Weeds in wheat stubble continue to be an increasing problem, especially in fields that had thin wheat stands and abundant rain this summer. Weeds in dry areas can be especially difficult to control.
Because of this, it may be unreasonable to expect complete control of many of these populations. A more realistic approach might consider management to reduce seed production and facilitate moisture conservation.
Figure 1. Kochia and pigweeds in August, in a field to be planted to wheat. Photo by Curtis Thompson, K-State Research and Extension.
Weeds grow rapidly this time of year, and 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 get complete coverage and 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 is especially important to manage spray drift, both droplet drift and vapor drift. Keep in mind, when adding ammonium sulfate (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 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 foolproof, even on big weeds, but that is no longer the case. Dicamba and 2,4-D probably were not 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 to apply those treatments before the weeds exceed 4 to 6 inches tall, but that often does not happen.
Higher rates of the 2,4-D and dicamba may improve control, but in most cases we probably do not 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 plus an ammonium source such as AMS or urea ammonium nitrate (UAN) and can provide some 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 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. It is recommended to apply at least of 0.75 pounds ai/ac of paraquat, especially when trying to control think stands of pigweed or kochia. Recent research has revealed that pigweed control with paraquat is optimized when paraquat is applied through a nozzle and pressure combination that delivers a medium to coarse spray droplet classification (Figure 2). While control did not differ between the treatment produced fine and medium spray droplets, the potential for herbicide drift with the fine droplet size was much greater; therefore, a medium to coarse droplet size should be utilized.
Figure 2. Palmer amaranth control two weeks after treatment in a Sedgewick County stubble field in 2017. Control was influenced by different spray droplet classifications of paraquat applied at 20 GPA with 0.25% v/v NIS. Treatments with different letters indicate a significant different α = 0.05. Dv50 (µm) can be linked to spray droplet classifications in the right side of the figure that would be found in a nozzle selection book. Adapted from Hay et al. 2017.
Paraquat also needs to be applied with a nonionic surfactant at 0.25% v/v to enhance surface coverage of the plant foliage. Research has not been able to demonstrate the value of adding an ammonium source to enhance paraquat activity. A tank mix with a photosystem II-inhibiting herbicide such as atrazine, metribuzin, or linuron will enhance paraquat control and could provide some residual weed control. Because most of these herbicides are lipophilic, nonionic surfactant should be substituted with crop oil concentrate at 1 pt/ac when tank mixed with paraquat. Special consideration should be given to rotational restrictions of tank mix partners. If winter wheat is to be planted this fall, atrazine, metribuzin, and linuron should not be applied this summer. Atrazine is an option 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. Paraquat generally provides poor control of grass weeds in stubble situations; therefore, if applying paraquat to control pigweed or kochia, it would be better to make a sequential application of glyphosate to control grass weeds. Research at K-State has shown that tank mixing glyphosate with paraquat produces an antagonistic response and should be avoided (see Table 1).
Table 1. Large crabgrass control three weeks after treatment.
|
Herbicide |
Control (%) |
|
Paraquat |
53 |
|
Glyphosate |
99 |
|
Paraquat + glyphosate |
58 |
|
Glyphosate fb paraquat |
99 |
|
LSD α = 0.05 |
14 |
Different paraquat combinations were applied to 4-inch tall large crabgrass in Kingman County in 2017 (Table 1). Treatments were visually evaluated 3 weeks after application. Paraquat alone provided poor control of the large crabgrass while glyphosate alone provided excellent control. These types of responses are what we would expect from paraquat and glyphosate. An antagonistic response (58% control) was observed when glyphosate was combined with paraquat. To contrast, when glyphosate and paraquat were split-applied with the glyphosate applied 24 hours in advance of the paraquat, excellent control was observed. While the exact mechanism is unknown, it is likely due to a chemical or physiological incompatibility between paraquat and glyphosate. Table 1 adapted from Hay and Peterson, 2018.
If planting wheat this fall, a tank mix with Sharpen is an option to provide some residual broadleaf control, especially if more than 1 oz/ac is applied. 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. On the other hand, spray tips that produce very coarse or larger droplets, like those required for application of the new dicamba products, will not provide thorough enough coverage for good weed control.
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. Burndown treatments probably will not affect the viability of weeds seeds that are already mature at the time of application. 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.
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.
An additional option that has become more common again due to the increase in glyphosate resistance has been the use of tillage to control thick stands of pigweed or kochia. While the use of tillage can cause additional concerns (i.e., water conservation, soil erosion, loss of organic matter, etc.), tillage could be an integral part of an integrated weed management plan that would reduce the selection pressure for herbicide resistance with late-season herbicide applications.
Dallas Peterson, Weed Management Specialist
dpeterso@ksu.edu
Marshall Hay, Weed Science Graduate Research Assistant
mmhay@ksu.edu
References:
Hay MM and DE Peterson (2018). Interactions of tank-mix partners with paraquat for enhanced grass control. Weed Science Society of America Annual Meeting. In Proceedings 58:158.
Hay MM, DE Peterson, GR Kruger, T Butts (2017). Paraquat efficacy as influenced by spray droplet size for Palmer Amaranth Control. North Central Weed Science Society Annual Meeting. In Proceedings 72:149.
Tags: weeds weed control wheat