Where there has been plenty of rain this summer in Kansas, some fields that will be planted to wheat this fall may have become overgrown with broadleaf and grassy weeds.
Figure 1. Kochia and pigweeds in August, in a field to be planted to wheat. Photo by Curtis Thompson, K-State Research and Extension.
In those cases, some of the broadleaf weeds will have flowered and formed seed by now, which will make chemical control more difficult. Another potential problem for chemical control will be getting the spray down through the canopy to reach any weeds or grasses underneath the taller weeds.
The standard treatment over the years to control weeds and volunteer wheat in wheat stubble has been glyphosate plus 2,4-D LVE. If kochia was present, we may have added some dicamba. Where susceptible crops are nearby, it’s especially important to manage spray drift, both droplet drift and vapor drift. Keep in mind, when adding AMS to a herbicide mixture containing dicamba, volatility of the dicamba increases greatly. This is true for all formulations of dicamba. If a crop sensitive to 2,4-D is adjacent to the weedy field, 2,4-D amine should be used instead of 2,4-D LVE to minimize the potential for damaging volatility drift. 2,4-D amine may not be safe around cotton fields.
Glyphosate plus 2,4-D and/or dicamba remain the primary options 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 fool-proof, 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 with acceptable control. Timing and weed size is much more critical with almost all other herbicides than it has been 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.
Higher rates of the 2,4-D and dicamba may improve control, but in most cases we probably don’t want to exceed 1 qt/acre of 2,4-D or a pint/acre of dicamba. Sharpen is another herbicide tank-mix partner that may help with control of the pigweeds and provide some residual control. Sharpen works best with the addition of methylated seed oil and can provide some pretty good burndown on smaller weeds. If the weeds are very big, Sharpen tends to burn the tops and plants eventually resume growth. Sharpen requires complete coverage so using 15 to 20 gallons/acre spray solution is important.
One herbicide alternative to glyphosate that has worked pretty well on pigweed and kochia the last couple of years is Gramoxone 2SL (paraquat). Gramoxone 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. Gramoxone 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 Gramoxone 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. Using nozzles and application pressures resulting in uniform droplet sizes that provide sufficient coverage while minimizing the number of fine droplets can provide good control and reduce the potential for off-target movement. Keep in mind that flat fan nozzles and high pressure produce fine droplet sizes which are prone to move off target and can cause very striking paraquat injury wherever the droplet lands.
Producers should not expect perfect control of weeds and grasses from any treatments if the stands are unusually tall and thick, or if many of the weeds have flowered or formed seed. Producers should also be prepared for a second flush of weeds, and possibly volunteer wheat, once the main canopy is killed, so follow up treatments may be required.
Producers should be aware that burndown herbicides will not affect the viability of mature seeds in broadleaf weeds.
Using a sulfonylurea herbicide such as Finesse or Rave could improve control of certain broadleaf weeds and provide some residual control if planting wheat this fall, but will limit recropping options to sorghum or ALS-tolerant soybeans next spring. Many pigweed and kochia populations are ALS-resistant and may not be controlled by the ALS herbicides.
Dallas Peterson, Weed Management Specialist
Curtis Thompson, Extension Agronomy State Leader and Weed Management Specialist