Italian ryegrass is the bane of many Palouse-region farming operations. It is the “poster child” for herbicide resistance. The heavy reliance on a narrow range of herbicides labeled in wheat has resulted in the development of Italian ryegrass populations resistant to many of the herbicides previously used in wheat to control Italian ryegrass, such as the ACCase inhibitors (Group 1), ALS inhibitors (Group 2), and very long-chain fatty acid synthesis inhibitors (Group 15).
Over the past few years, many Palouse growers have added spring canola to their crop rotations. Some of this shift has been driven by canola prices relative to other crop rotation options such as the pulse crops, but much of it has been driven by herbicide-resistant Italian ryegrass. Glyphosate-resistant, a.k.a. Roundup Ready, canola allows growers to apply glyphosate to control Italian ryegrass. The technology has been very effective. However, as we have seen many times before, the more we rely on a single herbicide option for weed control, the more quickly herbicide resistance is likely to develop.
Italian ryegrass biotypes resistant to glyphosate (Group 9) and glufosinate (Group 10) have developed in PNW orchards that relied heavily on these two chemistries for weed control. This fact does not bode well for the long-term sustainability of glyphosate for the control of Italian ryegrass in canola.
I expressed my concern about relying too heavily on glyphosate for the control of Italian ryegrass in canola in a Timely Topic from 2019. In 2022, we established a field research trial (PDF) at the Cook Agronomy Farm near Pullman to look at ways to incorporate more than one herbicide site of action into spring canola production.
What we found was that glyphosate is still a very effective tool for controlling Italian ryegrass in glyphosate-resistant canola. However, the addition of trifluralin as a preemergence herbicide greatly reduced the quantity of Italian ryegrass that was subsequently treated with glyphosate.
Herbicide resistance is a numbers game–that is, the more plants you treat with an herbicide, the more likely you are to find that rare individual with the mutation that confers resistance. Having a preemergence herbicide down to reduce the number of individuals treated with glyphosate lowers the probability of selecting a biotype resistant to glyphosate, and thus extending the life of this useful technology.
In the short term, relying on just glyphosate for Italian ryegrass control in glyphosate-resistant canola is effective; however, this approach is likely to quickly result in the selection of Italian ryegrass biotypes that we can no longer control with glyphosate. Growers who wish to retain this effective technology for as long as possible should consider adding another effective herbicide to the program. A recent Weeders of the West post discusses The Reemergence of Preemergence Herbicides for the management of herbicide resistance in weeds.
Let’s help canola remain a potent tool in the battle with herbicide-resistant Italian ryegrass by using glyphosate-resistant canola wisely.
For questions or comments, contact Drew Lyon by email at firstname.lastname@example.org or by phone at 509-335-2961.