Post-harvest weed control in wheat stubble is important to conserve soil moisture and prevent weeds from going to seed and adding to the weed seedbank. Excessively wet weather this spring, particularly in May and early June, will likely require earlier and more intensive weed management efforts this year. Also, where there are bare spots either because of hail damage or flooding, weeds may be growing quickly. Controlling these weeds while there is moisture and active growth, before they set seed, is a good idea.

The standard treatment for many years to control weeds and volunteer wheat in wheat stubble was glyphosate plus 2,4-D LVE. If kochia was present, we may have added some dicamba, but it generally wasn’t added in the eastern areas of Kansas because of drift concerns to soybeans. Glyphosate plus 2,4-D and/or dicamba remain a primary option for weed control in stubble, but with the development of glyphosate-resistant weeds, these options certainly don’t work as well or quickly as they used to. Glyphosate used to be fairly foolproof, even on big weeds, but that is no longer the case. Dicamba and 2,4-D probably weren’t contributing as much to the weed control in those tank mixes as we may have thought, so now we are struggling to achieve acceptable control.

Timing and weed size is much more critical with almost all other herbicides than it was with glyphosate. Consequently, it is very important to try and apply those treatments before the weeds exceed 4 to 6 inches tall, but that often doesn’t happen. In addition, treatment before weeds exceed 4 inches tall may require a number of applications to manage multiple flushes of weeds, which adds significantly to the cost of control.

One herbicide alternative to glyphosate that can work well to control emerged pigweed and kochia is paraquat. Paraquat is a contact herbicide, so spray coverage is critical. Spray volumes of 20 gallons/acre or higher are preferred, especially on larger and thicker weeds. Paraquat also needs to be applied with a nonioinic surfactant or oil concentrate to enhance surface coverage of the plant foliage. A tank mix with atrazine will enhance control and provide some residual weed control if planning to plant corn or sorghum next spring.

Likewise, metribuzin can be tank-mixed with paraquat if rotating to soybean to enhance control and provide some residual. If planting wheat this fall, a tank mix with Sharpen is an option to provide some residual control. Recent work at K-State suggests that making the paraquat application as soon as possible following wheat harvest allows for better coverage and more effective control, especially of the pigweeds.

Another herbicide that can be added to the burndown treatments for residual broadleaf weed control in wheat stubble is flumioxazin (Valor and others). Flumioxazin has been used as a pre-plant/pre-emergence treatment in soybeans for years, but it hasn’t been used much in wheat stubble because of the cost. However, with the recent reduction in flumioxazin prices, it may be worth considering as part of our stubble management treatments for residual weed control, especially the pigweeds.

Wheat can be planted 30 days after 2 oz/ac, or 60 days after 3 oz/ac Valor application, if at least 1 inch of rain occurs between application and planting. Corn, sorghum, cotton, sunflowers or soybeans can be planted the following spring following flumioxazin treatment. Residual weed control with flumioxazin will depend on rainfall for activation, just as with pre-plant treatment in soybeans.

 

— Ryan Flaming is a Kansas State Research and Extension Agent for Harvey County. Agriculture is his specialty. The Harvey County Extension Office can be contacted at 284-6930.