By STACY CAMPBELL
Cottonwood Extension District
With the 2025 wheat harvest in full swing it is time to think about weed control in wheat stubble. When soil moisture conditions are favorable, weeds that have been suppressed by the canopy will grow rapidly once crop competition is removed. In addition, weeds that have emerged through the wheat canopy will be damaged during harvest and will quickly begin regrowth.
Delaying control can result in lost soil moisture that could be used for crop production, as well as weed seed production, which will cause difficulties in the future.
When thinking about weed control in wheat stubble, there are two priorities – controlling already emerged weeds and preventing later flushes. Making applications before weeds exceed 4 to 6 inches is necessary for good control of already emerged weeds. Residual herbicides are necessary to reduce the number of herbicide applications required to control multiple flushes of weeds.
Despite a growing number of herbicide-resistant weeds, glyphosate plus 2,4-D LVE and/or dicamba continue to be important for weed control in wheat stubble. However, these herbicides alone are not likely to provide adequate control of pigweeds or kochia, especially when applied in the hot, dry conditions that are common after wheat harvest.
Some herbicide options commonly used to control weeds after wheat harvest are listed below.
Paraquat (Gramoxone, others) is a Group 22 herbicide that can work well in place of glyphosate for nonselective control of emerged pigweed and kochia.
If planning to plant corn or sorghum next spring, a tank mix of paraquat with atrazine (Group 5) will enhance the control of emerged weeds and provide some residual weed control. Atrazine labels have recently changed such that the only noncrop uses of atrazine permitted are for wheat stubble in wheat-fallow-wheat, wheat-corn-fallow, and wheat-sorghum-fallow rotations.
Metribuzin is another Group 5 herbicide that can be used instead of atrazine to enhance control and provide some residual activity.
One final note regarding paraquat. Limited research out of Australia suggests applying paraquat 2 weeks after a glyphosate application will increase weed control. This is called a ‘double knock’ strategy. This information is included here not as a recommendation per se, but to encourage careful thought about when you want to utilize contact herbicides in your fallow weed management system.
If paraquat were sprayed with or before glyphosate, the rapid damage to leaf tissue with prevent uptake and translocation of glyphosate. However, if a glyphosate application partially controls weeds, there will be sufficient leaf area 2 weeks after application for paraquat to be effective.
Saflufenacil (Sharpen), a Group 14 herbicide applied at one to two fluid ounces per acre, is an option to provide postemergence and short-term residual control of Palmer amaranth, kochia, and other broadleaf weeds.
Tiafenacil (Reviton) is a Group 14 very similar to Sharpen in terms of weed control and adjuvant use.
Flumioxazin (Valor, others) is a Group 14 herbicide that can be added to burndown treatments at rates of one to four fluid ounces per acre for activity on emerged broadleaf weeds and some residual activity on broadleaf and grass weeds in wheat stubble. Flumioxazin can be mixed with glyphosate or clethodim (Select Max) for enhanced grass control. It can also be mixed with 2,4-D, atrazine, metribuzin, or paraquat.
Another group 14 herbicide that can be considered is sulfentrazone (Spartan, others). Sulfentrazone can be applied to stubble and will control Palmer amaranth and kochia as well as other broadleaf weeds and some grasses. Of the Group 14 herbicides discussed, flumioxazin and sulfentrazone provide the greatest residual activity; however, saflufenacil provides greater postemergence kochia control than flumioxazin.
Research to evaluate Palmer amaranth control in wheat stubble was conducted in Hays during 2019 and 2020. Herbicides were applied approximately 3 weeks after harvest to Palmer amaranth that was 2 to 3 feet tall. Palmer amaranth control 4 weeks after application in general, treatments that included paraquat (Gramoxone) resulted in the greatest Palmer amaranth control.
Information provided by K-State weed control specialists.
Stacy Campbell is a Crop Production Extension agent in the Cottonwood District (which includes Barton and Ellis counties) for K-State Research and Extension. You can contact him by e-mail at [email protected] or by calling 785-628-9430.