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Multi-herbicide resistant ryegrass in Arkansas wheat

Steven Powles of Australia once famously said that herbicide resistance transformed him from a herbicide scientist to a weed scientist. New evidence from samples taken in the summer of 2008 is pushing those of us who work in wheat the same direction here in Arkansas.

My wheat research counterpart in Fayetteville, Dr. Nilda Burgos, for some time now has been monitoring ryegrass populations across the state for herbicide resistance. Although we have been testing for many things her research and screening program has primarily documented the spread of ACCase-resistant (Hoelon) ryegrass to every wheat-producing county in Arkansas.

Over the past few years the use of Osprey herbicide, as well as an increased reliance on Finesse herbicide, has proven an effective tool for the control of this Hoelon-resistant ryegrass. We have been concerned, however, about the development of resistance to these herbicides as well, as our reliance on them has increased. Both Osprey and Finesse (as well as PowerFlex, Finesse Grass and Broadleaf, Harmony and Glean) are all ALS inhibitors. They are effective on a very specific enzyme in plants. Resistance to this family of chemistry is relatively easy for plants to achieve and, therefore, very widespread.

It is not the fault of Osprey herbicide; however, after only two years of use, several fields were brought to the attention of county agents last fall and spring following the failure of Osprey to fully control ryegrass. In all, 12 populations of ryegrass where Osprey failed were sampled and sent in to Dr. Burgos’ screening program. Unfortunately, all but one of these populations was resistant to not only Osprey, but also other ALS herbicides, including Dow’s new PowerFlex and Beyond (for Clearfield wheat). In addition, all but one of the populations was cross-resistant to Hoelon.

These fields were located in Arkansas, Cross, Lawrence, Phillips, Prairie, and St. Francis counties. For those growers, choices for ryegrass control in wheat are now very limited.

If you feel that you have both Hoelon- and Osprey-resistant ryegrass, then two of the best recommendations that I can make are non-herbicidal. First, do not grow wheat in those fields, fallow the ground if possible and control ryegrass with fall residuals, tillage or glyphosate. Do not allow ryegrass to go to seed and control ryegrass populations in adjoining areas like roadsides and equipment lots, etc. If you are going to grow wheat in these fields, then allow a flush of ryegrass to come up prior to wheat planting or emergence, control that flush of ryegrass with tillage or a stiff dose of glyphosate, then plant. Either of these cultural practices will result in around 75 percent to 80 percent ryegrass control according to work that myself and Dick Oliver have previously reported.

After you have killed the first flush of ryegrass, a good option would be to apply Prowl H2O after wheat has emerged. Another option would be the product Axiom from Bayer. Both have moderately good activity on ryegrass and are neither ACCase nor ALS inhibitors. I would then come back POST to 4-lf to 1 tiller ryegrass with Axial herbicide as needed. Axial is basically the same mode of action as Hoelon; however, the molecule is slightly different and our data indicates that it will control some populations of Hoelon-resistant ryegrass. Do not rely on Axial alone for Hoelon-resistant ryegrass control.


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