in 2020 and results will be presented in the 2020 WSU Weed Control Report, which will be posted to the Wheat and Small Grains website ( in early January, 2021. The 2019 results are currently available on the website. Postemergence broadleaf herbicide trials have not looked quite so promising, but you can see the results on the Wheat and Small Grains website.

          1. Bob Venera says:

            Thanks Drew,
            We are direct seeders and want to try a continuous ww/wp rotation on one field. We average about 18″ of rain a year. My concern with chemicals is making sure whatever is applied doesn’t cause problems for the next crop. Any comments on that or anything else we should consider?

          2. Drew Lyon says:

            Bob, the first thing to consider is what are the primary weeds of concern for you in that field? Knowing the weed or weeds of concern will help me fine-tune my response. My next question is why are you interested in a continuous two-year rotation instead of a longer rotation? A two-year rotation only gives you one year between either crop, which is not long enough to have any significant effect on the soil weed seedbank. You really need at least two years between crops to gain the advantage that crop rotation can provide for weed control. While replacing summer fallow with winter peas will give you the benefit of crop competition with weeds that fallow does not, it may actually be more difficult to obtain the same high level of weed control in the winter pea crop as can be done in summer fallow. I would encourage you to think about a continuous three-year rotation, which should be feasible in an 18-inch annual precipitation zone, particularly in a direct-seed system. Depending on the weed species of greatest concern, the third crop could be spring canola, which provides the option for using a herbicide-resistant cultivar, or perhaps spring barley for grain or winter or spring triticale for grain or forage. Personally, I like three-year rotations, with or without summer fallow, much better than two-year rotations. I think in a direct-seed cropping system, longer and more diverse crop rotations are needed than in cropping systems that employ tillage. I’m happy to discuss herbicide selection once I know what the weeds of greatest concern are for you. Drew

  3. Mark Greene says:

    Drew, I’m all in with crop rotations using different mode of action to control not just grassy weeds but broadleaf’s as well.
    Thanks for encouraging us to stay involved in the fight against resistant weeds!

    1. Drew Lyon says:

      Thanks for the positive comment, Mark! I’m a big proponent for using crop rotation to help manage weeds. I know that short-term economics are sometimes a disincentive, but biology has proven itself all too capable of overcoming a chemical-only approach to weed management. Keep up the fight!!

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