By Christy Sprague

Over the next month, many winter wheat acres will be treated with herbicides for weed control. While there are several herbicide options available for use in wheat, there are many factors growers should consider prior to deciding when to spray and what products to use.

Cold temperatures
While it looks like we are finally moving past the cold temperatures of the 2018 spring, it is important to remember that most herbicides labeled for weed control in winter wheat have specific instructions that state herbicide applications should be made when weeds are actively growing. Herbicides should not be applied when the crop is under stress from very cold temperatures, when there are wide fluctuations in day/night temperatures, when a frost has occurred or when temperatures are below freezing prior to, at or immediately following herbicide applications. A good rule of thumb is to only apply herbicides to winter wheat when the daily temperature is 50 degrees F or higher. Following this rule of thumb helps avoid possible wheat injury from herbicide applications and improves weed control.

Spring herbicide applications on winter wheat frost-seeded with red clover
The time-honored tradition of frost-seeding red clover in winter wheat has regained prominence for many growers. However, one of the greatest challenges for growers who are frost-seeding clover in winter wheat is in finding a herbicide that can be applied for weed control without damaging the clover. While there are several herbicides that can be applied in the fall that have little impact on frost-seeded clover, the only herbicide that can be applied in the spring without negatively impacting clover is MCPA.

28% UAN (nitrogen) as a herbicide spray carrier
Spring conditions have generally been unfavorable for nitrogen applications to winter wheat this year. While in the next few weeks many growers will be applying nitrogen to wheat, some growers choose to split their nitrogen applications and apply a portion of their nitrogen with their herbicides. Several years ago, Michigan State University researchers conducted research to determine the effect of applying the herbicides Affinity BroadSpec, Huskie and 2,4-D with 28% UAN (nitrogen) as a spray carrier on weed control and wheat tolerance. From this research, they were able to formulate the following recommendations:

• 2,4-D amine or 2,4-D ester at 1 pint per acre can be applied with liquid nitrogen fertilizer solutions (28% UAN) as the spray carrier at 100% or a 50:50 28% UAN:water mixture. 2,4-D ester formulations generally mix easier with fertilizer solutions than 2,4-D amine formulations. When applying 2,4-D products with 28% UAN as the spray carrier, DO NOT include surfactant. The addition of other herbicides or fungicides to these mixtures will likely increase the risk for crop injury.

• MSU does not recommend applying Affinity BroadSpec or Huskie with 100% (56 pounds actual N) 28% UAN as the spray carrier. The risk of crop injury and potential yield reductions is higher with these combinations. The full load of surfactant at 0.25% v/v used in these combinations was likely the cause for increased injury.

• Affinity BroadSpec and Huskie can be applied with 50:50 ratio of liquid nitrogen fertilizer solutions (28% UAN) and water (28 pounds actual N). Reducing the surfactant rate in these mixtures to 0.125% v/v will also reduce the risk for crop injury. Wheat tolerance is also greater if applications of these combinations are made prior to wheat jointing (Feeke’s stage 6).

• Quelex is a newer herbicide that can be applied in spray solutions containing nitrogen fertilizer. While this herbicide was not available while we were conducting our spray carrier research, we have applied Quelex with 100% UAN in a previous research trial and observed a slight increase in injury to wheat compared with Quelex applied alone. However, this injury was only temporary and did not impact wheat yield. If using this practice, do not add more than a 0.25% v/v surfactant.

• We have never applied Talinor using 28% UAN as a carrier. However, since the use of ammonium sulfate (AMS) is prohibited on the label, our recommendations would be to not apply Talinor with 28% UAN as the herbicide carrier. 

Winter wheat growth stage and weeds controlled
All herbicides have a maximum wheat growth stages for application listed on the label. Late herbicide applications can lead to excessive crop damage that can cause kernel abortion and blank wheat heads that can ultimately reduce yield. Some of the more restrictive herbicides that are used in winter wheat are the plant growth regulator herbicides including 2,4-D amine, 2,4-D ester, dicamba (Banvel or Clarity), MCPA and Curtail (2,4-D amine + Stinger). The plant growth regulator herbicides are typically good on summer annual weeds like common lambs-quarters, pigweed and common ragweed, but vary in their control of some of the more common winter annual weeds like common chickweed. Curtail, 2,4-D and MCPA will not control chickweed.

All plant growth regulator herbicides need to be applied prior to winter wheat jointing (Feeke’s stage 6). Other herbicides that need to be applied prior to Feeke’s stage 6 are some of the more popular herbicides for control of grasses in wheat, like windgrass and the newer grass problem roughstalk bluegrass. These herbicides include Osprey, PowerFlex HL and Puma. PowerFlex also has good activity on many of the broadleaf weeds encountered in wheat, including common chickweed. If winter wheat is at jointing, these herbicides should no longer be used. Axial XL is another herbicide that has good control on many grasses in wheat and should be applied up to the pre-boot stage of wheat.

The herbicides Affinity BroadSpec, Harmony Extra, Harmony, Express, Huskie and the new herbicides Quelex and Talinor are not as restrictive as many of the plant growth regulator herbicides. These herbicides can be applied to wheat until just before the flag-leaf is visible (Feeke’s stage 7.9). Talinor can be applied up to Feeke’s stage 8. All of these herbicides also have better control of common chickweed than many of the growth regulator herbicides. Peak, another herbicide, is also an option for common chickweed control. However, longer rotation restrictions (22 months) to many crops including soybean often restrict the use of this herbicide.

Buctril, Stinger, Starane and Widematch (Stinger + Starane) are other herbicides that will control broadleaf weeds in winter wheat. These herbicides have the longest application window. They can all be applied to winter wheat up to the boot stage (Feeke’s stage 9). However, many of these herbicides have fairly narrow spectrums of weed control. Buctril provides better control of summer annual weeds and is not very effective against winter annuals. Starane has a very narrow weed control spectrum, but is excellent in controlling hemp dogbane. Stinger, on the other hand, provides excellent Canada thistle control.

More information on controlling winter and summer annual weeds, including windgrass, can be found in Chapter 3 of MSU Extension‘s “2018 Weed Control Guide for Field Crops” (E0434).

Sprague is with the Michigan State University Extension, Department of Plant, Soil and Microbial Sciences.