Herbicide Resistance: Looking Back and Forward


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 Integrated Management of Downy Brome in Winter Wheat, 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.[row layout=”side-right” padding_bottom=”1rem” ]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.

Herbicide Resistance BMP Poster.
Herbicide Resistance BMP Poster (pdf)

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 via email at drew.lyon@wsu.edu or phone at 509-335-2961.