Wheat Weeds

There are three categories of weeds which affect winter wheat: winter annuals, summer annuals, and perennials.

Winter Annuals: These include the various mustards, wild lettuce, downy brome (cheat grass) and jointed goatgrass. Winter annuals germinate in the fall, usually in years with at least one significant fall rain event.

Yield losses from winter annuals are mostly from the loss of moisture and nutrients, as they seldom cause appreciable harvest loss. Yield losses can be significant: one study says that only 1 mustard plant per square foot can reduce wheat yields 10-15%, and 4 per square foot can rob up to 40% of yield.

Mustards and lettuce infestations are often worse on wheat field boundaries, and sometimes limited to those areas.  Often, however, the mustards are found throughout the field, distributed in a patchy manner, with clean areas and weedy areas interspersed.  

Treatment of the broadleaf weeds--tansy mustard, blue mustard, and wild lettuce--is best done early, before the wheat joints. Pre-joint treatments can include dicamba, the active ingredient in Banvel™, thereby providing control of kochia which would emerge later in the season as a summer annual.  This is important, since kochia is often resistant to the sulfonylurea family of herbicides. 

If the wheat has jointed, but not yet in the boot stage, the dicamba can be replaced with Starane™, an excellent kochia herbicide which is more expensive.

These early treatments normally provide excellent control of the existing mustards and lettuce, as long as enough moisture exists for weed growth.  To obtain the residual weed control offered by the various residual herbicides, a post application rainfall or irrigation is required.

Control of downy brome (cheat) and jointed goatgrass is more problematic, as the available herbicides often do not provide complete control. Severe infestations can be suppressed, and sometimes control is very good, but crop rotation may be necessary to provide complete control of these weeds. 

One chemical program for cheat and jointed goatgrass is the use of Clearfield™ wheat and imidazolinone  (Beyond™) herbicide from BASF.  Clearfield™ wheat is a special wheat cultivar, and is not not a GMO.  The best is to treat emerged, actively growing downy brome and jointed goatgrass in the fall, when they have no more than 2 tillers and 1-5 leaves. This generally occurs in late October to mid-November.  Spring applications require a higher rate and are generally used only as a second application.

With this system, control of later emerging downy brome can be acceptable if rain or irrigation occurs after treatment. There are rotation restrictions with Beyond. Click here to read a CSU white paper for more information.

Another product labeled for downy brome is Olympus™ or Olympus Flex™ from Bayer. Olympus' active ingredient is propoxycarbazone sodium, which offers control of mustards and suppression (not control) of jointed goatgrass. 

Best timing is in the fall, when the downy brome has mostly emerged but is no larger than 2-3 tillers. Olympus Flex is more expensive, but Bayer says efficacy on downy brome is improved, and the rotation restrictions are less limiting.

For soil activity to be effective on later emerging flushes, 1/2" of rainfall or irrigation is required after application. 

To see a photo of a wheat field where we applied a spring Olympus treatment, click here.   To read more about Olympus on the Bayer website, click here.

Again, the best control of wheat grasses will be achieved with crop rotation to a crop such as corn, where tillage and herbicides such as glyphosate and atrazine can achieve very good control.

Summer Annuals: These include kochia, sunflower, Russian thistle, lambsquarter, and pigweed. The summer annuals germinate in the spring, and are normally not a problem in wheat with a thick stand.  In years when there is spring moisture and the wheat has thin spots, summer annuals can cause problems. 

Yield loss from summer annuals is partially from the loss of moisture and nutrients, but also from harvest losses.

If the wheat is treated early in the growing season for winter annuals (pre-joint or pre-boot) with a residual herbicide, the threat from summer annuals is largely eliminated. If the wheat is not treated prior to boot stage, treatment is considered pre-harvest, and can not be accomplished until the wheat is in the dough stage. These larger weeds can often be controlled even in the pre-harvest stage (if they have enough moisture for growth) but treatment should be done as soon as possible after the dough stage to allow maximum time for the weeds to dry down.   

Pre-harvest chemical options include tank mixes of 2,4-D and dicamba or a mixture of glyphosate (Roundup™) and 2,4-D.  

Perennials: The most common perennial weed in winter wheat in this area is field bindwind. This weed is difficult to control, and requires multiple treatments over a period of years.